Antioxidant Activity of Marine Algal Polyphenolic Compounds: A Mechanistic Approach

Abstract

Polyphenolic compounds isolated from marine algae exhibit a broad spectrum of beneficial biological properties, including antioxidant, anticancer, antimicrobial, anti-inflammatory, and antidiabetic activities, along with several other bioactivities centered on their antioxidant properties. Consequently, polyphenolic compounds are increasingly being investigated for their potential use in food, cosmetic, and pharmaceutical applications. The antioxidant activities of these compounds have been explored widely through experimental studies. Nonetheless, a theoretical understanding of the structural and electronic properties could broaden research perspectives, leading to the identification and synthesis of efficient structural analogs with prophylactic uses. This review briefly summarizes the current state of knowledge regarding antioxidant polyphenolic compounds in marine algae with an attempt to describe the structure–activity relationship.

Introduction

Oceans cover >70% of the Earth's surface and are a habitat for a wide variety of marine organisms with an immense biological diversity.¹ These species produce a variety of secondary metabolites that have different structural and functional properties compared to their terrestrial counterparts.² Among them, a distinctive place is reserved for phenolic compounds in marine algae, as these algae have been recognized as a rich source of biologically active phenolic compounds.³

Table 1 presents a list of phenolic compounds isolated from marine algae. Owing to their broad spectrum of antioxidant activities, these compounds have been recognized as having protective effects against many disease conditions, including cardiovascular diseases, diabetes, cancer, atherosclerosis, aging, and other degenerative diseases.^39,40 Due to the potential beneficial effects on human health, research on natural antioxidants such as these phenolic compounds remains an interesting area of study.⁴¹ This review mainly focuses on the antioxidant properties and active health benefits of the polyphenolic compounds derived from marine algae and their possible potential role in developing pharmaceutical, food, and cosmeceutical products.

Table 1.

A Comprehensive List of Phenolic Compounds Isolated from Marine Algae

	Seaweed	Identified compounds	References
Phenolic acids	Halimeda monile	Salicylic, gallic, and caffeic acids	⁴
	Laminaria digitata	Gallic, protocatechuic, gentisic hydroxybenzoic, chlorogenic, vanillic, caffeic	⁵
	Dictyota dichotoma	Gallic, protocatechuic, gentisic, p-hydroxybenzoic, chlorogenic, vanillic, syringic
	Fucus vesiculosus	Gallic, protocatechuic, gentisic, chlorogenic, vanillic, caffeic
	Fucus serratus	Gallic, protocatechuic, gentisic, chlorogenic, vanillic, caffeic
	Fucus distichus	Gallic, protocatechuic, gentisic, chlorogenic, caffeic
	Fucus spiralis	Gallic, protocatechuic, gentisic, chlorogenic, vanillic, caffeic
	Mastocarpus stellatus	Gallic, protocatechuic, gentisic, p-hydroxybenzoic, chlorogenic, vanillic, caffeic
	Polysiphonia fucoides	Gallic, protocatechuic, gentisic, chlorogenic, syringic, caffeic, salicylic
	Saccharina latissima	Gallic, protocatechuic, gentisic, p-hydroxybenzoic, chlorogenic, vanillic, syringic
	Gracilaria vermiculophylla	Gallic, protocatechuic, gentisic, p-hydroxybenzoic, vanillic, syringic, salicylic
	Palmaria palmata	Gallic, protocatechuic, gentisic, p-hydroxybenzoic, chlorogenic, syringic, caffeic
	Porphyra purpurea	Gallic, protocatechuic, gentisic, p-hydroxybenzoic, chlorogenic
	Chondrus crispus	Gallic, protocatechuic, gentisic, p-hydroxybenzoic, chlorogenic, syringic, caffeic, ferulic
	Ulva lactuca	Gallic, protocatechuic, gentisic, p-hydroxybenzoic, chlorogenic, vanillic, syringic, caffeic, salicylic, coumaric
	Sargassum muticum	Gallic, protocatechuic, gentisic, p-hydroxybenzoic, chlorogenic, vanillic, syringic, caffeic
	Enteromorpha intestinalis	Gallic, protocatechuic, gentisic, p-hydroxybenzoic, chlorogenic, vanillic, syringic, caffeic, salicylic
	Spongiochloris spongiosa	Protocatechuic, p-hydroxybenzoic, 2,3-dihydroxybenzoic, chlorogenic, vanillic, caffeic, p-coumaric, salicylic acid, and cinnamic acid	⁶
	Anabaena doliolum	Protocatechuic, p-hydroxybenzoic, 2,3-dihydroxybenzoic, chlorogenic, caffeic, p-coumaric, salicylic acid, and cinnamic acid
	Spirogyra sp.	Gallic acid, methyl gallate	⁷
Flavonoids	Acanthophora spicifera	Acanthophorin A, acanthophorin B, tiliroside, catechin, quercetin	⁸
	S. muticum, Hypnea spinella, Porphyra sp., C. crispus, Halopytis incurvus	Daidzin, genistin, ononin, daidzein, sissotrin, genistein, formononetin, biochanin	⁹
	Undaria pinnatifida	Daidzin, genistin, ononin, sissotrin, formononetin biochanin
	Acetabularia ryukyuensis	Catechin, epicatechin	¹⁰
	Tydemaniz expeditionis, Caulerpa serrulata	Catechin, epigallocatechin
	Caulerpa racemosa	Epigallocatechin
	Eisenia bicyclis	Catechin, epicatechin, epigallocatechin, epicatechin gallate, epigallocatechin gallate
	Hizikia fusiformis	Epigallocatechin
	Laminaria religiosa, Sargassaceae ringgoldianum	Catechin
	Ishige okamurae	Catechin, epigallocatechin
	Padina arborescens	Catechin, epigallocatechin, epigallocatechin gallate
	Padina minor	Catechin, epicatechin, epigallocatechin gallate
	Hydroclathrus clathratus	Catechin
	Sargassum thunbergii	Catechin, epigallocatechin
	S. muticum	Epicatechin, epigallocatechin, epicatechin gallate
	Turbinaria ornata	Catechin, epigallocatechin
	Porphyra yezoensis	Catechin, epigallocatechin gallate
	Chondrus verrucosus, Ceratodictyon spongiosum	Catechin
	Gloiopeltis complanata	Catechin, epigallocatechin
	Gelidium elegans	Catechin, epicatechin, catechin gallate
	Chondrococcus hornemannii	Catechin, epicatechin, epigallocatechin, epigallocatechin gallate
	Gracilaria texorii, Gracukarua asiatica	Epigallocatechin, epigallocatechin gallate
	Caulerpa sertularioides	Gallocatechin, epicatechin, catechin gallate	¹¹
	Turbinaria conoides	Epigallocatechin, catechin, epicatechin
	Sargassum polycystum	Epigallocatechin gallate
	Padina australis	Catechin
Phlorotannins	Ecklonia cava	Phloroglucinol, eckol, phlorofucofuroeckol A, dieckol and 8,8′-bieckol, 6,6′-bieckol, dioxinodehydroeckol	^12 –15
	E. bicyclis and Ecklonia Kurome	Phloroglucinol, eckol, phlorofucofuroeckol A, dieckol and 8,8′-bieckol	¹²
	Ecklonia stolonifera	Phlorofucofuroeckol A, dieckol, dioxinodehydroeckol, eckstolonol, triphlorethol-A, fucosterol, phloroglucinol, eckol, phlorofucofuroeckol-A, 2-phloroeckol, 7-phloroeckol	^16 –19
	F. distichus	A trimeric fucol, phloroethol, fucophloroethol	²⁰
	Eisenia arborea	Phlorofucofuroeckol-B	²¹
	I. okamurae	Diphlorethohydroxycarmalol	^22,23
	Ishige foliacea, Ishige sinicola	Octaphlorethol A	^24 –26
	Cystophora congesta	Phloroglucinol triacetate, diphlorethol pentaacetate, bromotriphlorethol-A1-heptaacetate, bromotriphlorethol-A2-heptaacetate, tetraphlorethol-C-nonaacetate, fucodiphlorethol-D-decaacetate, triphlorethol-A-heptaacetate	²⁷
	Analipus japonicus	Difucol hexaacetate, trifucol nonaacetate, and tetrafucol-A and B dodecaacetates	²⁸
Halogenated phenolic compounds	A. japonicus	Bromo- and chlorotrifucol nonaacetate, 5′-bromo- and 5′-chlorotetrafucol-A dodecaacetate, as well as 5′-bromo- and 5′-chloropentafucol-A pentadecaacetate (Fig. 6K–N)	²⁸
	E. arborea	Iodo phloroglucinol and bromo phloroglucinol	²⁹
	Symphyocladia latiuscula	Brominated and fully substituted mono- and bis-phenols	^30,31
	Polysiphonia urceolata	7-Bromo-9,10-dihydrophenanthrene-2,3,5,6-tetraol, 4,7-dibromo-9,10-dihydrophenanthrene-2,3,5,6-tetraol, 1,8-dibromo-5,7 dihydrodibenzo[c,e]oxepine-2,3,9,10-tetraol, urceolatol, 3-bromo-4,5-dihydroxybenzaldehyde, and 3,5-dibromo-4-hydroxybenzaldehyde	³²
	Rhodomela confervoides	3-(2,3-Dibromo-4,5-dihydroxybenzyl)pyrrolidine-2,5-dione, methyl 4-(2,3-dibromo-4,5-dihydroxybenzylamino)-4-oxobutanoate, 4-(2,3-dibromo-4,5-dihydroxybenzylamino)-4-oxobutanoic acid, 3-bromo-5-hydroxy-4-methoxybenzamide, and 2-(3-bromo-5-hydroxy-4-methoxyphenyl)acetamide (Fig. 6I, J, O–Q)	³³
	E. bicyclis E. kurome	2,4-Dibromophenol, 2,4,6-tribromophenol, dibromo-iodophenol	³⁴
	C. congesta	Bromodiphlorethol pentaacetate	²⁷
	Cystophora retroflexa	Halogenated phlorotannins (Fig. 6S, T)	³⁵
	Carpophyllum angustifolium	Chlorophloroglucinol, iododiphlorethol, chlorobifuhalol, and chlorodifucol	³⁶
	Laminaria ochroleuca	Monochlorodiphlorethol C heptaacetate	³⁷
	Analipus japonicus	Bromo- and chlorotrifucol nonaacetate, 5′-bromo- and 5′-chlorotetrafucol-A dodecaacetate, 5′-bromo- and 5′-chloropentafucol-A pentadecaacetate	²⁸
	Colpomenia sinuosa	Colpol	³⁸

Reactive Oxygen Species

“Reactive oxygen species” (ROS) encompass a group of oxygen containing compounds, which are highly reactive toward essential biomolecules, and, thus, pose a threat to the integrity of cells. It is a collective term that describes oxygen containing radicals such as hydroxyl (OH^•), peroxyl (ROO^•), superoxide (O₂ ^•), hydroperoxyl (HO₂ ^•), alkoxyl (RO^•), sulfonyl (ROS^•), thiyl peroxyl (RSOO^•), and nitric oxide (NO^•), as well as nonradical oxidizing agents such as hydrogen peroxide (H₂O₂), singlet oxygen (¹O₂), organic hydroperoxides (ROOH), and hypochlorous acid (HOCl) that can be easily converted into radicals.^42
–44 Although the term ROS exclusively describes molecules containing oxygen, the formation of free radicals or other reactive species in biological systems is not exclusively limited to oxygen containing compounds.⁴⁵ These destructive ROS can be generated in cells through a number of biological processes.^43,46 In fact, they are continuously produced during normal cellular metabolic processes depending on the type and function of the cells in different tissues.⁴⁷ In mammalian cells, the inner mitochondrial membrane, equipped with nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) complexes, has been identified as the most active site of ROS production.^48,49 However, the cells have the ability to tolerate these ROS under normal physiological conditions, as the cells are equipped with antioxidant defense mechanisms that regulate the balance between ROS production and scavenging.⁵⁰ However, a number of biotic and abiotic factors can affect this equilibrium in cells that can result in an abnormally increased production of ROS.^51,52

Elevated levels of ROS have been detected in cancer cells that promote tumor progression and in neutrophils and macrophages as a strategy of killing the tumor cells in the host body.^44,53 A dramatic increase of intracellular ROS production referred to as an “oxidative burst” could be observed during a pathogenic attack. This increased production of ROS, mainly superoxide and hydrogen peroxide at the site of an invasion, acts as a defense strategy against pathogens.⁵⁴ Apart from that, environmental pollution, exposure to xenobiotics, pesticides and heavy metals, consumption of alcohol and drugs, tobacco smoking, prolonged use of certain medications, and several other lifestyle habits have an adverse influence on ROS production.⁵⁵ Excessive ROS production induces oxidative stress causing damage to essential biomolecules and subsequently to DNA, causing mutations or cell death.^55,56 Oxidative stress causes a number of chronic disease conditions that include cancer, type II diabetes, neurodegenerative diseases, atherosclerosis, and inflammatory processes.⁵⁶

Phenolic Compounds

Polyphenols or phenolic compounds are a large and diverse class of secondary metabolites that consist of around 8000 naturally occurring compounds that possess vital biological functionalities related to their antioxidant properties and free radical scavenging abilities.^57,58 The common structural features shared by these molecules are the phenol groups. Based on the number of substituents, they can be found as simple phenols or polyphenols.⁵⁹ These well-known phytochemicals can be classified into different structural groups ranging from simple phenols to complex molecules such as phenolic acids, coumarins, tannins, lignins, lignans, stilbenes, flavonoids, and other classes.⁶⁰ Polyphenols are biosynthesized mainly through two basic pathways that include the shikimic acid and acetate–malonate pathways.⁶¹ Simple phenolic compounds and phenolic acids are considered as metabolites of the shikimic acid pathway. However, the biosynthesis of complex polyphenol compounds such as flavonoids requires both the shikimic acid and acetate–malonate pathways.^62,63

Radical Scavenging Antioxidant Activity of Phenolic Compounds

The antioxidant activity of a compound can be classified according to the mechanism of action as radical scavengers, metal ion chelators, or oxygen scavengers.⁶⁴ Among them, radical scavenging activity is an important determinant of the antioxidant activity of compounds.⁶⁵ Theoretical studies based on computational quantum chemical methods compared with experimental studies have revealed that the antioxidant radical scavenging activity of these compounds mainly proceeds through hydrogen atom transfer or through electron transfer mechanisms.^66
–68 The hydrogen atom transfer mechanism indicates the ability of the phenolic derivatives (ArOH) to transfer an H atom from the phenolic −OH group.⁶⁹ The stability of the resulting phenolic radicals (ArO^•) governs the radical scavenging ability of these compounds. Therefore, factors that enhance the stability of the resulting ArO^• enhance the radical scavenging activity of these compounds. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm{R}} \cdot{ \rm{ }} + { \rm{ArOH }}- \rightarrow { \rm{ RH }} + { \rm{ArO}} \cdot{ \rm{}}\end{align*} \end{document}

Considering the electron transfer mechanism, the ionization potential or the ability of transferring an electron governs the radical scavenging activity of phenolic derivatives. The stability of the resulting radical cation depends on its ionization potential value, whereas a lower value indicates better stability.⁶⁹ \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm{R}} \cdot + \ { \rm{ArOH }} - \rightarrow { \rm{ AROH}}{ \cdot^ + }{ \rm{ }} + {{ \rm{R}}^ - }\end{align*} \end{document}

Given the simplified reaction mechanism (Fig. 1), phenols can react with free radical species designated as R^• giving phenolic radicals that undergo resonance stabilization within the molecule by delocalizing the unpaired electron within the aromatic ring leading to a stable intermediate (indicated by the resonance hybrid).⁷⁰ The hydroxy groups in phenols are dually responsible for the observed antioxidant activity. Hydroxy groups arranged in the ortho position of the aromatic ring have been found to further increase the stability of the phenolic radicals through delocalizing the electrons. The presence of double bonds that conjugate with the aromatic ring further stabilizes the delocalization of electrons by extending the resonance stabilization.⁷¹ Furthermore, the substitution of the aromatic ring with alkyl groups that sterically hinders the molecule at 2 and 6 positions and attachment of groups that can provide additional resonance stabilization further stabilize the resonance increasing the radical scavenging ability.⁷⁰

FIG. 1.

The proposed reaction between phenol and a radical that produces a phenolic radical, which undergoes resonance stabilization. (The curved half-headed arrows represent the transfer of a single electron.)

Types of Polyphenolic Compounds

Phenolic acids

Phenolic acids consist of a phenolic ring with carboxylic acid functional groups.⁷² The presence of phenolic acids, including gallic, protocatechuic, gentisic, p-hydroxybenzoic, chlorogenic, vanillic, syringic, caffeic, salicylic, coumaric, and ferulic acids (Fig. 2), has been identified from 16 different algae species that possess profound antioxidant activity as revealed by antioxidant assays, including 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, Fe²⁺ chelating assay, reducing power assay and have been found to be inhibitors of lipid peroxidation in iron-induced liposomes. Furthermore, the contradictory results observed for some of the antioxidant assays were found to be explained by the synergistic effects displayed by these compounds.⁵ In vitro cultures of Anabaena doliolum and Spongiochloris spongiosa have been found to contain phenolic acids, including protocatechuic, p-hydroxybenzoic, 2,3-dihydroxybenzoic, chlorogenic, vanillic, caffeic, p-coumaric, salicylic acid, and cinnamic acid. Measurement of antioxidant activity of these fractions has revealed that the antioxidant capacity of these compounds relates with their structural properties.⁶

FIG. 2.

Structures of some phenolic acids found in seaweeds. (A) Salicylic acid, (B) gallic acid, (C) caffeic acid, (D) protocatechuic acid, (E) gentisic acid, (F) p-hydroxybenzoic acid, (G) vanillic acid, (H) chlorogenic acid, (I) syringic acid, (J) p-coumaric acid, and (K) ferulic acid.

The structural features of salicylic, p-hydroxybenzoic, gallic, protocatechuic, and gentisic acids indicate that these are derivatives of benzoic acid with an aromatic benzene ring with attached −OH moieties (Fig. 2A, B, D, E, F). The carboxyl group of benzoic acid has a negative effect on proton donation due to its electron-withdrawing properties. However, the attachment of hydroxy (–OH) groups at the meta position of benzoic acid increases the H-donating radical scavenging activity.⁷³ According to Peron and Nkiliza, the presence of OH groups in benzene rings with ortho substitution and the presence of a meta-carboxyl group can further increase the stability of radical intermediates.⁷¹ This feature could be identified in phenolic acids such as gallic, caffeic, protocatechuic, and chlorogenic acids (Fig. 2B, C, D, H). The chlorogenic acid (Fig. 2H) is an ester of caffeic acid and quinic acid, whereas the quinic acid is found to be an important intermediate in the shikimic acid pathway that produces mycosporine-like amino acids.⁷⁴ A higher antioxidant activity could be observed for phenolic acids with an attached −CH = CH-COOH moiety that increases the H atom donating ability and extends the radical scavenging ability.⁷⁵ Similar structural features can be identified in caffeic acid, p-coumaric acid, and ferulic acid (Fig. 2C, J, K). Hotta et al. and Fulcrand et al. suggest that the oxidation reactions in phenolic compounds such as caffeic acid could result in different oxidation products, whereas the generated radical species could be cancelled out with the formation of dimers.^76,77

Flavonoids

Flavonoids are polyphenolic compounds that are characterized by the C-15 skeleton, arranged into three rings with C6–C3–C6 units composed of two phenyl rings on either side with one heterocyclic ring (benzo-γ-pirone) in the middle that are generally abbreviated as A, B, and C, respectively.⁷⁸ Flavonoids can be classified into several groups based on the presence/absence of the carbon–carbon double bond between C₂ and C₃ positions, a hydroxy group on C₃ position, and a carbonyl carbon at the C₄ position.⁷⁹ These compounds represent a remarkably diverse group of secondary metabolites with a diverse array of biological activities mainly related to their antioxidant activity.⁸⁰ Flavonoids are found to be present mainly in land plants, but recent investigations indicate the presence of flavonoids in marine flora.⁸¹ Several flavonol glycosides, including quercetin derivatives, have been identified in the fresh water microalga Hematococcus pluvialis.⁸¹ Flavonol glycosides are compounds where a sugar molecule is bound with the flavonoid group through a glycosidic bond. In addition, isoflavones have also been identified from several marine and fresh water algae and cyanobacteria.^8,9 As suggested by Yan et al., the phenolic OH group in the flavonoid moiety can donate H^• radicals indicating radical scavenging effects.⁸² The biosynthesis of flavonoids proceeds through the phenylpropanoid metabolic pathway, whereas the amino acid phenylalanine acts as the precursor in producing 4-coumaroyl-CoA that combines with malonyl-CoA yielding chalcones.^81,83 The stereospecific cyclization of chalcones results in the formation of the general ring structure of flavonoids.⁸⁴

Flavonoids are well known for their superoxide and hydroxyl radical scavenging activities both in hydrophilic and lipophilic systems.^85
–87 These compounds can donate hydrogen radicals to peroxyl, hydroxyl, and peroxynitrite radicals, stabilizing them and producing a stable semiquinone radical that undergoes resonance stabilization as indicated in Figure 3.^88,89 Termination of further reactions proceeds through the flavonoid radical that reacts with available free radicals.⁹⁰ In general, the radical scavenging activities are mainly governed by their molecular structure and the substitution pattern of hydroxy moieties.⁷⁹ Attachment of hydroxy substituents to the phenyl rings mainly at the C₃ position and unsaturation of the C₂–C₃ position conjugated with the C₄ keto group extend the electron delocalization and increase the antiperoxidative properties.^88,91 In addition, the 3′4′-dihydroxy (catechol) structure in ring-B is found to enhance the lipid peroxidation inhibition of the flavan backbone coupled with its electron donating properties.^92,93 However, the presence of a double bond at the C₂–C₃ position causes a decrease in radical scavenging activity; nevertheless, the presence of OH groups at C₃ and C₅ positions with a C₄ keto group coinciding with that of C₂–C₃ unsaturation increases the scavenging effect.⁹³ Flavan-3-ol derivatives, which fulfill these requirements, exhibit better DPPH radical scavenging activity compared with several other flavonoids. Computational quantum chemical analysis has revealed that the most feasible radical scavenging mechanism of flavonoids could proceed with the donation of an H^• radical from the −OH group at C₃ position.⁸⁸ Furthermore, the antioxidant activity of flavonoids in biological systems could also be due to inhibition of several redox enzymes, including lipoxygenase, cyclooxygenase, and NADPH oxidase, preventing the generation of some intracellular ROS.⁹⁴ Other than the radical scavenging activities, flavonoids have the ability to act as antioxidants by chelating transition metal ions, thereby suppressing the Fenton type reactions that are believed to be some of the most active pathways of ROS generation.^86,95

FIG. 3.

A proposed radical scavenging reaction mechanism for a flavone derivative.⁸⁸ Illustrating the formation of the flavonoid radical by the loss of hydrogen atom in the hydroxyl group followed by resonance stabilization of the flavonoid radical. It further illustrates that one of the resonance structures gets isomerized into another flavonoid radical attaining an equilibrium, whereas further extension of the resonance stabilization takes place.

Phlorotannins

Phlorotannins are a class of marine algal polyphenolic compounds mainly confined to the brown algae.⁹⁶ These compounds are oligomers of phloroglucinol (1,3,5-trihydroxybenzene) monomers that get biosynthesized through the acetate–malonate pathway.⁹⁷ The intramolecular cross-linking between different phloroglucinol units in phlorotannins could be hypothesized as being due to apoplastic vanadium-dependent haloperoxidases on the connectivity of monomeric units as demonstrated by Berglin et al. ⁹⁸ Phlorotannins can be categorized into several classes, including fuhalols (phloroglucinol units joined with ether bridges at ortho and para positions with an extra OH-group in each third ring), phlorethols (phlorotannins, which contain an ether linkage), fucols (that contain a phenyl linkage), fucophloroethols (that contain an ether and phenyl linkage), and eckols (that contain a dibenzodioxin linkage).^99,100 Phlorotannins are highly hydrophilic components due to the presence of −OH groups that can form H bonds with water. The molecular sizes of these compounds range between 126 and 650 kDa.⁹⁹ Similar to other polyphenolic compounds, phlorotannins have been found to exert a range of biological activities, including antidiabetic, anti-inflammatory, anticancer, angiotensin-I-converting enzyme (ACE-I) inhibition, acetylcholinesterase (AChE) and butylcholinesterase (BChE) inhibition, tyrosinase inhibition, antimicrobial, and antiviral activities, whereas most of them are centered on their antioxidant activity.^{12,101

–104}

The brown alga Ecklonia cava is one of the most abundant sources of polyphenolic compounds, including phlorotannins such as phloroglucinol, eckol, fucodiphloroethol G, phlorofucofuroeckol A, dieckol, 7-phloroeckol, and 6,6′-bieckol (Fig. 4).¹⁰⁵ Phlorotannins, including phloroglucinol, eckol, phlorofucofuroeckol A, dieckol, and 8,8′-bieckol (Fig. 4A, B, E, F), which have been isolated from Eisenia bicyclis, E. cava, and Ecklonia kurome, have indicated superoxide anion and DPPH radical scavenging activities and lipid peroxidation inhibition activity.¹⁰⁶ Phlorofucofuroeckol A, dieckol, and dioxinodehydroeckol isolated from Ecklonia stolonifera have been found to possess strong DPPH radical scavenging activities, and phlorofucofuroeckol A and dieckol have been shown to inhibit intracellular ROS production.¹⁶

FIG. 4.

Chemical structures of some phlorotannins isolated from Ecklonia cava. (A) Phloroglucinol, (B) eckol, (C) fucodiphloroethol G, (D) 7-phloro eckol, (E) phlorofucofuroeckol A, (F) dieckol, and (G) 6,6′-bieckol.

According to a recent study by Mwangi et al., four phlorotannin derivatives that include phloroglucinol, eckol, 7-phloroeckol, and 2-phloroeckol isolated from the brown algae Ecklonia maxima (Fig. 5) have demonstrated radical scavenging activities that support theoretical predictions of their structural and electronic features.⁶⁹ Comparing the stabilization energies of compounds B, C, and D given in Figure 5, the increased order of radical scavenging effects has been identified as B < D < C (Fig. 5), which is consistent with the experimental findings. Furthermore, the bond dissociation enthalpy (BDE) values of each −OH group (the bond between O–H) have indicated that the −OH groups in phloroglucinol have a comparably higher BDE value, thereby a lower radical scavenging activity in accordance with hydrogen atom transfer mechanism than the rest of the phlorotannin derivatives. The −OH attached to C₇ in compound B and D had indicated a lower BDE value, suggesting the most reactive site of the H atom donation. In compound C, the −OH attached to C₄ indicates the reactive site of H atom donation. The electron delocalization in radical species generated by the donation of H atoms from the most reactive sites of aforementioned compounds could account for the observed order of radical scavenging activities.⁶⁹

FIG. 5.

Structures of phloroglucinol derivatives isolated from the brown alga, Ecklonia maxima. (A) Phloroglucinol, (B) eckol, (C) 7-phloroeckol, and (D) 2-phloroeckol.

Halogenated phenolic compounds

Halogenated phenolic compounds (Fig. 6) have been found in brown algae and in red algae species.¹⁰⁰ Most of them have indicated pronounced radical scavenging activities. Brominated and fully substituted mono- and bis-phenols (Fig. 6A–H, R) from the red alga Symphyocladia latiuscula have shown potential DPPH radical scavenging activities.^30,31 The red alga Polysiphonia urceolata was also found to contain bromophenols (BPs) with potent antioxidant activity to DPPH radical scavenging activity.³² Li et al. report the isolation of five nitrogen-containing BPs from Rhodomela confervoides with DPPH and 2,20-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) radical scavenging activity (Fig. 6I, J, O–Q).³³ According to recent studies, BPs have been identified as being one of the most likely potential candidates in the prevention of diseases and conditions related to free radicals, including diabetes, cancer, inflammation, and neurodegeneration.¹⁰⁷ According to Li et al., the radical scavenging antioxidant activities of BPs are related to the number of hydroxy groups, whereas bromination slightly decreases the radical scavenging activity by deactivating the aromatic benzene ring.^32,108

FIG. 6.

Structures of bromophenol derivatives isolated from marine algae. (A–J) Halogenated phenolic monomers identified in brown and red algae, (K–N) halogenated fucols from Analipus japonicus,^28,36 (I, J, and O–Q) nitrogen-containing bromophenols isolated from the red alga Rhodomela confervoides, (R) a brominated and fully substituted bis-phenol isolated from the red alga Symphyocladia latiuscula,³⁵ (S, T) halogenated fucophlorethols from Cystophora reflexa, and (U) Colpol from Colpomenia sinuosa.³⁸

Extraction and Purification Methods

The traditional methods used to prepare plant extracts include the powdering of plant material followed by organic solvent extraction. The ethanolic extracts of marine algae have shown a higher yield and variety of different phenolic compounds compared with their respective water extracts.⁵ Li et al. reported that the ethyl acetate solvent fraction contains a higher yield of BPs from the 95% ethanol extraction followed by solvent fractionation, respectively, into petroleum ether, ethyl acetate, and n-butanol. Xu et al. also reports that the ethyl acetate solvent fraction from the 95% methanol extract of the red alga R. confervoides is a convenient way to proceed with the isolation of BPs.¹⁰⁹ The use of pressurized-liquid extraction (PLE) combined with solid-phase extraction (SPE) is an efficient extraction technique for phenolic compounds.⁶ PLE is a technique in which the extraction solvent approaches supercritical conditions under elevated temperature and pressure that affects a higher recovery of the targeted compounds.¹¹⁰ Supercritical fluid extraction of marine and freshwater algae and cyanobacteria with methanol:water 1:9 extract using CO₂ at 35 MPa and 40°C has been found to be effective for the isolation of isoflavones.⁹ Microwave-assisted extraction could also be utilized for the efficient extraction of phenolic compounds.¹¹¹

Novel environmentally friendly extraction methods have attracted the attention of modern natural product research due to decreased usage of organic solvents, efficiency, and higher extraction yields of targeted compounds. Enzyme-assisted extraction (EAE) of algae is one such approach that can be implemented to extract phenolic compounds. The enzymes assist in breaking algae cell walls, releasing some of the phenolic compounds that remain attached to the cell wall proteins by intermolecular interactions. EAE should be carried out under constant pH and temperature conditions optimal for the used enzymes.¹¹² Wang et al. reported that protease enzymes can significantly enhance the extraction efficiency of phenolic compounds compared with carbohydrase enzymes and respective water extracts.¹¹³ Athukorala et al. indicate that EAE is an efficient and nontoxic technique for extracting phenolic compound.¹¹⁴

Although many studies have characterized methods for extraction and semipurification of phenolic compounds, a relatively limited number of studies have described the isolation and structural identification of these compounds. The isolation of phenolic compounds is generally considered a difficult strategy owing to their high structural diversity.¹¹⁵ Based on several previous reports, the ethyl acetate solvent fraction of the initial organic extracts of algae material reports higher amounts of phenolic content.^116
–118 Further purification and enrichment of phenolic compounds from these traditional methods follow several repetitive chromatography processes that employ the use of reverse phase and sephadex LH-20 cross-linked dextran gel filtration open column chromatography.^117,119,120 These traditional methods are found to be time-consuming and extravagant.¹²¹ Kim et al. report the use of macroporous adsorption resins (HP-20) for the enrichment and purification of phlorotannins from crude ethanolic extract of E. cava. This procedure was reported to be a highly efficient method to purify phlorotannins with a recovery rate of 92%.¹¹⁵ Lee et al. report centrifugal partition chromatography as an efficient method to simply purify bioactive phenolic compounds.¹²¹

Analysis of Phenolic Content

Most of the studies included in this review report that the method described by Yuan et al. is a convenient way of analyzing the total phenolic content in extracts.¹²² The sample is initially treated with 2% Na₂CO₃ solution. After 2 min, 50% Folin–Ciocalteu's phenol reagent, which contains a mixture of phosphomolybdate and phosphotungstate, is introduced to the mixture. Then, the absorbance is measured at 720 nm wavelength following a 30-min reaction time. In general, gallic acid is used as the calibration standard. The polyphenolic level is expressed as the percentage or, more commonly, as gallic acid equivalents.¹²² In addition, some studies report the use of the “Prussian blue assay” for the determination of total phenolic content.¹²³ Apart from generalized methods of detecting total polyphenolic content, specific assays are available for the quantitative analysis of specific polyphenolic compound categories. The vanillin assay described by Butler et al. is specific for mainly flavan-3-ols and other minor constituents, including dihydrochalcones and proanthocyanidins.¹²⁴ As described by Stern et al., 2,4-dimethoxybenzaldehyde (DMBA) assay provides quantitative measurement of phlorotannins using phloroglucinol as the reference standard.¹²⁵

Potential Industrial Applications of Marine Algal Phenolic Compounds

Over the past few decades, there has been an increased interest and growth in the use of marine algal polyphenolic compounds for industrial applications such as functional foods, pharmaceuticals, nutraceuticals, and cosmeceuticals.^126,127 The increased consumer awareness about the incorporation of functional molecules into consumer products has attracted the public concern in today's society.¹²⁸ With growing industrial applications and consumer demand, novel sources of antioxidant compounds are urgently needed.¹²⁹ In this regard, the antioxidant activities of polyphenolic compounds isolated from marine algae could be a potentially beneficial source of antioxidants with protective effects that can be used in industrial production of consumer and medicinal products. Although phenolic compounds exert beneficial effects on physiological well-being, their perceived role as “antinutrients” reduce the digestibility of proteins; potential adverse side effects due to excessive consumption should be studied in detail.^130,131

According to previous reports, antioxidant phlorotannins from E. cava (phloroglucinol, eckol, and dieckol) have potential use in cosmeceutical preparations.¹³² Furthermore, these phlorotannins have been found to exert tyrosinase inhibition and protective effects against photo-oxidative stress conditions induced by UV-B radiation.¹¹⁹ Phlorofucofuroeckol-B, isolated from the edible brown alga Eisenia arborea, has shown strong antioxidant and histamine release inhibitory activity, which suggests potential utilization in antiallergic drug preparations.²¹ Algal polyphenols with their antioxidant activities have the potential to be used in the preparation of health enhancing drinks with functional properties.¹³³ Taken together, polyphenolic compounds from marine algae with their broader therapeutic perspectives could be used in industry to manufacture a range of consumer products and pharmaceuticals.

Conclusions

Marine algae are rich sources of polyphenolic compounds with pronounced antioxidant activities. The antioxidant properties of these compounds mainly depend on the structure that signifies the geometric arrangement, number and positions of hydroxyl moieties, and the substitution of aromatic rings.⁷⁵ The beneficial biological functionalities of these phenolic compounds can be attributed to their pronounced antioxidant activity.⁸⁹ Nevertheless, further studies are needed to assess the harmful and adverse side effects of these phenolic compounds.^130,131 Understanding the underlying principles of reaction mechanisms responsible for their antioxidant activity could lead to synthesizing phenolic derivatives with optimum antioxidant efficiency. These phenolic derivatives of marine algae could lead in producing a wide variety of pharmaceuticals, cosmeceuticals, nutraceuticals, and functional foods with potential biological functionality.

Footnotes

Acknowledgment

This work was supported by a grant from the National Institute of Fisheries Science (R2016059).

Author Disclosure Statement

No competing financial interests exist.

References

Wijesekara

, Pangestuti

, Kim

: Biological activities and potential health benefits of sulfated polysaccharides derived from marine algae. Carbohydr Polym, 2011; 84:14–21.

Güven

, Percot

, Sezik

: Alkaloids in marine algae. Mar Drugs, 2010; 8:269–284.

Thomas

, Kim

: Potential pharmacological applications of polyphenolic derivatives from marine brown algae. Environ Toxicol Pharmacol, 2011; 32:325–335.

Mancini-Filho

, Novoa

, González

, et al.: Free phenolic acids from the seaweed Halimeda monile with antioxidant effect protecting against liver injury. Z Naturforsch C, 2009; 64:657–663.

Sabeena Farvin

, Jacobsen

: Phenolic compounds and antioxidant activities of selected species of seaweeds from Danish coast. Food Chem, 2013; 138:1670–1681.

Onofrejová

, Vašíčková

, Klejdus

: Bioactive phenols in algae: The application of pressurized-liquid and solid-phase extraction techniques. J Pharm Biomed Anal, 2010; 51:464–470.

Lee

, Han

, Ko

, et al.: Protective effect of a freshwater alga, Spirogyra sp., against lipid peroxidation in vivo zebrafish and purification of antioxidative compounds using preparative centrifugal partition chromatography. J Appl Phycol, 2016; 28:181–189.

Zeng

, Wang

, Su

, et al.: Flavonoids from the red alga Acanthophora spicifera . Chin J Chem, 2001; 19:1097–1100.

Klejdus

, Lojková

, Plaza

, Snóblová

, Stěrbová

: Hyphenated technique for the extraction and determination of isoflavones in algae: Ultrasound-assisted supercritical fluid extraction followed by fast chromatography with tandem mass spectrometry. J Chromatogr A, 2010; 1217:7956–7965.

10.

Yoshie

, Wang

, Petillo

, Suzuki

: Distribution of catechins in Japanese seaweeds. Fish Sci, 2000; 66:998–1000.

11.

Santoso

, Yoshie

, Suzuki

: The distribution and profile of nutrients and catechins of some Indonesian seaweeds. Fish Sci, 2002; 68:1647–1648.

12.

Shibata

, Yamaguchi

, Nagayama

, Kawaguchi

, Nakamura

: Inhibitory activity of brown algal phlorotannins against glycosidases from the viscera of the turban shell Turbo cornutus . Eur J Phycol, 2002; 37:493–500.

13.

Artan

, Li

, Karadeniz

, Lee

, Kim

: Anti-HIV-1 activity of phloroglucinol derivative, 6,6′-bieckol, from Ecklonia cava . Bioorg Med Chem, 2008; 16:7921–7926.

14.

Kong

, Kim

, Yoon

, Kim

: Induction of apoptosis by phloroglucinol derivative from Ecklonia cava in MCF-7 human breast cancer cells. Food Chem Toxicol, 2009; 47:1653–1658.

15.

Kim

, Kim

, Kang

, et al.: Polyphenol-rich fraction from Ecklonia cava (a brown alga) processing by-product reduces LPS-induced inflammation in vitro and in vivo in a zebrafish model. Algae, 2014; 29:165–174.

16.

Kim

, Shin

, Lee

, et al.: Isolation and identification of phlorotannins from Ecklonia stolonifera with antioxidant and anti-inflammatory properties. J Agric Food Chem, 2009; 57:3483–3489.

17.

Kang

, Chung

, Jung

, Son

, Choi

: A new phlorotannin from the brown alga Ecklonia stolonifera . Chem Pharm Bull, 2003; 51:1012–1014.

18.

Jung

, Hyun

, Kim

, Choi

: Angiotensin-converting enzyme I inhibitory activity of phlorotannins from Ecklonia stolonifera . Fish Sci, 2006; 72:1292–1299.

19.

Yoon

, Chung

, Kim

, Choi

: Acetyl- and butyrylcholinesterase inhibitory activities of sterols and phlorotannins from Ecklonia stolonifera . Fish Sci, 2008; 74:200–207.

20.

Kellogg

, Grace

, Lila

: Phlorotannins from Alaskan seaweed inhibit carbolytic enzyme activity. Mar Drugs, 2014; 12:5277–5294.

21.

Sugiura

, Matsuda

, Yamada

, et al.: Isolation of a new anti-allergic phlorotannin, phlorofucofuroeckol-B, from an edible brown alga, Eisenia arborea . Biosci Biotechnol Biochem, 2006; 70:2807–2811.

22.

Heo

, Hwang

, Choi

, et al.: Diphlorethohydroxycarmalol isolated from Ishige okamurae, a brown algae, a potent α-glucosidase and α-amylase inhibitor, alleviates postprandial hyperglycemia in diabetic mice. Eur J Pharmacol, 2009; 615:252–256.

23.

Heo

, Ko

, Kang

, et al.: Inhibitory effect of diphlorethohydroxycarmalol on melanogenesis and its protective effect against UV-B radiation-induced cell damage. Food Chem Toxicol, 2010; 48:1355–1361.

24.

, Jung

, Kang

, et al.: Angiotensin I-converting enzyme (ACE) inhibition and nitric oxide (NO)-mediated antihypertensive effect of octaphlorethol A isolated from Ishige sinicola: In vitro molecular mechanism and in vivo SHR model. J Funct Foods, 2015; 18:289–299.

25.

Lee

, Kang

, Kim

, et al.: Antidiabetogenic and antioxidative effects of octaphlorethol a isolated from the brown algae Ishige foliacea in streptozotocin-induced diabetic mice. Food Sci Biotechnol, 2014; 23:1261–1266.

26.

Kang

, Kim

, Chaminda Lakmal

, et al.: Octaphlorethol A isolated from Ishige foliacea prevents and protects against high glucose-induced oxidative damage in vitro and in vivo . Environ Toxicol Pharmacol, 2014; 38:607–615.

27.

Koch

, Gregson

: Brominated phlorethols and nonhalogenated phlorotannins from the brown alga Cystophora congesta . Phytochemistry, 1984; 23:2633–2637.

28.

Glombitza

, Zieprath

: Phlorotannins from the brown alga Analipus japonicus . Planta Med, 1989; 55:171–175.

29.

Glombitza

, Gerstberger

: Phlorotannins with dibenzodioxin structural elements from the brown alga Eisenia arborea . Phytochemistry, 1985; 24:543–551.

30.

Choi

, Park

, Jung

, Chung

, Jung

, Choi

: A cyclohexanonyl bromophenol from the red alga Symphyocladia latiuscula . J Nat Prod, 2000; 63:1705–1706.

31.

Duan

, Li

, Wang

: Highly brominated mono- and bis-phenols from the marine red alga Symphyocladia latiuscula with radical-scavenging activity. J Nat Prod, 2007; 70:1210–1213.

32.

, Li

, Ji

, Wang

: Bromophenols from the marine red alga Polysiphonia urceolata with DPPH radical scavenging activity. J Nat Prod, 2008; 71:28–30.

33.

, Li

, Gloer

, Wang

: New nitrogen-containing bromophenols from the marine red alga Rhodomela confervoides and their radical scavenging activity. Food Chem, 2012; 135:868–872.

34.

Shibata

, Hama

, Miyasaki

, Ito

, Nakamura

: Extracellular secretion of phenolic substances from living brown algae. J Appl Phycol, 2006; 18:787–794.

35.

Sailler

, Glombitza

: Halogenated phlorethols and fucophlorethols from the brown alga Cystophora retroflexa . Nat Toxins, 1999; 7:57–62.

36.

Glombitza

, Schmidt

: Nonhalogenated and halogenated phlorotannins from the brown alga Carpophyllum angustifolium . J Nat Prod, 1999; 62:1238–1240.

37.

Glombitza

, Koch

, Eckhardt

: Chlorierte phlorethole aus Laminaria ochroleuca . Phytochemistry, 1977; 16:796–798.

38.

Green

, Kashman

, Miroz

: Colpol, a new cytotoxic C₆-C₄-C₆ metabolite from the alga Colpomenia sinuosa . J Nat Prod, 1993; 56:1201–1202.

39.

, Cheng

, Wong

, Fan

, Chen

, Jiang

: Evaluation of antioxidant capacity and total phenolic content of different fractions of selected microalgae. Food Chem, 2007; 102:771–776.

40.

Cai

, Luo

, Sun

, Corke

: Antioxidant activity and phenolic compounds of 112 traditional Chinese medicinal plants associated with anticancer. Life Sci, 2004; 74:2157–2184.

41.

Haslam

: Natural polyphenols (vegetable tannins) as drugs: Possible modes of action. J Nat Prod, 1996; 59:205–215.

42.

Bayir

: Reactive oxygen species. Crit Care Med, 2005; 33:S498–S501.

43.

Halliwell

: Free radicals and other reactive species in disease. eLS, 2003, DOI: 10.1002/9780470015902.a0002269.pub3.

44.

Liou

, Storz

: Reactive oxygen species in cancer. Free Radic Res, 2010; 44, DOI: 10.3109/10715761003667554.

45.

Halliwell

: Reactive species and antioxidants. Redox biology is a fundamental theme of aerobic life. Plant Physiol, 2006; 141:312–322.

46.

Apel

, Hirt

: Reactive oxygen species: Metabolism, oxidative stress, and signal transduction. Annu Rev Plant Biol, 2004; 55:373–399.

47.

Foyer

, Harbinson

, Mullineaux

: Oxygen metabolism and the regulation of photosynthetic electron transport. In: Causes of Photooxidative Stress and Amelioration of Defense Systems in Plants. ( Foyer

, Mullineaux

, eds.). CRC Press, Boca Raton, FL, 1994, pp. 1–42.

48.

Muller

: The nature and mechanism of superoxide production by the electron transport chain: Its relevance to aging. J Am Aging Assoc, 2000; 23:227–253.

49.

Bedard

, Krause

: The NOX family of ROS-generating NADPH oxidases: Physiology and pathophysiology. Physiol Rev, 2007; 87:245–313.

50.

Alvarez

, Pennell

, Meijer

, Ishikawa

, Dixon

, Lamb

: Reactive oxygen intermediates mediate a systemic signal network in the establishment of plant immunity. Cell, 1998; 92:773–784.

51.

Elstner

: Mechanisms of oxygen activation in different compartments of plant cells. Curr Top Plant Physiol, 1991; 6:13–25.

52.

Maxwell

, Nickels

, McIntosh

: Evidence of mitochondrial involvement in the transduction of signals required for the induction of genes associated with pathogen attack and senescence. Plant J, 2002; 29:269–279.

53.

Babior

: The respiratory burst oxidase. Curr Opin Hematol, 1995; 2:55–60.

54.

Apostol

, Heinstein

, Low

: Rapid stimulation of an oxidative burst during elicitation of cultured plant cells. Plant Physiol, 1989; 90:109–116.

55.

Al-Gubory

: Environmental pollutants and lifestyle factors induce oxidative stress and poor prenatal development. Reprod Biomed Online, 2014; 29:17–31.

56.

Droge

: Free radicals in the physiological control of cell function. Physiol Rev, 2002; 82:47–95.

57.

Croteau

, Kutchan

, Lewis

: Natural products (secondary metabolites). In: Biochemistry and Molecular Biology of Plants. ( Buchanan

, Gruissem

, Jones

, eds.). John Wiley & Sons, Inc., Hoboken, NJ, 2000, pp. 1250–1318.

58.

Tomás-Barberan

, Ferreres

, Gil

: Antioxidant phenolic metabolites from fruit and vegetables and changes during postharvest storage and processing. In: Studies in Natural Products Chemistry ( Rahman

, ed.). Elsevier B.V., Philadelphia, PA, 2000, pp. 739–795.

59.

Clifford

: A nomenclature for phenols with special reference to tea. Crit Rev Food Sci Nutr, 2001; 41:393–397.

60.

Khoddami

, Wilkes

, Roberts

: Techniques for analysis of plant phenolic compounds. Molecules, 2013; 18:2328–2375.

61.

Tsao

: Chemistry and biochemistry of dietary polyphenols. Nutrients, 2010; 2:1231–1246.

62.

Grigelmo-Miguel

, Rojas-Graü

, Soliva-Fortuny

, Martín-Belloso

: Methods of analysis of antioxidant capacity of phytochemicals. In: Fruit and Vegetable Phytochemicals: Chemistry, Nutritional Value, and Stability. ( de la Rosa

, Alvarez-Parrilla

, Gonzalez-Aguilar

, eds.) Wiley-Blackwell, Hoboken, NJ, 2009, pp. 271–307.

63.

Fatland

, Ke

, Anderson

, et al.: Molecular characterization of a heteromeric ATP-citrate lyase that generates cytosolic acetyl-coenzyme A in Arabidopsis. Plant Physiol, 2002; 130:740–756.

64.

Sánchez-Moreno

, Larrauri

, Saura-Calixto

: A procedure to measure the antiradical efficiency of polyphenols. J Sci Food Agric, 1998; 76:270–276.

65.

Yamaguchi

, Takamura

, Matoba

, Terao

: HPLC method for evaluation of the free radical-scavenging activity of foods by using 1,1-diphenyl-2-picrylhydrazyl. Biosci Biotechnol Biochem, 1998; 62:1201–1204.

66.

Kozlowski

, Trouillas

, Calliste

, Marsal

, Lazzaroni

, Duroux

: Density functional theory study of the conformational, electronic, and antioxidant properties of natural chalcones. J Phys Chem A, 2007; 111:1138–1145.

67.

Trouillas

, Marsal

, Svobodová

, et al.: Mechanism of the antioxidant action of silybin and 2,3-dehydrosilybin flavonolignans: A joint experimental and theoretical study. J Phys Chem A, 2008; 112:1054–1163.

68.

Wright

, Johnson

, DiLabio

: Predicting the activity of phenolic antioxidants: Theoretical method, analysis of substituent effects, and application to major families of antioxidants. J Am Chem Soc, 2001; 123:1173–1183.

69.

Mwangi

, Van Der Westhuizen

, Marnewick

, Mabusela

, Kabanda

, Ebenso

: Isolation, identification and radical scavenging activity of phlorotannin derivatives from brown algae, Ecklonia maxima: An experimental and theoretical study. Free Radic Antioxid, 2013; 3:S1–S10.

70.

Janzen

, Wilcox

, Hinton

: Novel spin trap nitronyl hindered phenols. U.S. Patent WO1995011227 B1. May 18, 1995.

71.

Peron

, Nkiliza

: Stablised polyphenol derivatives, process for their manufacture, and uses thereof. U.S. Patent US 20130202545 A1. Aug 8, 2013.

72.

Kim

, Tsao

, Yang

, Cui

: Phenolic acid profiles and antioxidant activities of wheat bran extracts and the effect of hydrolysis conditions. Food Chem, 2006; 95:466–473.

73.

Rice-Evans

, Miller

, Paganga

: Structure-antioxidant activity relationships of flavonoids and phenolic acids. Free Radic Biol Med, 1996; 20:933–956.

74.

Yuan

, Walsh

: Antioxidant and antiproliferative activities of extracts from a variety of edible seaweeds. Food Chem Toxicol, 2006; 44:1144–1150.

75.

Balasundram

, Sundram

, Samman

: Phenolic compounds in plants and agri-industrial by-products: Antioxidant activity, occurrence, and potential uses. Food Chem, 2006; 99:191–203.

76.

Hotta

, Nagano

, Ueda

, Tsujino

, Koyama

, Osakai

: Higher radical scavenging activities of polyphenolic antioxidants can be ascribed to chemical reactions following their oxidation. Biochim Biophys Acta, 2002; 1572:123–132.

77.

Fulcrand

, Cheminat

, Brouillard

, Cheynier

: Characterization of compounds obtained by chemical oxidation of caffeic acid in acidic conditions. Phytochemistry, 1994; 35:499–505.

78.

Cook

, Samman

: Flavonoids—Chemistry, metabolism, cardioprotective effects, and dietary sources. J Nutr Biochem, 1996; 7:66–76.

79.

Gil

, Cout

: Flavonoid electrochemistry: A review on the electroanalytical applications. Rev Bras Farmacogn, 2013; 23:542–558.

80.

Stafford

: Flavonoid evolution: An enzymic approach. Plant Physiol, 1991; 96:680–685.

81.

Goiris

, Muylaert

, Voorspoels

, et al.: Detection of flavonoids in microalgae from different evolutionary lineages. J Phycol, 2014; 50:483–492.

82.

Yan

, Cao

, Yang

: HPLC-DPPH screening method for evaluation of antioxidant compounds extracted from Semen Oroxyli. Molecules, 2014; 19:4409–4417.

83.

Ververidis

, Trantas

, Douglas

, Vollmer

, Kretzschmar

, Panopoulos

: Biotechnology of flavonoids and other phenylpropanoid-derived natural products. Part I: Chemical diversity, impacts on plant biology and human health. Biotechnol J, 2007; 2:1214–1234.

84.

Winkel-Shirley

: Flavonoid biosynthesis. A colorful model for genetics, biochemistry, cell biology, and biotechnology. Plant Physiol, 2001; 126:485–493.

85.

Torel

, Cillard

, Pierre Cillard

: Antioxidant activity of flavonoids and reactivity with peroxy radical. Phytochemistry, 1986; 25:383–385.

86.

Afanas'ev

, Dcrozhko

, Brodskii

, Kostyuk

, Potapovitch

: Chelating and free radical scavenging mechanisms of inhibitory action of rutin and quercetin in lipid peroxidation. Biochem Pharmacol, 1989; 38:1763–1769.

87.

Arora

, Nair

, Strasburg

: Structure-activity relationships for antioxidant activities of a series of flavonoids in a liposomal system. Free Radic Biol Med, 1998; 24:1355–1363.

88.

Seyoum

, Asres

, El-Fiky

: Structure-radical scavenging activity relationships of flavonoids. Phytochemistry, 2006; 67:2058–2070.

89.

Heim

, Tagliaferro

, Bobilya

: Flavonoid antioxidants: Chemistry, metabolism and structure-activity relationships. J Nutr Biochem, 2002; 13:572–584.

90.

Robak

, Gryglewski

: Flavonoids are scavengers of superoxide anions. Biochem Pharmacol, 1988; 37:837–841.

91.

Ratty

, Das

: Effects of flavonoids on nonenzymatic lipid peroxidation: Structure-activity relationship. Biochem Med Metab Biol, 1988; 39:69–79.

92.

Mora

, Payá

, Ríos

, Alcaraz

: Structure-activity relationships of polymethoxyflavones and other flavonoids as inhibitors of non-enzymic lipid peroxidation. Biochem Pharmacol, 1990; 40:793–797.

93.

Amić

, Davidović-Amić

, Bešlo

, Trinajstić

: Structure-radical scavenging activity relationships of flavonoids. Croatica Chemica Acta, 2003; 76:55–61.

94.

Nijveldt

, van Nood

, van Hoorn

, Boelens

, van Norren

, van Leeuwen

: Flavonoids: A review of probable mechanisms of action and potential applications. Am J Clin Nutr, 2001; 74:418–425.

95.

Morel

, Lescoat

, Cogrel

, et al.: Antioxidant and iron-chelating activities of the flavonoids catechin, quercetin and diosmetin on iron-loaded rat hepatocyte cultures. Biochem Pharmacol, 1993; 45:13–19.

96.

Shibata

, awaguchi

, Hama

, Inagaki

, Yamaguchi

, Nakamura

: Local and chemical distribution of phlorotannins in brown algae. J Appl Phycol, 2004; 16:291–296.

97.

Roh

, Uddin

, Chun

: Extraction of fucoxanthin and polyphenol from Undaria pinnatifida using supercritical carbon dioxide with co-solvent. Biotechnol Bioprocess Eng, 2008; 13:724–729.

98.

Berglin

, Delage

, Potin

, Vilter

, Elwing

: Enzymatic cross-linking of a phenolic polymer extracted from the marine alga Fucus serratus . Biomacromolecules, 2004; 5:2376–2383.

99.

, Wijesekara

, Yong Li

, Kim

: Phlorotannins as bioactive agents from brown algae. Process Biochem, 2011; 46:2219–2224.

100.

La Barre

, Potin

, Leblanc

, Delage

: The halogenated metabolism of brown algae (Phaeophyta), its biological importance and its environmental significance. Mar Drugs, 2010; 8:988–1010.

101.

Siriwardhana

, Lee

, Jeon

: Radical scavenging potential of hydrophilic phlorotannins of Hizikia fusiformis . Algae, 2005; 20:69–75.

102.

Kim

, Ta

, Mendis

, et al.: Phlorotannins in Ecklonia cava extract inhibit matrix metalloproteinase activity. Life Sci, 2006; 79:1436–1443.

103.

Yoon

, Eom

, Kim

: Inhibitory effect of phlorotannins isolated from Ecklonia cava on mushroom tyrosinase activity and melanin formation in mouse B16F10 melanoma cells. J Agric Food Chem, 2009; 57:4124–4129.

104.

Lee

, Hou

, Huang

, et al.: Marine algal natural products with anti-oxidative, anti-inflammatory, and anti-cancer properties. Cancer Cell Int, 2013; 13:55.

105.

, Qian

, Ryu

, Lee

, Kim

: Chemical components and its antioxidant properties in vitro: An edible marine brown alga, Ecklonia cava . Bioorg Med Chem, 2009; 17:1963–1973.

106.

Shibata

, Ishimaru

, Kawaguchi

, Yoshikawa

, Hama

: Antioxidant activities of phlorotannins isolated from Japanese Laminariaceae. J Appl Phycol, 2008; 20:705–711.

107.

Liu

, Hansen

, Lin

: Bromophenols in marine algae and their bioactivities. Mar Drugs, 2011; 9:1273–1292.

108.

Lee

, Lee

, Kang

, Shin

, Lee

: The in vitro antioxidant activities of the bromophenols from the red alga Tichocarpus crinitus and phenolic derivatives. J Korean Magn Reson Soc, 2007; 11:56–63.

109.

, Fan

, Yan

, Li

, Niu

, Tseng

: Antibacterial bromophenols from the marine red alga Rhodomela confervoides . Phytochemistry, 2003; 62:1221–1224.

110.

Hawthorne

, Grabanski

, Martin

, Miller

: Comparisons of Soxhlet extraction, pressurized liquid extraction, supercritical fluid extraction and subcritical water extraction for environmental solids: Recovery, selectivity and effects on sample matrix. J Chromatogr A, 2000; 892:421–433.

111.

, Wang

, Zhang

, Qu

, Xu

, Li

: Preparation and antioxidant property of extract and semipurified fractions of Caulerpa racemosa . J Appl Phycol, 2012; 24:1527–1536.

112.

Wijesinghe

WAJP

, Jeon

: Enzyme-assistant extraction (EAE) of bioactive components: A useful approach for recovery of industrially important metabolites from seaweeds: A review. Fitoterapia, 2012; 83:6–12.

113.

Wang

, Jónsdóttir

, Kristinsson

, et al.: Enzyme-enhanced extraction of antioxidant ingredients from red algae Palmaria palmata . LWT—Food Sci Technol, 2010; 43:1387–1393.

114.

Athukorala

, Kim

, Jeon

: Antiproliferative and antioxidant properties of an enzymatic hydrolysate from brown alga, Ecklonia cava . Food Chem Toxicol, 2006; 44:1065–1074.

115.

Kim

, Yoon

, Yang

, et al.: Enrichment and purification of marine polyphenol phlorotannins using macroporous adsorption resins. Food Chem, 2014; 162:135–142.

116.

Heo

, Kim

, Jung

, et al.: Identification of chemical structure and free radical scavenging activity of diphlorethohydroxycarmalol isolated from a brown alga, Ishige okamurae . J Microbiol Biotechnol, 2008; 18:676–681.

117.

Wijesinghe

, Ko

, Jeon

: Effect of phlorotannins isolated from Ecklonia cava on angiotensin I-converting enzyme (ACE) inhibitory activity. Nutr Res Pract, 2011; 5:93–100.

118.

Eom

, Park

, Yu

, et al.: Antimicrobial activity of brown alga Eisenia bicyclis against methicillin-resistant Staphylococcus aureus . Fish Aquat Sci, 2011; 14:251–256.

119.

Heo

, Ko

, Cha

, et al.: Effect of phlorotannins isolated from Ecklonia cava on melanogenesis and their protective effect against photo-oxidative stress induced by UV-B radiation. Toxicol In Vitro, 2009; 23:1123–1130.

120.

Yeo

, Lee

, Tae

, et al.: Anti-hyperlipidemic effect of polyphenol extract (Seapolynol™) and dieckol isolated from Ecklonia cava in in vivo and in vitro models. Prev Nutr Food Sci, 2012; 17:1–7.

121.

Lee

, Ko

, Oh

, et al.: Preparative isolation and purification of phlorotannins from Ecklonia cava using centrifugal partition chromatography by one-step. Food Chem, 2014; 158:433–437.

122.

Yuan

, Bone

, Carrington

: Antioxidant activity of dulse (Palmaria palmata) extract evaluated in vitro . Food Chem, 2005; 91:485–494.

123.

Price

, Butler

: Rapid visual estimation and spectrophotometric determination of tannin content of sorghum grain. J Agric Food Chem, 1977; 25:1268–1273.

124.

Butler

, Price

, Brotherton

: Vanillin assay for proanthocyanidins (condensed tannins): Modification of the solvent for estimation of the degree of polymerization. J Agric Food Chem, 1982; 30:1087–1089.

125.

Stern

, Hagerman

, Steinberg

, Winter

, Estes

: A new assay for quantifying brown algal phlorotannins and comparisons to previous methods. J Chem Ecol, 1996; 22:1273–1293.

126.

Thomas

, Kim

: Beneficial effects of marine algal compounds in cosmeceuticals. Mar Drugs, 2013; 11:146–164.

127.

Holdt

, Kraan

: Bioactive compounds in seaweed: Functional food applications and legislation. J Appl Phycol, 2011; 23:543–597.

128.

Williamson

, Holst

: Dietary reference intake (DRI) value for dietary polyphenols: Are we heading in the right direction?. Br J Nutr, 2008; 99:S55–S58.

129.

Zheng

, Wang

: Antioxidant activity and phenolic compounds in selected herbs. J Agric Food Chem, 2001; 49:5165–5170.

130.

Ferguson

: Role of plant polyphenols in genomic stability. Mutat Res, 2001; 475:89–111.

131.

Mennen

, Walker

, Bennetau-Pelissero

, Scalbert

: Risks and safety of polyphenol consumption. Am J Clin Nutr, 2005; 81:326s–329s.

132.

Ahn

, Kim

, Cha

, et al.: Antioxidant activities of phlorotannins purified from Ecklonia cava on free radical scavenging using ESR and H₂O₂-mediated DNA damage. Eur Food Res Technol, 2007; 226:71–79.

133.

Nagai

, Yukimoto

: Preparation and functional properties of beverages made from sea algae. Food Chem, 2003; 81:327–332.