Exploring curcumin interactions with BN nanostructures: A DFT approach

1. Introduction

In recent years, applications of nanostructures in medicine have been investigated by performing numerous works to develop novel procedures of medications, in which nano-based drug delivery has been seen as an important process for this vital purpose [1–3]. Accordingly, investigating intermolecular communications have been seen important for recognition of new structures for participating in drug delivery process [4–6]. To this aim, interactions between curcumin and boron nitride (BN) nanostructures were investigated in this work to show benefits of BN surfaces for adsorbing the curcumin substance, which is a natural product derived from turmeric [7–9]. Indeed, numerous applications of curcumin have been investigated to this time, especially focusing on exploring benefits of traditional medicine for human life [10–12]. Accordingly, applications such as anti-oxidant, anti-inflammatory, anti-mutagenic, anti-microbial, and anti-cancer functions have been expected for curcumin [13–15]. However, low efficacy is still a restricting factor for employing the curcumin for medical purposes showing the need of exploring more features for this naturally known compound [16–18]. Earlier works indicated benefits of employing nanostructures for targeted drug delivery purposes to enhance the efficacy of medications for living systems [19–25]. Indeed, nanostructures have verities of applications in various fields from industry to biological living systems [26–28]. Besides the pure carbon nanostructures, those with other atomic compositions have been seen important for employing in the specified fields [29–31]. In this regard, exploring interacting complex systems is an advantage of detection the role of nanostructure as an adsorbent surface [32–34]. To approach this issue, the hypothesis of curcumin delivery by means of BN nanostructures was investigated in this work by employing the surfaces of nanosheet and nanotube (BNNS and BNNT). After the pioneering work of carbon nanotubes innovations, several other works have been done to show existence of other nanostructures, in which the BN composition has been seen appropriate for this purpose [35–39]. Indeed, heteroatomic composition of the BN nanostructure could yield advantage of providing an appropriate surface for involving in adsorption processes in comparison with those of homoatomic carbon ones [40–42].

Within this work, adsorption processes of keto and enol tautomers of curcumin at the surface of each of BNNS and BNNT models were investigated by performing density functional theory (DFT) calculations. It is important to mention that the formation of tautomers is a common occurring process for the organic structures yielding new features for the resulted tautomeric conformations [43–45]. Keto-enol tautomerism process is occurred by movement of the hydrogen atom of enol group to keto group and changing the positions of original keto – enol to new enol – keto tautomeric forms [45]. To approach the goal of this work, optimization processes were performed to stabilize the singular structures prior to detection of their participation in the formation of interacting nanostructure-curcumin bimolecular complexes. Indeed, this work was done to due important of employing nanostructures in the drug delivery process by emphasizing on adsorbing the curcumin substance of this work. As a results, the models geometries were prepared and their features were evaluated to recognize the adsorption processes of keto and enol forms of curcumin at the surface of BNNS and BNNT nanostructures (Table 1 and Figs. 1 and 2).

OPEN IN VIEWER

Figure 1.

The optimized structures of singular BNNS, BNNT, and curcumin models in addition to the visualized MEP surfaces and DOS diagrams.

OPEN IN VIEWER

Figure 2.

The optimized structures of bimolecular complexes of keto and enol forms of curcumin and each of BNNS and BNNT nanostructures in addition to the visualized MEP surfaces and DOS diagrams.

In the current research work, DFT calculations were performed to obtain stabilized structures and their corresponding features to approach the goal of employing BN nanostructures for delivery of curcumin. As shown in Fig. 1, the singular models of representative BNNS and BNNT nanostructures and keto and enol forms of curcumin were optimized to prepare the required structures for this work. Next, combinations of already optimized curcumin and nanostructures were re-optimized to obtain the interacting bimolecular models as shown in Fig. 2. In all cases of singular and bimolecular models, electronic molecular orbital features including energy levels of the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO), representations of molecular electrostatic potential (MEP) surfaces, and diagrams of density of states (DOS) were obtained for the optimized models. Moreover, natural bond orbital (NBO) atomic charges were calculated to show the magnitude of charge transfer (Q_T) between the interacting substances. To examine the strength of bimolecular complex formations, values of adsorption energy (E_ads) were evaluated for the interacting models, in which the basis set super position error (BSSE) was corrected for the obtained results. To show energy distances of HOMO-LUMO levels, values of energy gap (E_g) were evaluated for all cases of singular and bimolecular models. All calculations of this work were performed by employing the B3LYP exchange-correlation functional and the 6-31G(d) basis set as implemented in the Gaussian 09 program [46]. Indeed, this work was done by benefit of employing computational tools for obtaining required results about the investigated models at the smallest molecular and atomic scales [47–49]. Geometrical optimizations and features evaluations helped to characterize the investigated models for approaching the research goal. In the case of showing benefit of employing such system, the obtained results were evaluated for the singular and interacting models.

3. Results and discussion

The optimized singular models of BN nanostructures and curcumin were shown in Fig. 1. The investigated BNNS model was a planar sheet of BN composition and the BNNT model was a (8, 0) zigzag tubular structure. The advantage of employed models was their heteroatomic composition making them suitable for providing an activated surface for participating in interactions. Besides containing the heteroatomic composition, the shapes of nanostructures were also examined to see which one could be more favorable for use in such interacting system with the curcumin substance. Additionally, such polar BN bonds of nanostructures could help them to be dispersed in water media better than the pure non-polar carbon nanostructures showing the advantage of BN nanostructures for those biological applications [39]. In the current work, the planar and rolled-up models of BN nanostructures were investigated to examine their function of adsorption of curcumin substance. For both models, the edges were saturated using the hydrogen atoms avoiding the existence of dangling effects [50–52]. The obtained B-N bond lengths were recognized to be ∼1.45 Å acceptable for BN-related nanostructures [53]. The stabilized models were successfully obtained by performing optimization processes. Moreover, the keto and enol forms of curcumin were obtained by performing individual optimization processes. Analyzing the obtained features of HOMO and LUMO in addition the evaluated MEP surfaces and DOS diagrams could all show characteristic features of singular models for participating in interaction processes. Based on the obtained MEP results, the enol form of curcumin showed a red head with the negative charge whereas the keto form did not show such single-standing red heard because of conformational change of the keto form in comparison with the enol form. As listed in Table 1, the obtained values of E_g were calculated 6.22 eV and 5.30 eV for the BNNS and BNNT models showing different energy distances between the HOMO and LUMO levels of the investigated BN nanostructures. Accordingly, the evaluated DOS diagrams showed changes of such electronic molecular orbital features in a visual mode. As a consequence, not only availability of a BN composition, but also structural configuration could determine the features of BN nanostructures for involving in further functions, especially interactions with other substances.

To examine the optimum adsorption processes curcumin at the surface of BNNS and BNNT structures, different configurations of molecular orientations towards each other were tested. As a consequence, the optimum models were obtained and their configurations were shown in Fig. 2. The results indicated that the middle parts of both of BN nanostructures were appropriate for adsorbing the curcumin substance, in which the relaxation of enol form of curcumin was achieved better than the relaxation of keto form for obtaining stronger bimolecular complex. In this regard, the obtained values of E_ads showed that the BNNS model was seen as a better surface than the BNNT model for adsorbing curcumin. Based on such obtained energies, the BNNS model of BN nanostructure and the enol form of curcumin were the most suitable individual substances for the formation of interacting bimolecular complexes. As a consequence, the initial hypothesis of curcumin adsorption by means of BN nanostructures was affirmed based on the achieved results. It is important to mention that the type of interactions was physical for the interacting complexes revealing the advantage of these adsorbent models for releasing the drug to an appropriate target. It is indeed a type of reversible adsorption process meaning adsorption and releasing of the adsorbed substance could be done. For performing further analysis, variations of the HOMO and LUMO magnitudes and MEP surfaces were good evidences for observing the features of models after occurrence of adsorption processes in comparison with the singular models. Moreover, the obtained values of Table 1 also showed such variations by yielding different values for the electronic molecular orbital features. Based on the results of MEP surfaces, the B atomic type of investigated BN nanostructures was the most favorable position for attracting the nucleophilic agents. Moreover, the evaluated DOS diagrams indicated that the energy levels of HOMO and LUMO detected significant effects of interacting bimolecular complex formations for both of BNNS and BNNT counterparts. The evaluated magnitudes of Table 1 indicated that the valence (HOMO) and conduction (LUMO) levels of molecular orbitals were shifted to higher and lower energy levels, respectively. Consequently, the values of E_g of complexes were decreased in comparison with the singular models. The evaluated values of Q_T from NBO analysis showed that the sum of atomic charges were positive for all models indicating the direction of charge transfer from the curcumin molecule to the BN nanostructure in all the four investigated models. This would be an advantage of employing such BN nanostructures for drug delivery processes by protecting the adsorbed substance not to contribute to further interactions. As a result, the stabilized models were obtained and their features were evaluated to affirm the hypothesis of employing the BN nanostructure for delivery of curcumin. As a final achievement, the investigated BNNS nanostructure showed higher advantage of employing for adsorbing the curcumin substance.

4. Conclusion

In this study, occurrence of interactions between the curcumin and each of the representative BNNS and BNNT nanostructures were investigated by performing DFT calculations. The results indicated that the singular models of keto and enol forms of curcumin and each of BNNS and BNNT nanostructures were obtained and their electronic molecular orbital features showed characteristic features of the models for involving in interaction processes. The results of interacting curcumin and nanostructure bimolecular formations showed higher advantage of the BNNS model for adsorbing the curcumin substance in comparison with the BNNT model. Moreover, the enol form of curcumin was also in stronger interactions with both of the BN nanostructures. The electronic molecular orbital and charge transferring features approved the occurrence of curcumin adsorption by both of BN nanostructures, in which the BN nanostructure could even protect the adsorbed curcumin from contributing to further interactions. As a consequence, the hypothesis of employing BN nanostructure for delivery of curcumin was affirmed based on the obtained results proposing the investigated BNNS and BNNT nanostructures for approaching such vital purpose.