Characterization of the Interaction between 4-(Tetrahydro-2-Furanmethoxy)- N -Octadecyl-1,8-Naphthalimide and Human Serum Albumin by Molecular Spectroscopy and Its Analytical Application

A novel 4-(tetrahydro-2-furanmethoxy)-N-octadecyl-1,8-naphthalimide (TNN) was synthesized as a spectrofluorimetric probe for the determination of proteins. The effect of different solvents on the spectral characteristics of TNN was investigated. The results showed that TNN displayed dependent solvent polarity properties due to the effect of internal charge transfer. The interactions between TNN and human serum albumin (HSA) were studied by fluorescence and absorption spectroscopy. Fluorescence data revealed that the fluorescence quenching of HSA by TNN was the result of the formation of TNN–HSA complex. The binding parameters of interactions between TNN and HSA at different temperatures were obtained according to the Stern–Volmer equation. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS), for the interactions were calculated to be −7.31 kJ mol⁻¹ and 72.75 J mol⁻¹ K⁻¹ according to the van't Hoff equation, indicating that the hydrogen bonds and hydrophobic interactions were the dominant intermolecular force in stabilizing the complex. The effect of TNN on the conformation of HSA was analyzed by circular dichroism and synchronous fluorescence spectroscopy. Furthermore, the results of displacement experiments using warfarin indicated that TNN could bind to site I of HSA. The fluorescence of TNN could be largely quenched by HSA, based on which a new fluorometric method for detecting HSA in the HCl-Tris buffer solution (pH = 7.4) was developed. The linear ranges of the calibration curves were 0.1~14.2 μM for HSA, 0.1~13.0 μM for bovine serum albumin (BSA), 0.2~9.7 μM for γ-globulin, and 0.3~11.3 μM for hemoglobin (Hb), with detection limits (3σ) of 13.7 × 10⁻¹⁰ M for HSA, 1.84 × 10⁻¹⁰ M for BSA, 3.14 × 10⁻¹⁰ M for γ-globulin, and 6.86 × 10⁻¹⁰ M for Hb. The effect of metal cations on the fluorescence spectra of TNN in ethanol was also investigated. The method has been applied to the determination of total proteins in human serum samples collected from the hospital and the results were in good agreement with those reported by the hospital.