Investigating the interaction mechanism of fluorescent whitening agents to human serum albumin using saturation transfer difference-NMR, multi-spectroscopy, and docking studies

Abstract

In this study, 2,2′-(4,4′-biphenylylenebisvinylene)bisbenzenesulfonicacid (CBS-X) and its disodiumsalt (CBS) were used as model compounds to investigate the interaction mechanism between 4,4′-distyrylbiphenyl based fluorescent whitening agents (DSBP-FWAs) and human serum albumin (HSA) through various techniques, including ¹H saturation transfer difference nuclear magnetic resonance (¹H STD-NMR), fluorescence studies, UV-vis absorption, Fourier transform infrared (FT-IR) spectroscopy, circular dichroism (CD) spectroscopy, and molecular docking. The ¹H STD-NMR analyses indicated that CBS and CBS-X can bind to HSA at the favored Sudlow's sites II and I, respectively. Fluorescence emission spectra showed that CBS and CBS-X quenches HSA fluorescence through a dynamic mechanism, and this was further verified by fluorescence lifetime experiments and UV-vis absorption. Moreover, the effective binding constant values of the two compounds at the same temperature decreased in the order CBS > CBS-X. Furthermore, the energy transfer efficiency for CBS and CBS-X were 50.5% and 40.6%, respectively. Thermodynamic analyses indicated that the binding of CBS and CBS-X with HSA are both primarily controlled by hydrophobic forces. FT-IR and CD spectroscopy provided complementary information on the micro-environmental and conformational changes of HSA with the additions of CBS and CBS-X. Molecular docking further confirmed the NMR and spectroscopic results. Overall, the comparative studies on the interaction mechanism of CBS and CBS-X when binding to HSA may provide useful information for evaluating their effects on the human body.