Molecular mechanism of the binding of 3,4,5-tri-O-caffeoylquinic acid to human serum albumin: Saturation transfer difference NMR, multi-spectroscopy, and docking studies

As a natural dietary polyphenol, 3,4,5-tri-O-caffeoylquinic acid (3,4,5-triCQA) exhibits numerous stronger pharmacological activities than that of its analogues. Studies on interaction between 3,4,5-triCQA and protein are very helpful for understanding the mechanism of these enhanced biological functions. In this study, H-1 saturation transfer difference NMR (H-1 STD-NMR) combined with multi-spectroscopy were used to probe the interaction of 3,4,5-triCQA with human serum albumin (HSA). Both qualitative and quantitative H-1 STD-NMR indicated that 3,4,5-triCQA can specifically bind to HSA at the favored Sudlow's site II with caffeoyl groups as the main recognizable moiety. Fluorescence emission spectra showed that Stern-Volmer quenching constant (K-sv) decreases from 10.132 x 10(4) M-1 to 9.711 x 10(4) M-1 with temperature raise, indicating that 3,4,5-triCQA quenches HSA fluorescence through a static mechanism. Binding constant (K-b = 5557 x 10(5) M-1) and the number of binding sites (n approximate to 1) at 298 K suggested that 3,4,5-triCQA only occupies one site in HSA with high affinity. Enthalpy (Delta H = 28.802 kJ/mol) and entropy (Delta S = 12.429 J/mol/K) change proved the dominant role of electrostatic interaction in binding process. Multi-spectroscopic analysis also confirmed that the protein secondary structure and hydrophobicity were significantly affected. Molecular docking further verified the NMR and spectroscopic results. Overall, 3,4,5-triCQA exhibited a strong albumin affinity owing to the plural caffeoyl groups, which lead to the enhanced pharmacological activities. This study clarified the molecular mechanism of 3,4,5-triCQA in binding to HSA, and the findings are beneficial for the research on polyphenol-like drugs and antioxidants in foods or cosmetics. (C) 2016 Elsevier B.V. All rights reserved.