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

- Polyphenol-protein interaction was analyzed by saturation transfer difference NMR.
- Electrostatic force led 3,4,5-triCQA binding to human serum albumin at site II.
- Affinity of 3,4,5-triCQA reached to 105 level due to the plural caffeoyl groups.
- Protein surface hydrophobicity decreased from 1.22 × 1014 to 1.12 × 1014 level.
- Protein secondary structure was changed obviously after binding with 3,4,5-triCQA.

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, 1H saturation transfer difference NMR (1H 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 1H 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 (KSV) decreases from 10.132 × 104 M− 1 to 9.711 × 104 M− 1 with temperature raise, indicating that 3,4,5-triCQA quenches HSA fluorescence through a static mechanism. Binding constant (Kb = 5.557 × 105 M− 1) and the number of binding sites (n ≈ 1) at 298 K suggested that 3,4,5-triCQA only occupies one site in HSA with high affinity. Enthalpy (ΔH = − 28.802 kJ/mol) and entropy (ΔS = 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.

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