Deciphering the spectroscopic and thermodynamic aspects of binding of biologically important antioxidants with the alkali induced state of human serum albumin

Abstract

Protein–ligand interactions are crucial for developing and identifying novel therapeutic targets. In this study, we investigate the interaction of the alkali induced state of human serum albumin (pH 11.2) with three hydroxycinnamic acid derivatives (HCDs), ferulic acid (FA), sinapic acid (SA) and trans-o-coumaric acid, which are biologically important antioxidants, and compare the outcomes with the results obtained at physiological pH (7.4). This study aims to explore the interaction of altered protein conformation with small molecules. Spectroscopic characterization methods show that the conformation of HSA and the ionic properties of HCDs are pH-dependent. Fluorescence, FRET and lifetime measurements reveal that the binding of HCDs with HSA is different at both pH 7.4 and 11.2. Despite the moderate binding of HCDs to HSA, circular dichroism and thermal denaturation studies report no conformational changes in HSA in the presence of HCDs. Isothermal titration calorimetry is employed to assess their binding based on structure and energetics using thermodynamic parameters. Standard molar enthalpy change (ΔH⁰_m) and standard molar entropy change (ΔS⁰_m) values vary with the change of pH from 7.4 to 11.2 with the contributions from the exothermicity and hydrophobicity of functional and aromatic groups of HCDs. Ferulic acid (FA) and sinapic acid (SA) binding to HSA is entropically driven, whereas trans-o-coumaric acid (CA) acid binding is enthalpically favourable. Our ITC studies also reveal that the involvement of –OH functional groups present in CA in binding with HSA is greater than that present in FA and SA at pH 11.2. Overall, this experimental study shows the comparable binding strength of HCDs to both the alkali-induced state of HSA and native HSA (pH 7.4). However, the mechanism of their binding is different.