Unveiling the thermodynamic signature underlying the interaction of human serum albumin with sub-micellar concentrations of a surface active ionic liquid

Abstract

The interaction of the transport protein human serum albumin (HSA) has been studied with sub-micellar concentrations of the surface active ionic liquid 1-butyl-3-methylimidazolium octylsulfate ([BMIM][OS]). The prominent quenching of intrinsic Trp fluorescence of HSA together with a discernible hypsochromic shift of the maximum fluorescence wavelength from λem ~ 347 nm in native HSA to λem ~ 335 nm in the presence of ~15.6 mM of the [BMIM][OS] suggests disruption of the native protein conformation following interaction with the ionic liquid. This is further substantiated from far-UV circular dichroism spectroscopy which reveals the denaturing influence of [BMIM][OS] through lowering of α-helicity content of the protein. However, the major impetus of the present work lies in a detailed calorimetric investigation of the HSA-[BMIM][OS] interaction process. The isothermal titration calorimetry results delineates the predominant role of hydrophobic effect governing the interaction as characterized by negative change of heat capacity (∆Cp = − 160.65 ± 8.5 J mol−1 K−1). However, the temperature-dependent calorimetric results show an enthalpy-driven (ΔH < 0, exothermic) hydrophobic effect with gradually decreasing contribution from the entropy counterpart with increasing temperature. These results are counterintuitive to the notion of classical hydrophobic effect characterized by favorable entropy contribution (ΔS > 0) and unfavorable enthalpy contribution (ΔH > 0, endothermic), and are rationalized in terms of the ‘nonclassical hydrophobic effect’.