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In the present work we study and compare unfolding of a small protein,chicken villin headpiece (HP-36) in two different aqueous binary mixtures,namely water-ethanol (EtOH) and water-dimethyl sulphoxide (DMSO). In both thebinary mixtures, HP-36 is found to unfold (fully or partially, depending on themixture) under ambient conditions, that otherwise requires temperature as highas ~600 K to denature in pure aqueous solvent. In all the cases, first step ofunfolding is found to be similar, i.e. separation of the cluster formed bythree hydrophobic (phenylalanine) residues, namely Phe-7, Phe-11 and Phe-18,which constitute the hydrophobic core, thereby initiating melting of helix-2 ofthe protein. Subsequent unfolding steps follow different paths in differentchemical environments. As both water-DMSO and water-ethanol show compositiondependent anomalies, so do the details of unfolding dynamics. With an increaseof co-solvent concentration different partially unfolded intermediates arefound to be formed in both the cases. This is reflected in a remarkablenon-monotonic composition dependence of several order parameters, includingfraction of native contacts and protein-solvent interaction energy. Theemergence of such partially unfolded states is particularly attributed to thepreferential solvation of the hydrophobic residues by the ethyl groups ofethanol and methyl groups of DMSO. While in DMSO the protein gradually attainsa completely unfolded state at xDMSO=0.30, unfolding in water-ethanol appearsto be more complex and sensitive to solvent composition.

Solvent Sensitivity of Protein Unfolding: Study of Chicken Villin Headpiece Subdomain in Water-Ethanol and Water-DMSO Mixtures