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

TL;DR

This study investigates how chicken villin headpiece unfolds in water-ethanol and water-DMSO mixtures, revealing solvent-dependent unfolding pathways and intermediate states that differ from unfolding in pure water.

Contribution

It provides new insights into solvent-specific effects on protein unfolding, highlighting the role of co-solvent composition in modulating unfolding pathways and intermediates.

Findings

Unfolding initiated by hydrophobic core separation in both solvents.

Partially unfolded intermediates depend on co-solvent concentration.

Unfolding pathways differ significantly between water-ethanol and water-DMSO.

Abstract

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 the binary mixtures, HP-36 is found to unfold (fully or partially, depending on the mixture) under ambient conditions, that otherwise requires temperature as high as ~600 K to denature in pure aqueous solvent. In all the cases, first step of unfolding is found to be similar, i.e. separation of the cluster formed by three hydrophobic (phenylalanine) residues, namely Phe-7, Phe-11 and Phe-18, which constitute the hydrophobic core, thereby initiating melting of helix-2 of the protein. Subsequent unfolding steps follow different paths in different chemical environments. As both water-DMSO and water-ethanol show composition dependent anomalies, so do the details of unfolding dynamics. With an increase of co-solvent concentration different partially unfolded intermediates are found to be formed in both the cases. This is reflected in a remarkable non-monotonic composition dependence of several order parameters, including fraction of native contacts and protein-solvent interaction energy. The emergence of such partially unfolded states is particularly attributed to the preferential solvation of the hydrophobic residues by the ethyl groups of ethanol and methyl groups of DMSO. While in DMSO the protein gradually attains a completely unfolded state at xDMSO=0.30, unfolding in water-ethanol appears to be more complex and sensitive to solvent composition.