On the solvent role in alcohol-induced α-helix formation of chymotrypsin inhibitor 2

Abstract

The circular dichroism (CD) spectra of chymotrypsin inhibitor 2 (CI2) have been measured as a function of alcohol mole fraction in aqueous mixtures of methanol, ethanol, trifluoroethanol (TFE), and hexafluoro-iso-propanol (HFIP). Small-angle X-ray and neutron scattering (SAXS and SANS) of CI2 was also measured as a function of ethanol mole fraction in ethanol-water mixtures. The CD spectra have shown that the secondary structure of CI2 changes from β-strand to α-helical structure at alcohol mole fractions characteristic of the individual alcohols in an order of HFIP > TFE > ethanol > methanol in effectiveness, where the structure transition of solvent clusters takes place from the typical tetrahedral-like water to the chain-like alcohol clusters in alcohol-water mixtures previously reported. The radius of gyration of CI2, obtained from the analysis of the SANS data, increased with an increase in ethanol mole fraction up to around 0.2 and then gradually decreased. The SAXS data have shown that the shape of CI2 changes from a sphere to a rod-like one at a 0.1 ethanol mole fraction. A possible role of solvent clusters played in alcohol-induced α-helix formation of CI2 is discussed from a viewpoint of the solvent clusters.