Secondary solvent effects on the circular dichroism spectra of polypeptides in non-aqueous environments: influence of polarisation effects on the far ultraviolet spectra of alamethicin.

Secondary solvent effects on the far ultraviolet circular dichroism spectra of the polypeptide alamethicin have been studied systematically in a series of alcohols. The magnitudes of the shifts have been correlated with the physical properties of the solvents in an attempt to discover the underlying physical principles responsible for these shifts. The solvent effect in non-aqueous solvents generally produces spectral transitions with peaks found at longer wavelengths than those in aqueous solution, and is correlated with increasing refractive indices and with decreasing dielectric constants of the solvents. It appears that polarisation effects are the major contributors to the interactions between the chromophore and solvent molecules, and hence give rise to the red shift. It is clear that this secondary solvent effect is an important factor which should be considered in the examination and estimation of polypeptide secondary structures in non-aqueous solvents and membranes.