Effects of local environment on the circular dichroism spectra of polypeptides

The secondary structures of proteins may be estimated by analysis of their circular dichroism spectra using techniques which empirically fit component spectra derived from water-soluble protein databases. However, local environment effects red- or blue-shift peptide energy transitions primarily through solvent polarization effects and may be an especially important factor for proteins in nonaqueous environments. In this study the reference basis spectra were deconvoluted into three component Gaussian curves corresponding to each of the peptide absorbances in the wavelength range from 190 to 240 nm. These Gaussians were then systematically shifted and utilized in the analyses of the spectra of crambin solubilized in a variety of nonaqueous solvents, including that of the polypeptide associated with small unilamellar phospholipid vesicles. Crambin was selected since its well-characterized structure was found to retain native structure in a variety of nonaqueous environments. This method was more successful in estimating the helical content of this protein than conventional methodologies. This type of analysis may prove to be appropriate for estimating the secondary structure of proteins found in nonaqueous environments, such as membrane proteins.