Solvent effects on the conformation and far UV CD spectra of gramicidin

Solvent effects on the far-uv CD spectra of the polypeptide gramicidin have been studied systematically in a series of alcohols of increasing chain length, ranging from methanol to dodecanol. The effects observed are of two types: primary, involving a change in the equilibrium mixture of conformers present, and secondary, involving a shift in the spectral peak positions as a function of solvent polarizability. To quantitate the primary effect, the ratio of the individual conformers present was estimated by deconvolution of the spectra into their component species. For short chain length alcohols, both parallel and antiparallel double helices are found in considerable abundance. As the solvent chain length is increased and its polarity is decreased, the left-handed antiparallel double helical species is favored. For all alcohols with chain lengths of four or more carbon atoms, the ratio of the conformers present remains relatively constant. To quantitatively examine the secondary effect, the magnitudes of the spectral shifts on the dominant conformer (species 3) have been correlated with the dielectric constants and refractive indices of the solvents, thereby indicating what underlying physical properties are responsible for these shifts. This work thus demonstrates that for gramicidin, a flexible polypeptide, the solvent effects on the CD spectra can be resolved into two types: changes due to the mixture of conformers present and shifts in the spectral characteristics. Both effects need to be considered when interpreting CD spectra in terms of secondary structure for this and other polypeptides in nonaqueous solutions.