Analysis of the delocalized Raman modes of conformationally disordered polypeptides

Bands associated with delocalized vibrational modes were identified in the isotropic Raman spectra of a series of polyglycine oligomers in aqueous solution as zwitterions and as cations. The dependence of these bands on conformational disorder and chain length was determined. The observed dependence is closely mimicked in spectra calculated for a series of corresponding model polypeptides. The simulated spectra were calculated in a skeletal approximation for ensembles of conformationally disordered chains. As the chain length of the conformationally disordered polypeptides increases, the observed isotropic spectra rapidly approach the spectrum of the infinitely long disordered chain. Convergence is nearly complete at the tripeptide for both the zwitterion and the cation. The stimulated spectra behave in essentially the same way. Convergence to the spectrum of the infinitely long chain is much more rapid for the conformationally disordered polyglycines than for the ordered polyglycines because of the mode localization that results from disorder. In the low-frequency region the bands in the calculated spectra have frequencies that are systematically dependent on chain length. These bands are related to the longitudinal acoustic modes of the ordered chain.