Demonstration of the Ring Conformation in Polyproline by the Raman Optical Activity

Raman and Raman optical activity (ROA) spectra of poly-l-proline were recorded in a wide
frequency range and analyzed with respect to the proline side chain conformation. The analysis was based
on comparison to ab initio simulations of spectral band positions and intensities. The presence of two
conformer states of the five-member ring was found, approximately equally populated in the polypeptide.
Additionally, Raman and ROA spectral shapes indicated that the peptide adopts the polyproline II helical
conformation, in both aqueous and TFE solutions. The helix, however, is perturbed by fluctuations, which
affects the vibrational coupling among amino acid residues and broadens the ROA bands. Contributions
of the side and main peptide chains to the polyproline ROA intensities have comparable magnitudes. Thus
understanding of the origins of both signals is important for determination of the peptide structure by ROA.