Ultraviolet resonance Raman study of proline isomerization

The ultraviolet resonance Raman spectra of aqueous solutions of proline dipeptides obtained with 200- and 218-nm excitation at pH 1.5 and 6.5 are reported. The cis/trans isomer ratio for these dipeptides is known to depend on pH. The amide II′ like mode of the X-Pro peptide (imide) linkage at 1487 cm^-1, involving predominantly the motions of the peptide carbon and nitrogen, is the most strongly enhanced band in the 200-250-nm region. We observe that this band does not shift appreciably with the change in isomer ratio associated with the change in pH. The same behavior is observed with the 1467-cm^-1 band of aqueous polyproline, which has undergone a salt-induced isomerization. We conclude from these observations that contrary to earlier evidence the position of the amide II′ like band is not diagnostic for the isomeric state of the imide bond.