Raman spectroscopy and normal vibrations of peptides. Characteristic normal modes of a type-II beta turn.

Raman and infrared spectra have been recorded on the peptide Gly-LPro-Gly-Gly in its zwitterionic form and on its protected homolog Z-Gly-LPro-Gly-Gly-OMe (Z = N-benzyloxycarbonyl and OMe = methyl ester). Both peptides adopt a type-II beta-turn conformation into appropriate solvents, that is in dimethylsulfoxide and chloroform respectively. A normal coordinate treatment has been performed on Gly-LPro-Gly-Gly using a modified Urey-Bradley force field and the characteristic normal modes of vibrations. Their corresponding frequencies have been extracted from experimental investigations.