Laser Raman studies on the conformation of Pro-Leu-Gly-NH2.

Laser Raman spectra of Pro-Leu-Gly-NH2, the factor that inhibits release of pituitary melanotropin, have been obtained in the solid state, in dimethylsuloxide and in aqueous solution. The amide I frequencies were observed at 1650 and 1687 cm-1 in the solid state and at 1669 cm-1 in dimethylsulfoxide. The conformation of this tripeptide has been proposed by 1H-NMR studies in [2H]-dimethylsulfoxide and revealed by X-ray analysis to be type II beta-turn. These observed amide I frequencies thus are characteristic of type II beta-turn conformation. The relatively lower amide I frequency, 1645 cm-1, observed in 2H2O indicates that the conformation of the peptide in aqueous solution could be different from those in solid state and in dimethylsulfoxide. pH changes hae no significant effect on aqueous solution spectra, except for the shape of the amide III band. The maximum of the amide III band shifted from 1259 cm-1 at pH 2.0 to 1242 cm-1 at pH 11.7. The amide III peaks in the solid state were at 1234 and 1268 cm-1.