ENHANCEMENT OF C-ALPHA HYDROGEN VIBRATIONS IN THE RESONANCE RAMAN-SPECTRA OF AMIDES

Ultraviolet resonance Raman (UVRR) spectra of polypeptides and proteins exhibit a band at ca. 1390 cm-1 whose intensity is sensitive to helical content. The assignment of this band, designated amide S, has been contentious. To study the vibrational origin of amide S, relevant amide model compounds and their selectively deuterated isotopomers were synthesized. N-Methylaeetamide, N-methylpropionamide and N-methylisobutyramide possess a methyl, methylene and methine group, respectively, adjacent to the amide carbonyl. The UVRR spectrum of the natural abundance species show resonantly enhanced vibrational bands in the region 1350-1380 cm-1. These bands disappear on deuteration of the (C)C(alpha) hydrogen atoms, while the amide III modes shift up to a common position (ca. 1335 cm-1) and increase in intensity. These results suggest that amide S can be assigned to a (C)C(alpha) hydrogen bending vibration, which acquires resonance intensity by vibrational mixing with the CN stretch of the nearby amide III mode. Variation of this mixing with the C(alpha)-H/C-O dihedral angle can explain the sensitivity of amide III to protein secondary structure. Additional UVRR and Fourier transform IR data reveal that (C)C(alpha)-deuteration has a smaller effect on the amide I, I', II and II' vibrational modes. Finally, it is established that UVRR spectra for NMA in H2O and D2O show an absence of torsional fundamentals or overtones. These results are interpreted as indicating an energy barrier to excited-state isomerization, which argues against an alternative assignment of amide S to the amide V overtone.