Vibrational analysis of peptides,polypeptides, and proteins

The normal modes have been calculated for three kinds of low energy γ-turn structures resulting from recent conformational energy calculations by Némethy. Frequencies have been computed for a γ-turn, a mirror-related γ-turn, and an inverse γ-turn of CH₃-CO-(L-Ala)_n-NH-CH₃, with n = 3 and n = 5, and for certain ¹⁴C and ¹⁵N derivatives of the n = 3 molecule. Correlations are evident between amide frequencies and γ-turn structures, and it is found that only amide I modes of peptide groups in the turn are relatively insensitive to the lengths of attached chains.