Vibrational analysis of peptides, polypeptides, and proteins: XXX. Normal mode analyses of -turns

The normal modes have been calculated for three kinds of low energy y-turn structures resulting from recent conformational energy calculations by Nkmethy. Frequencies have been computed for a y-turn, a mirror-related 7-turn, and an inverse y-turn of CH3-CO-(L-Ala),-NH-CH3, with n = 3 and n = 5 , and for certain l4 C and l5 N derivatives of the n = 3 molecule. Correlations are evident between amide frequencies and y-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.