Proline for alanine substitutions in the C-peptide helix of ribonuclease A

The effect on overall alpha-helix content of substituting proline for alanine has been determined at 5 positions (1, 2, 4, 5, and 13) of a 13-residue peptide related in sequence to residues 1-13 of ribonuclease A. The helix content falls off rapidly as proline is moved inward, and the proline residue effectively truncates the helix. No helix-stabilizing effect of proline is found at positions 2 or 4 within the first turn of the helix. Proline substitution at either end position (1, 13) has little effect on overall helix content, in agreement with an earlier study of glycine for alanine substitutions. The two end residues of the helix appear to be strongly frayed.