Effect of the substitution Ala .fwdarw. Gly at each of five residue positions in the C-peptide helix

The substitution Ala----Gly has been studied in a unique-sequence peptide (related in sequence to the C-peptide of ribonuclease A) to determine its effect on C-peptide helicity at different residue positions. There is a substantial decrease in helicity for Ala----Gly at residue position 4, 5, or 6 but only a small decrease in helicity for Ala----Gly at end residue 1 and no decrease at end residue 13. The change for Ala----Gly is similar at position 4, 5, or 6; the change is caused chiefly by the difference in s, the helix growth parameter in the Zimm-Bragg model for alpha-helix formation, between Ala and Gly. Thus, the helicity of C-peptide depends sensitively on s at interior positions. The small change in helicity found for Ala----Gly at either end position suggests that the end residues are largely excluded from the helix, with the result that helicity is relatively unaffected by replacement of an end residue. Another possibility is that some helix-stabilizing effect is exerted by Gly only at an end position. Exclusion of an end residue from the helix might be caused either by fraying of the helix ends or by helix termination at an interior residue, resulting from a helix stop signal such as the Glu-2- -Arg-10+ salt bridge or the Phe-8-His-12+ ring interaction.