HELIX CAPPING PROPENSITIES IN PEPTIDES PARALLEL THOSE IN PROTEINS

Helix content of peptides with various uncharged nonaromatic amino aci ds at either the N-terminal or C-terminal position has been determined . The choice of N-terminal amino acid has a major effect on helix stab ility: asparagine is the best, glycine is very good, and glutamine is the worst helix-stabilizing amino acid at this position. The rank orde r of helix stabilization parallels the frequencies of these amino acid s at the N-terminal boundary (N-cap) position of helices in proteins f ound by Richardson and Richardson [Richardson, J. S. & Richardson, D. C. (1988) Science 240, 1648-1652], and the N-terminal amino acid in a peptide composed of helix-forming amino acids may be considered as the N-cap residue. The choice of C-terminal amino acid has only a minor e ffect on helix stability. N-capping interactions may be responsible fo r the asymmetric distribution of helix content within a given peptide found by various workers. An acetyl group on the N-terminal alpha-amin o function cancels the N-cap effect and the acetyl group is equivalent to N-terminal asparagine in an unacetylated peptide. Our results demo nstrate a close relationship between the mechanisms of alpha-helix for mation in peptides and in proteins.