ESTIMATING THE RELATIVE POPULATIONS OF 3(10)-HELIX AND ALPHA-HELIX INALA-RICH PEPTIDES - A HYDROGEN-EXCHANGE AND HIGH-FIELD NMR-STUDY

Recent experimental and theoretical work suggests that alanine-rich pe ptides fold as a mixture of 3(10)-helix (i --> i + 3 hydrogen bonding) and alpha-helix (i --> i + 4 hydrogen bonding). In order to assess th e relative proportions of the two conformers, NMR studies were perform ed on the 16 residue sequences: Ac-AAAAKAAAAKAAAAKA-NH2 (3K) and Ac-AM AAKAWAAKAAAARA-NH2 (MW). Hydrogen/deuterium-exchange kinetics measured for the first three amide protons of the 3K peptide indicate that the NH of Ala3 is partially protected from exchange. This result is consi stent with the presence of an i --> i + 3 hydrogen bond between the ca rbonyl group of the acetyl blocking group and the NH group of Ala3. Th e MW peptide is a modified version of the 3K peptide, designed to incr ease alpha H signal dispersion. H-1 NMR spectra of the MW peptide at 7 50 MHz reveal a series of intermediate range (NOEs) consistent with a mixture of 3(10)-helix and alpha-helix. The relative intensities of th e alpha N(i,i + 3) and alpha beta(i,i + 3) (nuclear Overhauser enhance ments) NOEs suggest that 3(10)-helix is present throughout the peptide , but with the greatest contribution at the termini. A model was devel oped to determine the relative contributions of 3(10)-helix and alpha- helix. Lower bounds for the population of 3(10)-helix are approximatel y 50% at the termini and 25% in the middle of the peptide. The greates t alpha-helical content is between the middle of the peptide and the N terminus. (C) 1997 Academic Press Limited.