ANALYSIS OF ASYMMETRY IN THE DISTRIBUTION OF HELICAL RESIDUES IN PEPTIDES BY H-1 NUCLEAR-MAGNETIC-RESONANCE

Peptides that assume full or partial helical structure in aqueous solution have provided useful models for investigating the determinants of alpha-helical structure. Circular dichroism (CD) spectroscopy, the usual measure of helicity, affords an estimate of the mean helix content when calibrated against suitable standards. Analysis of these systems by means of H-1 NMR makes it possible to determine precisely the location and extent of helix structure in a chain. NMR criteria for identifying helical domains include the following: NOE's between adjacent NH protons and between an NH proton at position i and the C-alpha proton at i + 3; values of the three-bond coupling constants 3J-alpha-N; and the relative chemical shift of the C-alpha protons. Application of these criteria to members of the series of partially helical synthetic peptides, succinylTyrSerGlu4Lys4XXXGlu4Lys4NH2, in which sets of three amino acids are inserted between blocks of glutamic acid and lysine side chains, shows that the helix is located preferentially near the N terminus in chains with the central substitution Ala3, Leu3, as well as in the parent species lacking any substitution. The degree of helicity rises sharply at the N terminus to a maximum near residue 8 and diminishes gradually from Glu14 to the C terminus. Application of the NMR criteria to the peptide containing Gly3 reveals very little helical structures in this peptide. These results that helix formation in short chains does not conform to an all-or-none reaction.