NMR EVIDENCE FOR MULTIPLE CONFORMATIONS IN A HIGHLY HELICAL MODEL PEPTIDE

A monomeric model peptide, acetyl-WEAQAREALAKEAAARA-amide, has been st ructurally characterized using the complementary techniques of H-1 2D NMR and circular dichroism. Temperature-dependent CD measurements are consistent with a two-state helix/coil transition model and indicate a 65% contribution of helical conformers at 5-degrees-C. Homonuclear 2D NMR experiments allowed the assignment of all proton resonances. The analysis of NOE-type cross-relaxation data established a large number of specific short- and medium-range NOE connectivities throughout the peptide, confirming the highly helical character of the peptide. Howev er, the observation of long-range NOEs between the methyl protons of l eucine-9 and backbone and side-chain protons of amino acids located at the N-terminus, as well as other unusual NOEs, unambiguously reflects the existence of significantly populated nonhelical structured confor mers, indicating a multiconformational equilibrium. Implications of th ese observations with regard to secondary structure quantitation and c urrent method limitations are discussed.