NMR evidence for multiple conformations in a highly helical model peptide

A monomeric model peptide, acetyl-WEAQAREALAKEAAARA-amide, has been structurally characterized using the complementary techniques of 1H 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. However, the observation of long-range NOEs between the methyl protons of leucine-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 conformers, indicating a multiconformational equilibrium. Implications of these observations with regard to secondary structure quantitation and current method limitations are discussed.