Nuclear Overhauser effects as probes of peptide structure. A diagnostic for left-handed helical conformations

The conformational dependence of the interresidue interproton distances in peptides, C alpha H ... Ni + 1 H and NiH ... Ni + 1 H, have been used to identify zones of sterically allowed phi, psi space, where both distances are less than 3A and expected to yield nuclear Overhauser effects (NOEs). L-residues in left-handed helical conformations are expected to yield both interresidue NOEs and also an appreciable intraresidue NiH----C alpha iH NOE. The effect of cutoff distances has been evaluated. Experimental results on three model peptides illustrate the utility of these NOEs in identifying L-residues at the i + 2 position of Type II and I' beta-turns. Simultaneous observation of both interresidue NOEs may also be indicative of conformational heterogeneity in specific cases, as illustrated for a single residue in a decapeptide.