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

Abstract

The conformational dependence of the interresidue interproton distances in peptides, C_i^αH ... N_i+1H and N_iH ...N_i+1H, have been used to identify zones of sterically allowed φ, ψ space, where both distances are < 3Å 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 N_iH ←→ C_i^αH 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′ β-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.