CONFORMATIONAL MOBILITY IN CYCLIC OLIGOPEPTIDES

Analysis of two isomeric cyclic hexapeptides of composition (Asp, Arg, Gly2, Pro, D-Pro) by a nuclear Overhauser effect constrained distance geometry conformation search yielded a narrowly defined backbone conf ormation for one and considerable ambiguity about the conformation in part of the other. Preliminary C-13 relaxation studies of these peptid es suggest that it is possible that this difference may correspond to a physical difference in internal mobility. In connection with this ob servation, other experimental evidence bearing on the backbone conform ational mobility of cyclic oligopeptides with 4-10 residues, frequentl y considered to have well-defined backbones, is reviewed. Conformation al heterogeneity involving rotation of a peptide bond plane relative t o the overall ring plane is identified as a common phenomenon. Nuclear magnetic resonance line-shape studies at temperatures down to 200 K c an detect backbone motions with activation free energy barriers down t o about 10 kcal/mole, but conformational exchange with lower barriers, though detectable in other ways, will not be obvious from nmr spectra alone.