Simultaneous observation of positive and negative nuclear overhauser effects in oligopeptides due to segmental motion

Abstract

Nuclear Overhauser effect (NOE) studies of the symmetrical cystine peptides Boc-Cys-(Val)_n-Trp-OMe (n=1-3) in dimethylsulfoxide, Boc-Cys-(Val)_n-Trp-OMe have resulted in the simultaneous observation of both positive and negative NOEs. Positive NOEs are observed on the Trp C2H and C4H protons of the indole ring upon irradiation of Trp C ^α H and C ^β H₂ resonances in the peptides where N=1 and 2. Negative NOEs are observed between backbone NH and C^α H protons. The magnitudes of the observed NOEs are sensitive to changes in molecular size and solvent viscosity. The results demonstrate that NOEs may be a useful probe of sidechain segmental motion in oligopeptides. Boc, t-butyloxycarbonyl; OMe, methyl ester; NOE, nuclear Overhauser effect.