Intramolecular 1H nuclear Overhauser effect study of the solution conformation of valinomycin in dimethyl sulfoxide

Determination of the mechanism of intramolecular nuclear Overhauser effects (NOE) in peptides and depsipeptides is essential to the use of this technique in conformational analysis of these and related biomolecules. Towards this end, 1H NMR double-resonance studies were conducted on valinomycin in (CD3)2SO at 90 MGZ (FT mode) and 250 MGZ (correlation mode). The NOE's are positive at the lower frequency and negative at the higher frequency. Consideration of the theoretical dependence of the NOE on the proton-proton internuclear correlation time and on the resonance frequency indicates that these results are explained by a predominantly dipolar relaxation mechanism. It is demonstrated that exchange modulation of scalar coupling does not contribute significantly to the NOE. A formalism for the NOE's of loosely coupled spin systems is presented which takes into account the effects of high magnetic-field strengths and long correlation times. An approximate analysis of the NOE data assuming a single correlation time for the entire molecule and ignoring cross-relaxation effects was used to evaluate various models that have been proposed for the conformation of valinomycin. The III-1 model of Patel and Tonelli (Patel, D.J., and Tonelli, A.E. (1973), Biochemistry 12, 486) fits the NOE and peptide NHCalphaH coupling constant data and is probably a preferred orientation in dimethyl sulfoxide. These experiments illustrate how intramolecular NOE data provide a valuable auxiliary method to other techniques for delineating the preferred solution conformation of peptides, depsipeptides, and other biomolecules.