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

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Abstract
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. 1H NMR double-resonance studies were conducted on valinomycin in (CD3)2SO at 90 MHz (FT mode) and 250 MHz (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. 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 were proposed for the conformation of valinomycin. The III-1 model of Patel and Tonelli fits the NOE and peptide NHC.alpha.H coupling constant data and is probably a preferred orientation in dimethyl sulfoxide. Intramolecular NOE data provide a valuable auxiliary method to other techniques for delineating the preferred solution conformation of peptides, depsipeptides and other biomolecules.

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