QUENCHING SPIN-DIFFUSION IN SELECTIVE MEASUREMENTS OF TRANSIENT OVERHAUSER EFFECTS IN NUCLEAR-MAGNETIC-RESONANCE - APPLICATIONS TO OLIGONUCLEOTIDES

In high-resolution nuclear magnetic resonance (NMR), the transfer of l ongitudinal magnetization from one spin to another (A squiggly arrow p ointing right X) under the effect of cross relaxation (nuclear Overhau ser effect) is often complicated by spin-diffusion pathways through ot her spins K in the vicinity (e.g., A squiggly arrow pointing right K s quiggly arrow pointing right X). It is shown how these undesirable pat hways can be quenched by manipulating the magnetization of the two sit es A and X with doubly selective inversion pulses. At the beginning of the experiment, after selective inversion of the ''source'' spin A, t he longitudinal magnetization tends to migrate not only to the 'target ' nucleus X but also to various other 'clandestine'' nuclei K, K, ... ([I(z)A] squiggly arrow pointing right [I(z)K], [I(z)K'], ...). In the middle of the interval tau(m), the longitudinal magnetization compone nts of both A and X are inverted simultaneously, without affecting the spins K, K', .... The direct flow of magnetization from A to X is not perturbed by this manipulation, but the indirect flow via K, K', ... is reversed in sign and almost perfectly canceled at the end of the re laxation interval tau(m). If the signal of the target spin X overlaps with other resonances, the polarization [I(z)x] may be monitored indir ectly by a doubly selective magnetization transfer to a 'spy'' proton M through a scalar coupling J(MX). The methods are illustrated by appl ications to Overhauser effects in the palindromic deoxyribonucleic aci d d(CGCGAATTCGCG)2, Which forms a B-type double helix.