Quenching spin diffusion in selective measurement of transient Overhauser effects in nuclear magnetic resonance. Applications to nucleotides

In high-resolution nuclear magnetic resonance (NMR), the transfer of longitudinal magnetization from one spin to another (A squiggly arrow pointing right X) under the effect of cross relaxation (nuclear Overhauser effect) is often complicated by spin-diffusion pathways through other spins K in the vicinity (e.g., A squiggly arrow pointing right K squiggly arrow pointing right X). It is shown how these undesirable pathways can be quenched by manipulating the magnetization of the two sites A and X with doubly selective inversion pulses. At the beginning of the experiment, after selective inversion of the ''source'' spin A, the 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 components 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 relaxation interval tau(m). If the signal of the target spin X overlaps with other resonances, the polarization [I(z)x] may be monitored indirectly by a doubly selective magnetization transfer to a 'spy'' proton M through a scalar coupling J(MX). The methods are illustrated by applications to Overhauser effects in the palindromic deoxyribonucleic acid d(CGCGAATTCGCG)2, Which forms a B-type double helix.