Nuclear Overhauser Effect

Abstract

The nuclear Overhauser effect (NOE) is defined as the change in intensity of one NMR resonance that occurs when another is saturated. It results from dipole–dipole cross-relaxation between nuclei, and its usefulness arises because the strength of a given NOE enhancement is approximately correlated with internuclear separation (actually r −6 where r is the internuclear distance). However, this relationship is approximate as the NOE also depends on other factors, particularly molecular motions. In small molecules in solution, the NOE is positive (i.e., it causes affected resonances to increase in intensity) and is generally measured using one-dimensional experiments. In larger molecules, the NOE is negative (affected resonances decrease in intensity) and is usually measured using the two-dimensional NOESY experiment or one of its multidimensional variants. Using the NOE to derive three-dimensional structural information generally relies on interpretation of an overlapping, redundant network of enhancements, rather than on calibrating precisely the distance dependence of individual enhancements. In small molecules it is particularly useful for determining particular stereochemical relationships, such as substitution or ring fusion patterns in largely rigid systems. For biomacromolecular systems such as proteins and their complexes with DNA, RNA, or other proteins, it can determine accurate three-dimensional solution structures.