NMR saturation factors in the presence of chemical exchange

Abstract

Rapid pulsing is frequently employed in NMR spectroscopy to increase signal-to-noise. In general, this results in partial saturation of spectral lines, so that observed magnetization amplitudes must be corrected with saturation factors to obtain true concentrations. Saturation factors may be obtained experimentally or derived from known T 1 values, provided that there is no exchange between observed chemical species. However, this is frequently not the case in biological samples and in other samples of biochemical interest. In this paper, the theory of saturation factors has been extended to allow for the effects of chemical exchange between two species. The saturation factors are not independent of concentration, in contrast to the case without exchange, and the error in concentration measurements which results from ignoring exchange depends on the reaction rate, on the difference in T 1 values of the exchanging species, on changes in the ratio of their concentrations, and on the interpulse delay and flip angle used. For reasonable values of kinetic and pulse parameters, this error can reach 10% or more. However, if the reaction rate and T 1 values are known, the effect of exchange on apparent concentrations can be derived. Alternatively, pulse parameters can be defined which will result in a predetermined upper limit on the error resulting from neglect of exchange. One may derive numerically the correct concentrations of exchanging species directly from observational data.