Compatibility of the two-site exchange model and 1H NMR relaxation rates

We demonstrate a formalism for determining whether 1H longitudinal and transverse magnetic relaxation decay curves are compatible with a model of proton exchange between two sites with inherently different relaxation rates. We apply the method to published data in which both decay curves have been fit to normalized biexponential functions, a prediction of the simplest two-site exchange model. The analysis is applied to relaxation data of adsorbed water vapor on silica gel (J.R. Zimmerman and J.A. Lasater, J. Phys. Chem. 62, 1157 (1958] and to relaxation data reported in mycelia of Botrytis cinerea Persoon and pupae of the tobacco cutworm (M. Yoshida and K. Nose, Agric. Biol. Chem. 51, 3399 (1987]. Exchange parameters are calculated form these data sets and the compatibility of the two-site exchange model with the data is discussed for each case. The sensitivity of the calculated exchange parameters to errors in the bioexponential fitting parameters is treated in some detail.