Proton exchange rates measured by saturation transfer using delayed randomization of the solvent magnetization.

The spin-lock saturation transfer experiment introduced by B. Adams and L. Lerner (J. Magn. Reson. 96, 604-607, 1992) is analyzed in terms of the Bloch equations. It is shown that the T1rho relaxation of the solvent is introduced in the decay of the exchangeable protons under conditions of relatively rapid exchange. An alternative experiment is suggested that randomizes the solvent magnetization with a pulsed field gradient before the observe pulse. This gives a single exponential intensity decay for the exchanging protons at all exchange rates. In addition, efficient water suppression and an even excitation profile are obtained.