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 theT_1ρrelaxation 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.