Quantitation of chemical exchange rates using pulsed-field-gradient diffusion measurements

A new approach to the quantitation of chemical exchange rates is presented, and its utility isillustrated with application to the exchange of protein amide protons with bulk water. Theapproach consists of a selective-inversion exchange HMQC experiment in which a short spinecho diffusion filter has been inserted into the exchange period. In this way, the kinetics ofexchange are encoded directly in an apparent diffusion coefficient which is a function of theposition of the diffusion filter in the pulse sequence. A detailed theoretical analysis of thisexperiment indicates that, in addition to the measurement of simple exchange rates, theexperiment is capable of measuring the effect of mediated exchange, e.g. the transfer ofmagnetization from bulk water to an amide site mediated by an internal bound water moleculeor a labile protein side-chain proton in fast exchange with bulk water. Experimental resultsfor rapid water/amide exchange in acyl carrier protein are shown to be quantitativelyconsistent with the exchange rates measured using a selective-inversion exchange experiment.