Measuring fast hydrogen exchange rates by NMR spectroscopy

We introduce a method to measure hydrogen exchange rates based on the observation of the coherence of a neighboring spin S such as N-15 that has a scalar coupling J(IS) to the exchanging proton I. The decay of S-x coherence under a Carr-Purcell-Meiboom-Gill (CPMG) multiple echo train is recorded in the presence and absence of proton decoupling. This method allows one to extract proton exchange rates up to 10(5) s(-1). We could extend the pH range for the study of the indole proton in tryptophan, allowing the determination of the exchange constants of the cationic, zwitterionic, and anionic forms of tryptophan. (c) 2006 Elsevier Inc. All rights reserved.