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 15N that has a scalar coupling JIS to the exchanging proton I. The decay of Sx 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 105s−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.