WATER EXCHANGE FILTER (WEX FILTER) FOR NUCLEAR-MAGNETIC-RESONANCE STUDIES OF MACROMOLECULES

A pulsed field gradient NMR method is described in which only exchange able proton signals are selectively observed. In this so-called water exchange filter (WEX filter), complete saturation of water is attained rapidly (within 0.5-1.0 ms) by frequency selective gradient phase enc oding of water protons. The WEX filter has several advantageous featur es. First, if exchangeable proton peaks are well resolved, exchange ra tes can be calculated from the dependency of the peak intensities on t he exchange mixing time in a 1D approach instead of in a conventional 2D experiment. Second, to resolve overlapping exchangeable peaks, the WEX filter can be combined with any phase-sensitive 1D and 2D experime nt. Third, complete saturation is achieved within a millisecond, avoid ing sensitivity losses. Finally, contrary to conventional methods like inversion transfer and 2D EXSY, the WEX filter does not suffer from r adiation damping. In this paper we demonstrate the use of the WEX filt er for a sample of 2 mM ubiquitin in 90/10 H2O/D2O and calculate the e xchange rate for a side-chain proton in the unfolded consensus zinc fi nger peptide CP1.