Water Exchange Filter (WEX Filter) for Nuclear Magnetic Resonance Studies of Macromolecules

A pulsed field gradient NMR method is described in which only exchangeable 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 encoding of water protons. The WEX filter has several advantageous features. First, if exchangeable proton peaks are well resolved, exchange rates can be calculated from the dependency of the peak intensities on the 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 experiment. Third, complete saturation is achieved within a millisecond, avoiding sensitivity losses. Finally, contrary to conventional methods like inversion transfer and 2D EXSY, the WEX filter does not suffer from radiation damping. In this paper we demonstrate the use of the WEX filter for a sample of 2 mM ubiquitin in 90/10 H₂O/D₂O and calculate the exchange rate for a side-chain proton in the unfolded consensus zinc finger peptide CP1.