Improved pulse sequences for pure exchange solid-state NMR spectroscopy

Spin-exchange experiments are useful for improving the resolution and establishment of sequential assignments in solid-state NMR spectra of uniformly N-15-labeled proteins oriented macroscopically in phospholipid bilayers. To exploit this advantage fully, it is crucial that the diagonal peaks in the two-dimensional exchange spectra are suppressed. This may be accomplished using the recent pure-exchange (PUREX) experiments, which, however, suffer from up to a threefold reduction of the cross-peak intensity relative to experiments without diagonal-peak suppression. This loss in sensitivity may severely hamper the applicability for the study of membrane proteins. In this paper, we present a two-dimensional exchange experiment (iPUREX) which improves the PUREX sensitivity by 50%. The performance of iPUREX is demonstrated experimentally by proton-mediated N-15-N-15 spin-exchange experiments for a N-15-Iabeled N-acetyl-L-valyl-L-leucine dipeptide. The relevance of exchange experiments with diagonal-peak suppression for large, uniformly N-15-labeled membrane proteins in oriented phospholipid bilayers is demonstrated numerically for the G-protein coupled receptor rhodopsin. Copyright (C) 2004 John Wiley Sons, Ltd.