Improved measurement of N-15-{H-1} NOEs in the presence of H(N)-water proton chemical exchange

A simple method is presented to accurately determine N-15-{H-1} NOEs in bio molecules in the presence of H(N)-water proton chemical exchange. Three mea surements are required: one with nonselective proton saturation and two wit h different water saturation conditions to determine the equilibrium value of the N-15 signal. This approach is exemplified with data on two peptides, one helix-forming 17-mer and one compactly folded 56-mer. Results indicate that N-15-{H-1} NOEs determined using the standard approach with short rec ycle times (3 to 4 s) can be significantly in error when H(N)-water proton chemical exchange is relatively rapid, water proton relaxation is relativel y slow, and N-15-{H-1} NOEs are away from the value of -1. This new method avoids such inaccuracies resulting from the use of short recycle times. (C) 2001 Academic Press.