Improved measurement of (15)N-[(1)H] NOEs in the presence of H(N)-water proton chemical exchange.

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