Overcoming solvent saturation-transfer artifacts in protein NMR at neutral pH. Application of pulsed field gradients in measurements of 1H-15N Overhauser effects.

Artifacts due to solvent saturation-transfer effects result in incorrect measurements of 1H-15N heteronuclear NOE (HNOE). These artifacts are commonly observed in aqueous protein solutions at neutral pH. We describe the application of pulsed field gradients (PFGs) together with long recycle delays in overcoming errors in HNOE measurements which arise from H2O solvent preirradiation and solvent saturation transfer. Even in the absence of explicit solvent irradiation, the HNOE pulse sequence itself results in a nonequilibrium spin-state distribution of solvent nuclei which can then be transferred by chemical exchange into amide-proton sites. This effect can be avoided by using PFGs for suppression of solvent H2O together with a recycle delay sufficiently long for the magnetization of water to relax back to equilibrium values during the preparation period. These effects were studied in 15N-enriched human type alpha transforming growth factor at pH 7.1. Comparisons of PFG-HNOE experiments with and without selective H2O irradiation and with different recycle times provide estimates of the effects of solvent irradiation on HNOE measurements, which are different for different amide nitrogen-15 nuclei. The amplitudes of these artificial HNOE enhancements are roughly correlated with solvent accessibilities of amide sites in the three-dimensional structure of hTGF alpha.