15N-{1H} NOE experiment at high magnetic field strengths

The heteronuclear N-15-{H-1} NOE values are typically determined by taking the ratio of N-15 signal intensities recorded in the presence and absence of H-1 saturation prior to evolution of N-15 magnetization. Since the intensity ratio of two independent experiments is taken, complete recovery of N-15 magnetization during the scan repetition delay is critical to obtain reliable NOE values. Because it may not be practical to wait for the complete recovery of magnetization at high magnetic fields, Solomon equations may be used to correct measured NOE values. Here, based on experiments and simulations, we show that since the cross-correlation between H-1-N-15 dipole and N-15 chemical shift anisotropy becomes significant at high fields for small or deuterated proteins, measured NOE values can not be accurately corrected based on the Solomon equations. We also discuss ranges of rotational correlation times and proton spin-flip rate, in which the NOE values can be corrected by the equations.