Minimizing the water resonance in biological NMR: Characterization and suppression of intermolecular dipolar interactions by multiple-axis gradients

Anomalous crosspeaks and additional resonances in the indirectly detected dimension have been previously observed in a number of 2D NMR experiments applied to samples having at least one concentrated species. These unexpected peaks exhibit all the characteristics of intermolecular multiple-quantum coherences. Since these coherences are possible within the concentrated species alone, their creation and subsequent detection may be one of the causes for poor water suppression in a variety of biomolecular NMR experiments, e.g.,the conventional MQ-filtered- (MQF) and MQ-COSY experiments applied to proteins/peptides in 90% water. In this report, we experimentally characterize the creation/observation of intermolecular water-water MQ coherences using variable-angle pulsed field gradients. We show that the existing theoretical picture is consistent with all of our experimental observations, thereby validating the predictive power of the intuition gained from this picture. We also provide an increased understanding of the effect that variable-angle gradients can have on the intensity of observable magnetization arising from these intermolecular MQ coherences. Finally, we establish a basis on which one may reasonably speculate concerning the impact of these coherences on water suppression within the repertoire of gradient-enhanced heteronuclear experiments that are currently being applied to C-13/N-15 isotopically labeled proteins in 90% water.