MINIMIZING THE WATER RESONANCE IN BIOLOGICAL NMR - CHARACTERIZATION AND SUPPRESSION OF INTERMOLECULAR DIPOLAR INTERACTIONS BY MULTIPLE-AXISGRADIENTS

Anomalous crosspeaks and additional resonances in the indirectly detec ted dimension have been previously observed in a number of 2D NMR expe riments applied to samples having at least one concentrated species. T hese unexpected peaks exhibit all the characteristics of intermolecula r multiple-quantum coherences. Since these coherences are possible wit hin the concentrated species alone, their creation and subsequent dete ction 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/observat ion of intermolecular water-water MQ coherences using variable-angle p ulsed field gradients. We show that the existing theoretical picture i s consistent with all of our experimental observations, thereby valida ting the predictive power of the intuition gained from this picture. W e 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 o f these coherences on water suppression within the repertoire of gradi ent-enhanced heteronuclear experiments that are currently being applie d to C-13/N-15 isotopically labeled proteins in 90% water.