Minimisation of sensitivity losses due to the use of gradient pulses in triple-resonance NMR of proteins

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Abstract
The use of pulsed field gradients in multiple-pulse NMR experiments has many advantages, including the possibility of obtaining excellent water suppression without the need for selective presaturation. In such gradient experiments the water magnetization is dephased deliberately; exchange between the saturated protons of the solvent water and the NH protons of a protein transfers this saturation to the protein. As the solvent is in large excess and relaxes relatively slowly, the result is a reduction in the sensitivity of the experiment due to the fact that the NH proton magnetization is only partially recovered. These effects can be avoided by ensuring that the water magnetization remains intact and is returned to the +z-axis at the start of data acquisition. General procedures for achieving this aim in any triple-resonance experiment are outlined and two specific examples are given. Experimental results confirm the sensitivity advantage of the modified sequences.