Effective combination of gradients and crafted RF pulses for water suppression in biological samples

Abstract

The selection of multiple-quantum coherence wiith field gradients has been shown to be an effective method for suppressing the water resonance in aqueous solutions and for reducing t, noise and other artifacts associated with the phase-cycle selection of coherence (l-3). Recently, some of these advantages have been demonstrated for a phase-sensitive, double-quantum-filtered COSY (DQF COSY) experiment (3). However, for large, dilute biomolecules in water, .the gradient pulse durations and amplitudes must be large to eliminate the water signal. In situations where the gradient amplitude is limited, the gradient duration must be increased, which invariably leads to loss of signal due to spin-spin relaxation. The requirement on the gradient duration may be reduced by combining coherence selection with some other technique which achieves partial water suppression. Traditional presaturation methods require irradiation periods on the order of seconds, which may result in the loss or attenuation of a desired signal due to magnetization transfer. A succe:ssful strategy for water elimination in volume-localized spectroscopy has been the use of selective water excitation followed by gradient dephasing (4, 5). The selectivity of water elimination using this approach can be improved by the use of crafted RF pulse design. In this Communication, we have used this selective water-elimination strategy to attenuate the water signal prior to the coherence-selection sequence, thereby allowing the use of shorter gradient pulses. A phase-sensitive, gradient-enhanced, DQF COSY experiment is demonstrated here using a medium-sized protein in water. The pulse sequence and the corresponding coherence-level diagram are shown in Fig. 1. The water resonance is selectively excited, followed by a gradient pulse along the x axis which dephases the transverse magnetization. This is repeated a second time to remove any remaining magnetization with the exception that the dephasing gradient is applied along the y axis to avoid gradient-recalled echoes. The flip angle of the second selective pulse is adjusted so that the water magnetization is partially inverted to allow for T, recovery during the dephase gradients. This delay also allows the fasterrelaxing protein signals at the water chemical shift to recover. The water-elimination sequence is followed by a phase-sensitive, DQF COSY sequence with short, doublequantum, coherence-selection gradients G, and GZ along the x, y, and z axes. Multiplequantum coherence orders labeled by the gradient G, are then converted into observable antiphase magnetization by the last RF pulse. The relative ratios of the gradient pulses