NMR experiments with biological macromolecules are generally performed in aqueous solutions. A variety of techniques have been developed to prevent the extremely intense water resonance from contributing to the measured signal (Wider
1998). In NMR experiments with large macromolecules additional attention has to be given to the water suppression scheme. For example, an NMR experiment recorded with water presaturation can have up to 10 times less sensitivity than an experiment that conserves the water polarization by using a selective water flip-back pulse. Strong interactions between the water magnetization and the protein magnetization that affect the effective longitudinal proton relaxation rates are responsible for the dramatic influence of the water handling on the intensities of the protein signals (Riek et al. 2002). tterefore, it is strongly recommended to use water flip-back techniques (Grzesiek and Bax 1993) for water suppression in all experiments with large biological macromolecules, to keep the water magnetization along the +z-axis during the entire duration of the measurement.
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