Chapter 8: Use of high power spin-lock purge pulses in high resolution NMR spectroscopy

This chapter discusses three different experiments, where short, high-power spin-lock pulses are used to purge the spectrum from undesired resonances. The experiments are (1) the 13C heteronuclear single quantum coherence (HSQC) experiment, (2) experiments with 13C half-filter elements, and (3) nuclear Overhauser enhancement spectroscopy (NOESY) and rotating-frame Overhauser effect spectroscopy (ROESY) experiments for the observation of water-protein nuclear Overhauser effects (NOEs). In the first two experiments, spin-lock purge pulses are used to suppress the signals from protons not directly bound to 13C. In the last experiment, spin-lock pulses are used to suppress the solvent signal immediately before the detection period. The chapter concludes that although radio-frequency coils are designed for optimum radio frequency field homogeneity, the spatial distribution of the radio-frequency field over the sample is still sufficiently inhomogeneous that their defocusing effect is virtually complete after only 0.5 to 3 ms. Spin-lock purge pulses are thus versatile tools for the suppression of undesired magnetization. They are often used with several experiments and applications—for example, as trim pulses preceding and following TOCSY mixing sequences, for water suppression in three-dimensional (3-D) NOESY–HSQC experiments, for water suppression by total scrambling of magnetization, and for the suppression of radiation damping.