Effects of radiation damping for biomolecular NMR experiments in solution: A hemisphere concept for water suppression

Abstract

Abundant solvent nuclear spins, such as water protons in aqueous solution, cause radiation damping in NMR experiments. It is important to know how the effect of radiation damping appears in high-resolution protein NMR because macromolecular studies always require very high magnetic field strengths with a highly sensitive NMR probe that can easily cause radiation damping. Here, we show the behavior of water magnetization after a pulsed-field gradient (PFG) using nutation experiments at 900 MHz with a cryogenic probe: when water magnetization is located in the upper hemisphere (having +Z component, parallel to the external magnetic field), dephasing of the magnetization by a PFG effectively suppresses residual water magnetization in the transverse plane. In contrast, when magnetization is located in the lower hemisphere (having −Z component), the small residual transverse component remaining after a PFG is still sufficient to induce radiation damping. Based on this observation, we designed 1H-15N HSQC experiments in which water magnetization is maintained in the upper hemisphere, but not necessarily along Z, and compared them with the conventional experiments, in which water magnetization is inverted during the t1 period. The result demonstrates moderate gain of signal-to-noise ratio, 0–28%. Designing the experiments such that water magnetization is maintained in the upper hemisphere allows shorter pulses to be used compared to the complete water flip-back and, thereby, is useful as a building block of protein NMR pulse programs in solution.

Design and applications of water irradiation devoid RF pulses for ultra-high field biomolecular NMR spectroscopy

“…4 More elaborate pulse schemes have been implemented in the past decades, thanks to the parallel development of magnetic field gradients and shaped RF pulses. [4][5][6][7][8][9][10][11] These advanced techniques mitigate signal losses by aligning the water polarization along the z-axis and performing water suppression before signal acquisition. Among the most effective methods is the water-suppression by gradient-tailored excitation (or WATERGATE 12 ).…”

Coherence Transfer by Radiation Damping Combined with J‐Couplings in 2D Correlation Spectroscopy at High‐Field Solution NMR

“…Radiation damping is also an ongoing problem in biological NMR applications, where the solvent peak is usually much larger than others. It generates many artifacts in 2D experiments and, hence, its suppression has been intensively investigated . However, it may be constructively used to suppress the water signal in some cases .…”