Longitudinal Relaxation Optimization Enhances 1 H‐Detected HSQC in Solid‐State NMR Spectroscopy on Challenging Biological Systems

Solid‐state (SS) NMR spectroscopy is a powerful technique for studying challenging biological systems, but it often suffers from low sensitivity. A longitudinal relaxation optimization scheme to enhance the signal sensitivity of HSQC experiments in SSNMR spectroscopy is reported. Under the proposed scheme, the 1 H spins of 1 H–X ( 15 N or 13 C) are selected for signal acquisition, whereas other vast 1 H spins are flipped back to the axis of the static magnetic field to accelerate the spin recovery of the observed 1 H spins, resulting in enhanced sensitivity. Three biological systems are used to evaluate this strategy, including a seven‐transmembrane protein, an RNA, and a whole‐cell sample. For all three samples, the proposed scheme largely shortens the effective 1 H longitudinal relaxation time and results in a 1.3–2.5‐fold gain in sensitivity. The selected systems are representative of challenging biological systems for observation by means of SSNMR spectroscopy; thus indicating the general applicability of this method, which is particularly important for biological samples with a short lifetime or with limited sample quantities.