Progress in proton-detected solid-state NMR (SSNMR): Super-fast 2D SSNMR collection for nano-mole-scale proteins

Proton-detected solid-state NMR (SSNMR) spectroscopy has attracted much attention due to its excellent sensitivity and effectiveness in the analysis of trace amounts of amyloid proteins and other important biological systems. In this perspective article, we present the recent sensitivity limit of ¹H-detected SSNMR using "ultra-fast" magic-angle spinning (MAS) at a spinning rate (ν_R) of 80-100 kHz. It was demonstrated that the high sensitivity of ¹H-detected SSNMR at ν_R of 100 kHz and fast recycling using the paramagnetic-assisted condensed data collection (PACC) approach permitted "super-fast" collection of ¹H-detected 2D protein SSNMR. A ¹H-detected 2D ¹H-¹⁵N correlation SSNMR spectrum for ∼27 nmol of a uniformly ¹³C- and ¹⁵N-labeled GB1 protein sample in microcrystalline form was acquired in only 9 s with 50% non-uniform sampling and short recycle delays of 100 ms. Additional data suggests that it is now feasible to detect as little as 1 nmol of the protein in 5.9 h by ¹H-detected 2D ¹H-¹⁵N SSNMR at a nominal signal-to-noise ratio of five. The demonstrated sensitivity is comparable to that of modern solution protein NMR. Moreover, this article summarizes the influence of ultra-fast MAS and ¹H-detection on the spectral resolution and sensitivity of protein SSNMR. Recent progress in signal assignment and structural elucidation by ¹H-detected protein SSNMR is outlined with both theoretical and experimental aspects.