Proton-detected solid-state NMR Spectroscopy of fully protonated proteins at 40 kHz magic-angle spinning

Remarkable progress in solid-state NMR has enabled complete structure determination of uniformly labeled proteins in the size range of 5-10 kDa. Expanding these applications to larger or mass-limited systems requires further improvements in spectral sensitivity, for which inverse detection of C-13 and N-15 signals with H-1 is one promising approach. Proton detection has previously been demonstrated to offer sensitivity benefits in the limit of sparse protonation or with similar to 30 kHz magic-angle spinning (MAS). Here we focus on experimental schemes for proteins with similar to 100% protonation. Full protonation simplifies sample preparation and permits more complete chemical shift information to be obtained from a single sample. We demonstrate experimental schemes using the fully protonated, uniformly C-13 N-15-labeled protein GB1 at 40 kHz MAS rate with 1.6-mm rotors. At 500 MHz proton frequency, 1-ppm proton line widths were observed (500 +/- 150 Hz), and the sensitivity was enhanced by 3 and 4 times, respectively, versus direct 13C and 15N detection. The enhanced sensitivity enabled a family of 3D experiments for spectral assignment to be performed in a time-efficient manner with less than a micromole of protein. CANH, CONH, and NCAH 3D spectra provided sufficient resolution and sensitivity to make full backbone and partial side-chain proton assignments. At 750 MHz proton frequency and 40 kHz MAS rate, proton line widths improve further in an absolute sense (360 +/- 115 Hz). Sensitivity and resolution increase in a better than linear manner with increasing magnetic field, resulting in 14 times greater sensitivity for H-1 detection relative to that of N-15 detection.