Sensitivity enhancement in 13C solid-state NMR of protein microcrystals by use of paramagnetic metal ions for optimizing 1H T1 relaxation

We discuss a simple approach to enhance sensitivity for 13C high-resolution solid-state NMR for proteins in microcrystals by reducing 1H T1 relaxation times with paramagnetic relaxation reagents. It was shown that 1H T1 values can be reduced from 0.4-0.8 s to 60-70 ms for ubiquitin and lysozyme in D2O in the presence of 10 mM Cu(II)Na2EDTA without substantial degradation of the resolution in 13C CPMAS spectra. Faster signal accumulation using the shorter 1H T1 attained by paramagnetic doping provided sensitivity enhancements of 1.4-2.9 for these proteins, reducing the experimental time for a given signal-to-noise ratio by a factor of 2.0-8.4. This approach presented here is likely to be applicable to various other proteins in order to enhance sensitivity in 13C high-resolution solid-state NMR spectroscopy.