Optimization of hardware and software for solid state nuclear magnetic resonance at high magnetic fields.

摘要

This research presents hardware and software solutions to many of the problems facing biological solid state nuclear magnetic resonance (ssNMR) spectroscopy at high fields. The low-E 750 MHz magic angle spinning (MAS) probe was designed, constructed, and thoroughly characterized. Under normal operating conditions, a proton (hydrogen, isotope weight 1) RF field nutation rate of 93 kHz and homogeneity (810 degrees/90 degrees) of 93% can be obtained with a sample length of 8.4 mm corresponding to a volume of 80 uL. With a higher power amplifier, we should be able to exceed 110 kHz decoupling fields based on bench measurements. Carbon (isotope weight 13) RF field nutation rates greater than 70 kHz with a homogeneity (810 degrees/90 degrees) of 70% are routinely observed for this sample length; the carbon RF homogeneity can be increased to 89% with a 6.7 mm sample length. Under full proton decoupling for long periods of time, sample heating due to the high RF field is minimal even for samples containing physiological levels of salt. We have not noticed any sample degradation in heat sensitive samples after extensive experimentation. The power handling characteristics, RF fields, and homogeneities make this an ideal probe for applying the full range of MAS solid state NMR experiments, including sequences which use extended periods of continuous RF pulsing on both channels, to biological samples which are inherently dilute.;A system for optimizing pulse sequences for ssNMR was also developed, demonstrated, and is running. This system was demonstrated on the two standard pulse sequences used to test pulse optimization systems: the inversion experiment and the refocusing experiment. In both cases, pulse sequences were derived which had a wider bandwidth than existing pulse sequences and had extremely good agreement between experiment and simulation. These pulse sequences should be useful in maintaining high signal strength and phase coherence in future research. The methods of optimization and verification allow them to be easily extended to more complex situations in future research.;The combination of the new probe and the method for optimizing pulse sequences for use at higher fields opens many opportunities for new research on biological solids.