HIGH FREQUENCY DYNAMIC NUCLEAR POLARIZATION OF BIOLOGICAL SYSTEMS

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We have been performing Dynamic Nuclear Polarization (DNP) experiments using a high-power cyclotron resonance maser (gyrotron) operating at 140 GHz. Previous results have shown NMR signal enhancements of -185 in the static CP spectrum of glycine and of -50-100 for the CP/M[AS spectra of arginine and T4-lysozyme, all in glycerol/water doped with 4-amino TEMPO, at 20-50K. In order to obtain detailed structural information from spinning samples, recoupling techniques must be incorporated with DNP. Successful incorporation is critically dependent on stable microwave power (i.e. constant enhancements) and stable spinning speeds over extended periods of time. We have implemented two ma or changes to improve the stability of the gyrotron output power. We have replaced the electron gun with one equipped with a newer cathode stem. The newer stem has stabilized both the short term (degradation over a day of long pulsing) and the long term (general degradation of emitted power over months) behavior of the gyrotron. In addition, the pumping efficiency near the gun has been increased, leading to a decrease in the maximum pressure (from _ 10-6 Torr to -10-' Toff) attained during long pulse (>30s). Since the output power is proportional to the base pressure, we can now perform longer pulses and/or higher duty cycles while maintaining a stable power output. We have also extended DNP to the study of quadrupolar nuclei. Due to the crucial role of oxygen in numerous biological functions and the low sensitivity of "0 NMR experiments, "0 is a particularly good candidate for DNP. Preliminary static DNP experiments on a glycerol/H2"O frozen solution doped with 4-amino TEMPO produced signal enhancements of 50-80 at 20K.