Dynamic nuclear polarization of biological systems at high magnetic fields

Abstract

Dynamic nuclear polarization methods were studied at high magnetic field strength and were applied to improve the sensitivity of the nuclear magnetic resonance spectroscopy of biological solids. Studies of the dynamics of electron-nuclear polarization transfer via the solid effect and thermal mixing at 5 Tesla are described for two systems: the free radical BDPA doped into polystyrene and the nitroxide TEMPO in a water:glycerol matrix. A model for thermal mixing at high magnetic fields in paramagnetic systems such as TEMPO which exhibit partially inhomogeneous EPR lines is developed in which electron-electron cross relaxation across the EPR line is explicitly included. The TEMPO/water/glycerol matrix is exploited for polarization transfer to biological solutes. As a demonstration, enhancements of up to two orders of magnitude were exhibited in the high-resolution "1N magic-angle spinning spectra of the protein T4- lysozyme. The potential of this method as a general signal enhancement tool for biological systems is assessed. These dynamic nuclear polarization experiments at 5 Tesla require high-power microwave irradiation at or near the EPR frequency. To that end, a cyclotron resonance maser, or gyrotron, is described. This 140 GHz gyrotron, which under conventional operation produces millisecond pulses, has been adapted to operate at -100 W in a quasi-CW mode for tens of seconds, the time required for electron-nuclear polarization transfer.