Sensitivity-enhanced nuclear magnetic resonance of biological solids

Abstract

Dynamic Nuclear Polarization (DNP) enhances the sensitivity of solid-state Nuclear Magnetic Resonance experiments via transfer of polarization from electrons to nuclei. The polarization transfer is driven by microwave irradiation near the electron Larmor frequency. Signal enhancements of up to approx. 185 at 5 Tesla have previously been reported using a nitroxide radical in water/glycerol and a high-power gyrotron as a microwave source. This thesis describes the continuation of these experiments with a focus towards developing methodology for structural studies of biological solids. DNP experiments are typically performed at low temperatures since the DNP efficiency increases with the electronic and nuclear spin-lattice relaxation times. Many biological systems, especially membrane proteins, have relatively short nuclear relaxation times even at 10 K. DNP experiments on fd bacteriophage and purple membrane show that significant sensitivity enhancements can still be obtained on systems with short relaxation times. DNP-enhanced spectra of the coat protein and DNA of fd bacteriophage demonstrate that proton spin diffusion evenly distributes the enhanced polarization throughout a large macromolecular assembly.

(cont.) High-Field DNP can be combined with Magic Angle Spinning (MAS) for high-resolution spectroscopy. The gyrotron has been redesigned to enable MAS/DNP experiments at temperatures approaching liquid nitrogen. The DNP efficiency was investigated as a function of a number of different parameters and signal enhancements of up to approx. 20 at 90 K are reported. The initial two-dimensional chemical shift correlation spectra obtained with DNP at high field illustrate the stability of the gyrotron and low-temperature spinning apparatus.