Improving the sensitivity and utility of pulsed dipolar experiments in EPR at 94 GHz

Abstract

Pulsed dipolar spectroscopy (PDS) is an electron paramagnetic resonance (EPR) technique, used to conduct long range distance measurements in proteins in the nanometre range. This thesis presents a number of methodological and instrumental techniques to improve the sensitivity and utility of PDS experiments using a home-built high power pulsed spectrometer, HiPER, operating at 94 GHz. These include the implementation of phase-modulated composite pulses, which correct for imperfections arising due to inhomogeneity, and offer increased excitation bandwidth as well as experimental protocols such as annealing and glassing of samples. A theoretical study into the use of matched filtering to reduce echo noise during measurements, has predicted gains of up to a factor of 3 enhancement in signal-to-noise. Using such techniques we demonstrate sensitivity enhancements of more than 30 on PDS experiments, between nitroxides and Fe centres, in haem-proteins, corresponding to a reduction in averaging time of almost 1,000, in comparison to standard commercial spectrometers operating at X-band. The use of composite pulses in PDS experiments on nitroxide biradicals were also investigated, including their limitations due to intramolecular effects. The thesis then describes a single frequency dipolar modulation experiment, RIDME, and uses high field measurements to determine both the distance and relative orientation of a cobalt-nitroxide system, for the first time. Finally, a design study is conducted to implement frequency and amplitude modulated pulses on a spectrometer at 9/34 GHz to improve sensitivity.