Improving the sensitivity and utility of pulsed dipolar experiments in EPR at 94 GHz
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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.
Thesis, PhD Doctor of Philosophy
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