High-frequency time domain EPR

Abstract

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2004.Vita.Includes bibliographical references.There are numerous advantages to high frequency (high field) electron paramagnetic resonance (EPR) spectroscopy. Two of the most important are improved sensitivity and the improved resolution of field dependent interactions. In addition, there are many attractive features to time domain spectroscopy. Pulsed EPR allows for the design of experiments, which can specifically be used to study structure and dynamics of paramagnetic species and provide utmost resolution by separating interactions from each other. The combination of pulsed techniques and high frequencies is not only complimentary to continuous wave (CW) low frequency EPR but it also greatly increases the accessible information on paramagnetic species. High frequency, time domain EPR is still in its infancy. Spectrometers at W-band ([approximately] 95 GHz) are now available commercially but to date very few spectrometers operating at higher frequencies have been described. The spectrometer developed in the Francis Bitter Magnet Laboratory operates at a microwave (MW) frequency of 139.5 GHz corresponding to [approximately] 5 T magnetic field. The applications presented in this thesis illustrate the potential of high frequency, time domain EPR spectroscopy at 139.5 GHz in obtaining structural and mechanistic insights of several paramagnetic systems. Well resolved EPR spectra observed at 139.5 GHz of the stable tyrosine radical in ribonucleotide reductase (RNR) revealed the existence of a hydrogen bond in RNR from yeast, chapter 1. The bond length and orientation were determined from the nuclear frequencies of the proton, detected by orientation selective electron nuclear double resonance (ENDOR).(cont.) The advantage of the time domain detection scheme is demonstrated in chapters 4, 5 and 6. A stimulated echo sequence is used to separate different organic radicals associated with the reduction chemistry and inhibition mechanisms of RNR. Using the dispersion in relaxation rates at high temperature ([approximately] 60 K) it is possible to filter the multi component spectrum. The assignment of new radicals is possible at high field, 5 T, due to the high resolution in g anisotropy. The findings support earlier proposals for the mechanism of nucleotide reduction and inhibition of this very important enzyme. To study photoexcited triplet molecules a light source was coupled to the high frequency spectrometer and the pulsed mode detection scheme was used to acquire EPR spectra. The new technique is demonstrated on several model systems. In addition to the basic advantages described above, high frequency EPR opens new frontiers for high spin systems, S >[or equal to] 1, with large spin-spin interaction. Because of the inverse field dependency of the zero field splitting, such systems may be totally EPR-silent at normal EPR frequencies. However their EPR spectra are accessible at high frequencies due to the reduction of linewidth. The Mn(II), S = 5/2, in superoxide dismutase (SOD) is a good example for such system.by Galit Bar.Ph.D