Electron Paramagnetic Resonance Imaging and Probes

EPR imaging at low frequency is an important method to measure in vivo physiology. Because native paramagnetic species exist at low concentrations in living systems, exogenous paramagnetic species are needed for in vivo EPR. Selection of the appropriate EPR probe allows the experimenter to obtain information about the environment of the imaged probe and its spatial distribution. Relaxation times for several nitroxide radicals were studied with the aim of understanding the relaxation mechanisms, which provides criteria for design of suitable in vivo imaging probes. Amino-substituted trityl radicals have the potential to monitor pH in vivo, and the suitability for this application depends on spectral properties. Electron spin relaxation times T1 and T2 were measured at X-band for the protonated and deprotonated forms of two amino-substituted triaryl methyl radicals. T1 exhibits little dependence on protonation, which makes it useful for measuring O2 concentration. Values of T2 vary substantially with pH, reflecting a range of dynamic processes, and thus T2 is a potentially useful monitor of pH. The spin-lattice relaxation rates at 293 K for three anionic semiquinones (2,5-di-t-butyl-1,4-benzosemiquinone, 2,6-di-t-butyl-1,4-benzosemiquinone, and 2,3,5,6-tetramethoxy-1,4-benzosemiquinone) were studied at up to 8 frequencies between 250 MHz and 34 GHz in ethanol or methanol solution containing high concentrations of OH-. The relaxation rates are about a factor of 2 faster at lower frequencies than at 9 or 34 GHz when measured in solvent with normal isotope abundance. However, in perdeuterated alcohols the relaxation rates exhibit little frequency dependence. The relaxation rates were modeled as the sum of two frequency-independent contributions (spin rotation and a local mode) and two frequency-dependent contributions (modulation of dipolar interaction with solvent nuclei and a much smaller contribution from modulation of g anisotropy). The correlation time for modulation of the interaction with solvent nuclei is longer than the tumbling correlation time and is attributed to hydrogen bonding of the alcohol to the oxygen atoms of the semiquinones. Rapid scan imaging was shown, for the same data acquisition time, to give higher signal-to-noise than continuous wave for the nitroxide probe 15N-perdeuterated tempone. The narrow spectral widths for the amino-substituted triaryl methyl radicals facilitate spectral-spatial EPR imaging. A phantom was imaged using rapid scan to test the feasibility of mapping the pH and to test a new algorithm for full spectrum imaging. Finally, a spin coherence phenomenon was observed at X-band in semiquinone rapid scan spectra, which arises from closely-spaced nuclear hyperfine lines