D. S. Tipikin

Magnetic properties of metal–quinone high-spin complexes prepared by solid-state mechano-activation and by chemical synthesis in solution. Comparative study with very high-field and low-temperature EPR and ENDOR

EPR spectroscopy at high frequencies and low temperatures and ENDOR spectroscopy at X- and W-bands have been applied to investigate two- and three-spin complexes of the type MQ2˙ and MQ3˙ generated by mechanochemical treatment (by grinding or by the action of elastic waves) of solid mixtures of quinones and metals (M = Al, Ga, In, Cd, Sn, Zn). In addition, complexes obtained in liquid-phase reactions of metal amalgams with quinones were investigated. The magnetic-resonance parameters and the observed thermal spin polarization provide evidence for the identity of the magnetic complexes produced in solutions and by solid-state mechanochemical treatment. In particular, the presence of ground-state triplet and quartet species has been demonstrated in both cases. In addition, the exchange and the dipole–dipole interaction parameters were evaluated, their comparison providing information about the electronic intramolecular interaction.

ESE-detected EPR of triplet radical pairs produced by mechano-chemical treatment of 2,6-di-t-butylquinone diazide mixed with 3,5-di-t-butylpyrocathechon

Abstract Electron spin echo experiments at high microwave frequency (95 GHz) have been performed to study mechanically induced radical pairs. It is shown that the sign of the spin—spin interaction of the radical pairs produced by mechanical treatment of a crystalline-powdered mixture of 2,6-di-t-butylquinone diazide and 3,3-di-t-butylpyrocathechon is positive. The asymmetry of the relaxation rates between the triplet sublevels is explained by the presence of a nearby singlet state.

POSSIBLE NATURE OF THE RADIATION-INDUCED SIGNAL IN NAILS: HIGH-FIELD EPR, CONFIRMING CHEMICAL SYNTHESIS, AND QUANTUM CHEMICAL CALCULATIONS

Exposure of finger- and toe-nails to ionizing radiation generates an Electron Paramagnetic Resonance (EPR) signal whose intensity is dose dependent and stable at room temperature for several days. The dependency of the radiation-induced signal (RIS) on the received dose may be used as the basis for retrospective dosimetry of an individuals fortuitous exposure to ionizing radiation. Two radiation-induced signals, a quasi-stable (RIS2) and stable signal (RIS5), have been identified in nails irradiated up to a dose of 50 Gy. Using X-band EPR, both RIS signals exhibit a singlet line shape with a line width around 1.0 mT and an apparent g-value of 2.0044. In this work, we seek information on the exact chemical nature of the radiation-induced free radicals underlying the signal. This knowledge may provide insights into the reason for the discrepancy in the stabilities of the two RIS signals and help develop strategies for stabilizing the radicals in nails or devising methods for restoring the radicals after decay. In this work an analysis of high field (94 GHz and 240 GHz) EPR spectra of the RIS using quantum chemical calculations, the oxidation-reduction properties and the pH dependence of the signal intensities are used to show that spectroscopic and chemical properties of the RIS are consistent with a semiquinone-type radical underlying the RIS. It has been suggested that semiquinone radicals formed on trace amounts of melanin in nails are the basis for the RIS signals. However, based on the quantum chemical calculations and chemical properties of the RIS, it is likely that the radicals underlying this signal are generated from the radiolysis of L-3,4-dihydroxyphenylalanine (DOPA) amino acids in the keratin proteins. These DOPA amino acids are likely formed from the exogenous oxidation of tyrosine in keratin by the oxygen from the air prior to irradiation. We show that these DOPA amino acids can work as radical traps, capturing the highly reactive and unstable sulfur-based radicals and/or alkyl radicals generated during the radiation event and are converted to the more stable o-semiquinone anion-radicals. From this understanding of the oxidation-reduction properties of the RIS, it may be possible to regenerate the unstable RIS2 following its decay through treatment of nail clippings. However, the treatment used to recover the RIS2 also has the ability to recover an interfering, mechanically-induced signal (MIS) formed when the nail is clipped. Therefore, to use the recovered (regenerated) RIS2 to increase the detection limits and precision of the RIS measurements and, therefore, the dose estimates calculated from the RIS signal amplitudes, will require the application of methods to differentiate the RIS2 from the recovered MIS signal.

MECHANOCHEMICAL GENERATION OF STABLE RADICAL SPECIES. OXIDATION OF PYROCATECHOLS

Abstract We have investigated the temperature and time dependences of the D -value and the intensity of mechanochemically generated radical pairs in order to establish the nature of these radical products. The method of synthesis of those pairs, based on the oxidation of pyrocatechols, is outlined. We have confirmed the biradical nature of the products (no signal in one-third field was detected). We have outlined ideas about the nature of the products, which probably are radical pairs of association.

Dielectric-Backed Aperture Resonators for X-Band in vivo EPR Nail Dosimetry

A new resonator for X-band in vivo EPR nail dosimetry, the dielectric-backed aperture resonator (DAR), is developed based on rectangular TE102 geometry. This novel geometry for surface spectroscopy improves at least a factor of 20 compared to a traditional non-backed aperture resonator. Such an increase in EPR sensitivity is achieved by using a non-resonant dielectric slab, placed on the aperture inside the cavity. The dielectric slab provides an increased magnetic field at the aperture and sample, while minimizing sensitive aperture resonance conditions. This work also introduces a DAR semi-spherical (SS)-TE011 geometry. The SS-TE011 geometry is attractive due to having twice the incident magnetic field at the aperture for a fixed input power. It has been shown that DAR provides sufficient sensitivity to make biologically relevant measurements both in vitro and in vivo Although in vivo tests have shown some effects of physiological motions that suggest the necessity of a more robust finger holder, equivalent dosimetry sensitivity of approximately 1.4 Gy has been demonstrated.