No abstract

8.9 GHz 10 K EPR studies of three low-symmetry centres in X-irradiated zircon are reported. Precise spin-Hamiltonian parameters are given for a Zr3+ centre, including 91Zr hyperfine and 89Y superhyperfine matrices, where the ideal tetragonal symmetry of the Zr site is lowered by charge compensator(s) to Laue class 1. Precise g-values are given for two previously unreported electron-hole centres shown to be located respectively at short- and long-bonded O-ligand sites, each of Laue class 2/m, monoclinic.

A d ' Ti3+ centre formed by x-irradiation of a synthetic zircon single crystal at 77 K has been studied by electton paramagneticpsonance (EPR) at 10 K. The Ti3t ion occupies n substitutional site of point group symmetry 42m (Dd. Absence of both site splitting in genenl crystal orientations and of superhyperhne structue from a +I ion indicated that a nearby charge compensator was not involved. Comparison with EPR measurements on Ti3t far quartz centres, point charge calculations and 29Si hyperfine spliuings indicated that the Ti3'' centre results from electron czpNre by Ti4+ occupying a &+ site in the crystal: the designation [TiOBI-is suggested. Precise spin-Hamiltonian parameters. including "Ti (I = z, 7.4%) and 49Ti (I = 1 2 ' 5.4%) hyperfine. nuclear electric quadrupole and nuclear Zeeman parameter matrices. together with high-spin nuclear term of dimension 14, 13S, IsS. Bf' and B f s were determined. The set of term B I , Bf' and B I s were found to be crucial in obtaining a good fit to the many 'forbidden' hyperfine lines observed. This resulted in determination of nuclear quadrupole and n u c l w Zeeman panmeters with precisions not hitherto attained in conventional single-crystal EPR measurements.

The Ge3+ centers [GeO−4(I)]− ≡Ge(I), [GeO4−(II)]− ≡Ge(II), and B, which are produced by x irradiation of α-quartz containing germanium, have been studied by EPR. The spin-Hamiltonian parameters ḡ, Ā73Ge, Ā29Si(1)⋅⋅ ⋅Ā29Si(4) for Ge(I), and ḡ, Ā73Ge for Ge(II), both sets measured at ≈15 K, and ḡ, Ā73Ge for B at ≈220 K are reported. The direction of the sp hybridized germanium orbital of the unpaired electron is found to coincide with the bisector of one of the O–Si–O angles [106.3° from the crystal c axis for Ge(I); crystal twofold axis for Ge(II)]. Centers Ge(I) and Ge(II) are found to be different, energetically inequivalent versions of the same basic center. The B center appears as a result of rapid electron jumping between Ge(I) and Ge(II) above ≈150 K. The observed line positions of B coincide with ones calculated from motional averaging among three states: two of Ge(I) and one of Ge(II). The relative Ge(I) and Ge(II) concentrations are obtained from signal intensity ratios at low temperatures (14:1 at ≈15 K, 4.4:1 at ≈80 K) and from the line positions of center B at sufficiently high temperatures (3.0:1 at ≈220 K, 2.7:1 at ≈300 K).

1. Menadione was found to react with both the haem groups and the beta-93 thiol groups of haemoglobin. 2. It oxidized the haem groups of oxyhaemoglobin, giving mainly methaemoglobin and a smaller amount of haemichrome. The reaction rate was decrease in the presence of catalase and markedly accelerated in the presence of superoxide dismutase. It is proposed that the overall reaction involves the initial reversible formation of methaemoglobin and the semiquinone, and that the effect of superoxide dismutase is to prevent the reverse reaction, by removing superoxide and hene O2-. E.s.r. evidence for the information of the semi-quinone and its reactions is presented. 3. The reaction of menadione with the beta-93 thiol groups of haemoglobin appeared to be similar to that with other thiols, forming the 3-thioether derivative of menadione, but it was also accompanied by reduction of methaemoglobin. This reduction was prevented by superoxide dismutase, but appeared to be caused by the semiquinone radical, which was produced as an intermediate. 4. Reduced glutathione functioned only to a limited extent as a scavenger of the menadione semiquinone. Its main reaction was directly with menadione to form the thioether. Ascorbate was a more efficient scavenger, and accelerated the oxidation of oxyhaemoglobin by menadione. 5. The significance of these findings in relation to menadione-induced erythrocyte haemolysis is discussed.