An experimentally determined limit function for long-range ferromagnetic and antiferromagnetic superexchange is proposed, the form of which comprises a quantitative statement of present knowledge of the "range" of superexchange effects in insulating solids, A possible functional form for this limit function, as well as for the general distance dependence of long-range superexchange, is also proposed. The fitting of this general function to experimental data for superexchange coupling vs. distance requires the fitting of three independent parameters. The possibility is considered that one of the parameters so obtained may be sensitive to the degree of extended character of the orbitals which mediate the superexchange effect. The superexchange limit function is shown to be important for characterizing the assignments of alternative paths of exchange interaction. It also serves to point out the need for improved theoretical methods for the analysis of the relative strength of competing paths of interaction in exchange coupled cluster complexes, and shows that there is a need for studies of superexchange interactions acting over distances greater than 6 Á.

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder associated with premutation alleles of the FMR1 gene that is characterized by progressive action tremor, gait ataxia, and cognitive decline. Recent studies of mitochondrial dysfunction in FXTAS have suggested that iron dysregulation may be one component of disease pathogenesis. We tested the hypothesis that iron dysregulation is part of the pathogenic process in FXTAS. We analyzed postmortem choroid plexus from FXTAS and control subjects, and found that in FXTAS iron accumulated in the stroma, transferrin levels were decreased in the epithelial cells, and transferrin receptor 1 distribution was shifted from the basolateral membrane (control) to a predominantly intracellular location (FXTAS). In addition, ferroportin and ceruloplasmin were markedly decreased within the epithelial cells. These alterations have implications not only for understanding the pathophysiology of FXTAS, but also for the development of new clinical treatments that may incorporate selective iron chelation.

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When ribonucleotide reductase from Lactobacillus leichmannii is incubated with its coenzyme (5 '-deoxyadenosylcobalamin), a thiol such as glutathione and certain nucleoside triphosphates, formation of paramagnetic products can be Dr. Lester J. Reed. The selenolipoate was reduced in the same manner as lipoate (Gunsalus and Razzell, 1957). The arabino analog of ATP (AraATP) and 2'-0-MeATP were synthesized 4696 BIOCHEMISTRY, VOL.

An analysis is given of the general spin-spin dipolar interaction in the presence of a general bilinear superexchange interaction, and its effects on a rigid lattice EPR line shape are calculated. The general magic angle effect is considered, numerically, for specific cases of both single crystal spectra and randomly oriented microcrystallite samples. Analysis of the number of determinable spin-spin coupling coefficients suggests that a component analysis can be developed which can yield useful information about the geometrical arrangement of two interacting spin = 7/2 molecules, as well as covalency information about the interaction. This analysis is applied to a Mo(V)-iron sulfur interaction in xanthine oxidase and xanthine dehydrogenase, which has been described by Lowe and Bray. Line shape calculations show that the observed small amount of dipolar anisotropy is consistent with a separation of the Mo(V) and Fe/S groups of 14 A or less. Considerations of the long-range nature of superexchange, from existing experimental data on superexchange, are consistent with this estimate.

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The electronic absorption spectra of single crystals of toluene, dibenzyl, diphenylmethane, and biphenyl have been photographed at 20 °K. using polarized light in the range 3000 to 2500 Å. The fluorescence spectrum of biphenyl has also been examined. This is the first time that the excited states of biphenyl analogous to the B2u state of benzene have been observed. The main features of the spectra are similar for all four compounds, and the differences may be understood in terms of the weak coupling between the benzene rings of the "double molecules".

Articles you may be interested inEPR and magnetic susceptibility investigation of iron-zinc-phosphate glass ceramics AIP Conf. Proc. 1565, 250 (2013); 10.1063/1.4833738 Low temperature magnetization study on high spin iron(III) porphyrins J. Chem. Phys. 78, 915 (1983); 10.1063/1.444794 Mössbauer effect study of the magnetic properties of the intermediatespin complex perchlorato (m e s o tetraphenylporphinato ) iron (III) Magnetic susceptibility and high field Mössbauer effect studies of some spin triplet iron (IV) compounds Mossbauer, magnetic susceptibility, and EPR measurements have been performed in the series of intermediate spin [Fe(S2C202hX]2-, (X = CI, Br, I) complex anions. All members of the series are paramagnetic down to 1.4 oK. Miissbauer and EPR results establish the axial character of the electronic spin Hamiltonian for the iodide derivative and increasing rhombicity for Br and Cl. Axial zero field splitting parameters D are positive and near 5 OK. The quadrupole splittings are large in the range 3.4-3.6 mm/sec and typical for the S = 3/2 state of Fe(III). Isomer shifts are near 0.30 mm/sec indicating substantial d electron delocalization in the 7T system of the ligand. The magnetic hyperfine constant decreases from the Cl to the I derivative indicating increasing covalency of the metal-halide bond. Pr?nounced relaxation effects are present in the Miissbauer spectra varying with the halogen ligand. Spectra 10 apphed fields up to 60 kG have been fitted with a relaxation theory based on a stochastic model. o