Physical analysis of the through-ligand long-distance magnetic coupling: spin-polarization versus Anderson mechanism.

The physical factors governing the magnetic coupling between two magnetic sites are analyzed and quantified as functions of the length of the bridging conjugated ligand. Using wave-function-theory based ab initio calculations, it has been possible to separate and calculate the various contributions to the magnetic coupling, i.e. the direct exchange, the spin polarization and the kinetic exchange. It is shown in model systems that while the Anderson mechanism brings the leading contribution for short-length ligands, the spin polarization dominates the through-long-ligand couplings. Since the spin polarization decreases more slowly than the kinetic exchange, highly spin polarizable bridging ligands would generate a good magneto-communication between interacting magnetic units.

Influence of topology on the long-range electron-transfer phenomenon.

Intramolecular electron-transfer phenomena in the radical anions derived from the partial reduction of diradicals (E,E)-p-divinylbenzene-beta,beta'-ylene bis(4-tetradecachlorotriphenylmethyl) diradical (1) and (E,E)-m-divinylbenzene-beta,beta'-ylene bis(4-tetradecachlorotriphenylmethyl) diradical (2) have been studied by optical and ESR spectroscopy. The synthetic methodology used allows for complete control of the geometry of diradicals 1 and 2, which have para and meta topologies, respectively, as well as of their E/Z isomerism. This fact is used to show the influence of the different topologies on the ease of electron transfer, which is larger for the para than for the meta isomer, in which a small or negligible electronic coupling is observed. A related monoradical compound (E)-bis(pentachlorophenyl)[4-(4-bromophenyl-beta-styryl)-2,3,5,6-tetrachlorophenyl]-methyl radical (3), which has only one such redox site, has also been obtained and studied for comparison purposes.

Theoretical study of the multiline EPR signal from the S(2) state of the oxygen evolving complex of photosystem II: Evidence for a magnetic tetramer.

The Oxygen evolving complex of plant photosystem II is made of a manganese cluster that gives rise to a low temperature EPR multiline signal in the S(2) oxidation state. The origin of this EPR signal has been addressed with respect to the question of the magnetic couplings between the electron and nuclear spins of the four possible Mn ions that make up this complex. Considering Mn(III) and Mn(IV) as the only possible oxidation states present in the S(2) state, and no large anisotropy of the magnetic tensors, the breadths of the EPR spectra calculated for dimers and trimers with S = (1/2) have been compared with that of the biological site. It is concluded that neither a dinuclear nor a trinuclear complex made of Mn(III) and Mn(IV) can be responsible for the multiline signal; but that, by contrast, a tetranuclear Mn complex can be the origin of this signal. The general shape of the experimental spectrum, its particular hyperfine pattern, the positions of most of the hyperfine lines and their relative intensities can be fit by a tetramer model described by the following six fitting parameters: g approximately 1.987, A(1) approximately 122.4 10(-4) cm(-1), A(2) approximately 87.2 10(-4) cm(-1), A(3) approximately 81.6 10(-4) cm(-1), A(4) approximately 19.1 10(-4) cm(-1) and deltaH = 24.5 G. A second model described by parameters very close to those given above except for A(4) approximately 77.5 10(-4) cm(-1) gives an equally good fit. However, no other set of parameters gives an EPR spectrum that reproduces the hyperfine pattern of the S(2) multiline signal. This demonstrates that in the S(2) state of the oxygen evolving complex, the four manganese ions are organized in a magnetic tetramer.

Magnetization and electron paramagnetic resonance studies of reduced uteroferrin and its "EPR-silent" phosphate complex.

The exchange coupling of reduced uteroferrin has been measured (19.8(5) cm-1 S1.S2) using recently developed techniques for studying metalloprotein magnetization. A spin Hamiltonian describing the coupled binuclear Fe(II).Fe(III) center has been used to fit the low and high field magnetization data, the EPR g values, and the highly anisotropic effective hyperfine tensor of the ferric site. The exchange coupling of the phosphate complex of reduced uteroferrin has also been measured (6.0(5) cm-1 S1.S2) using the same techniques. The smaller exchange coupling of the phosphate complex is comparable with the zero field splittings of the iron sites. This results in increased sensitivity of the system g values (found by calculation from the spin Hamiltonian) to variations of the zero field splitting parameters arising from heterogeneities in the protein microenvironment. Consequently, there is a very significant (9-fold) increase in the "effective g strain" of the system compared to the situation in the absence of phosphate. This, together with the larger g anisotropy (g = (1.06, 1.51, 2.27)), gives rise to an EPR signal for the phosphate complex of reduced uteroferrin which is extremely broad and difficult to detect but which has now been identified for the first time.

Magnetization of the sulfite and nitrite complexes of oxidized sulfite and nitrite reductases: EPR silent spin S = 1/2 states.

The saturation magnetizations of the sulfite complex of oxidized sulfite reductase and the nitrite complex of oxidized nitrite reductase have been measured to determine their spin state. Each shows the saturation magnetization signal of a spin S = 1/2 state with sigma g2 = 16, which is typical of low-spin ferrihemes. However, the EPR spectra of these complexes lack the expected signal intensity of a spin S = 1/2 state. Indeed, one of these complexes is EPR silent. The reasons for this unexpectedly low EPR signal intensity are considered.

Magnetization of the oxidized and reduced three-iron cluster of Desulfovibrio gigas ferredoxin II.

The saturation magnetizations of the three iron cluster of ferredoxin II of Desulfovibrio gigas in both the oxidized and reduced states have been studied at fixed magnetic fields up to 4.5 tesla over the temperature range from 1.8 to 200 K. The low field (0.3 tesla) susceptibility of oxidized ferredoxin II obeys the Curie law over this entire temperature range. This establishes -2Jox greater than 200 cm-1 as the lower limit for the antiferromagnetic exchange coupling of oxidized ferredoxin II. The saturation magnetizations of reduced ferredoxin II at several fixed fields yield a nested family of curves which can be fit with spin S = 2 and D = -2.7(4) cm-1 (with E/D assigned the value 0.23 as determined by Mössbauer and EPR spectra). The low field susceptibility of reduced ferredoxin II also obeys the Curie law from approximately 4 up to 200 K. This establishes -2Jred greater than 40 cm-1 as the lower limit for the antiferromagnetic coupling of reduced ferredoxin II.