A recently developed density functional (DFT) approach for the calculation of electronic g-tensors has been applied to semiquinone radical anions in the different protein environments of photosynthetic reaction centers. Supermolecular models have been constructed, based on combined crystallographic and quantum chemical structure data, for the QA and QB active sites of bacterial reaction centers, for the A1 site of photosystem I, as well as for ubisemiquinone in frozen 2-propanol. After scaling of the computed Δg x components by 0.92, both Δg x and Δg y components computed at gradient-corrected DFT level with accurate spin−orbit operators agree with high-field EPR reference data essentially to within experimental accuracy in all four systems studied. The influence of the various semiquinone−protein noncovalent interactions has been studied by successive removal of individual residues from the models. The effects of hydrogen bonding to the two carbonyl oxygen atoms of the semiquinones are nonadditive, due to compensating spin-polarization effects. The effects of tryptophan−semiquinone π-stacking are different for QA and A1 sites. This may be traced back to a different alignment of the interacting fragments and to differential spin polarization.

Reactions of SO 3 •radicals with complexes of Ni(II and I) and Cu(II and I) were investigated under anaerobic conditions. Reactions of CuCR + , Cu(TIM) + , and Ni(Me 6 [14]dieneN 4 ) + with SO 3 •radicals were investigated by time-resolved spectroscopy. The kinetics of the processes is in accordance with an outer-sphere electrontransfer mechanism. Intermediates with a ligand-radical nature were observed in SO 3 •reactions with Cu(2,9-dimethyl-1,10-phenanthroline) 2 + , Cu(Me 6 [14]dieneN 4 ) 2+ , NiCR 2+ , or NiCRH 4 2+ . Two products were isolated in the reaction of NiCR 2+ with SO 3 •radicals, one product with a sulfonated macrocycle and the other product with one additional double bond in the macrocycle. These products confirmed the attack of the ligand by SO 3 •radicals and the formation of intermediates with a ligand-radical nature. A mechanism based on the oxidative addition of SO 3 •radicals to the ligand accounts also for the kinetics of product formation.