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 Q(A) and Q(B) active sites of bacterial reaction centers, for the A(1) site of photosystem I, as well as for ubisemiquinone in frozen 2-propanol. After scaling of the computed Deltag(x) components by 0.92, both Deltag(x) and Deltag(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-semi quinone pi-stacking are different for Q(A) and A(1) sites. This may be traced back to a different alignment of the interacting fragments and to differential spin polarization.