Electronic structure studies of quinones and semiquinones: accurate calculation of spin densities and electron paramagnetic resonance parameters.

The application of electronic structure methods to the prediction of geometries, spin densities, and hyperfine couplings for biologically relevant quinones and semiquinones is reviewed. It is demonstrated that hybrid-type density functional methods are particularly suitable for such studies. Hydrogen bonding to the semiquinone oxygen by appropriate donors is shown to lead to a redistribution of spin density in the carbonyl group of the semiquinone. Experimental trends are well reproduced by the calculated values. Symmetric and asymmetric models of hydrogen bonding are modelled. It is shown that the symmetric models give good agreement with solution studies in vitro. The asymmetric models of hydrogen bonding give quite good agreement with values measured for in vivo semiquinones generated in the reaction centres of the purple photosynthetic bacterium, Rb sphaeroides, and also for the phyllosemiquinone free radical formed during electron transfer in Photosystem I of green plants. These recent advances in electronic structure calculations, in particular the applicability of density functional methods to the study of free radical properties, have opened up an exciting avenue for the complete characterisation of their electronic properties. In particular, the combination of experimental methods of electron paramagnetic resonance and such calculations should in future provide a clearer understanding of free radical chemistry in many areas of biology.