Density functional calculated 13C and 17O hyperfine couplings for the ubisemiquinone anion radical in an alcohol solvent model and in a model of the Qa binding site of Rb sphaeroides

Hybrid density functional calculations are used to calculate the spin density distribution and the 17O and 13C hyperfine coupling constants for the ubisemiquinone anion radical in model complexes simulating the radicalʹs environment in alcohol solvents and in the Qa binding site of Rb sphaeroides. Good agreement is observed between the alcohol solvent model calculated couplings and experimental determinations. For the Qa site model, best agreement is observed for a Qa site model containing a positively charged imidazole hydrogen bonding interaction at the O4 atom of the semiquinone. This may suggest that the asymmetry observed for the Qa site of Rb sphaeroides is due to strong hydrogen bonding by the O4 oxygen atom of the semiquinone to an imidazole carrying a partial positive charge. The obvious candidate to fulfil this role in the reaction centre is the HIS M219 ligand of an Fe(II) complex.