An ONIOM study of the QA site semiquinone in the Rhodobacter sphaeroides photosynthetic reaction centre

ONIOM (QM/MM) calculations are performed to investigate the spin density distribution for the ubisemiquinone anion radical in the QA binding site of the photosynthetic bacterium Rhodobacter sphaeroides. The calculated spin density in the QA site model suggests that differential hydrogen bonding strength to the O1 and O4 oxygen atoms of the radical results in an asymmetric spin density distribution in the semiquinone anion free radical form. The origin of the spin density asymmetry is attributed to the presence of the divalent iron or zinc ion situated between the QA and QB sites.