The origin of the spin density asymmetry at the QA binding site of type II photosynthetic reaction centres

Hybrid density functional calculations are used to calculate the spin density distribution for the ubisemiquinone anion radical in the QA binding site of the photosynthetic bacterium Rb 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. Good agreement is found between calculated and experimental 13C and 17O hyperfine couplings lending support to the calculation. 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. The asymmetry is proposed to play a role in the unique one-electron reduction mechanism of the QA site quinone and also in redirecting electron transfer parallel to the membrane plane.