Hybrid density functional calculations are used to calculate the spin density distribution for the ubisemiquinone anion radical in the Q(A) 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 01 and 04 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 C-13 and O-17 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 Q(A) and Q(B) sites. The asymmetry is proposed to play a role in the unique one-electron reduction mechanism of the Q(A) site quinone and also in redirecting electron transfer parallel to the membrane plane. (C) 2003 Published by Elsevier B.V.