An ONIOM study of the spin density distribution of the QA site plastosemiquinone in the photosystem II reaction center.

ONIOM (QM/MM) calculations are used to calculate the spin density distribution for the plastosemiquinone anion radical in the Q(A) binding site of photosystem II. A number of models are examined that explore the effect of iron depletion on the Q(A) site semiquinone spin density distribution and resultant hyperfine couplings. For a model system with a divalent metal ion in the nonheme site the calculated spin density in the Q(A) 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 hydrogen bond to the proximal O1 atom is significantly stronger. This is similar to the situation shown to exist previously in the bacterial reaction center of Rba sphaeroides. Various models of depleted nonheme site metal show the profound effect that the presence of a divalent ion in this site has on the spin density distribution of the Q(A) site semiquinone. The variation in calculated spin density distribution of the Q(A) site plastosemiquinone as a function of the occupancy of the nonheme site needs to be taken into account in the interpretation of experimental paramagnetic resonance data. For Type II reaction centers a major role for Fe(2+) in the nonheme site may be the raising of the redox potential of the Q(A)/Q(A)(-) couple to ensure that electron transfer from the (bacterio)pheophytin anion free radical occurs at a sufficient rate to compete with wasteful back-reactions.