School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom.
J Phys Chem B. 2011 Apr 14;115(14):4227-33. doi: 10.1021/jp112163w. Epub 2011 Mar 23.
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.
ONIOM(QM/MM)计算用于计算光合作用系统 II 的 Q(A)结合位点中半醌阴离子自由基的自旋密度分布。研究了多种模型，以探讨铁耗竭对 Q(A)位点半醌自旋密度分布和由此产生的超精细偶合的影响。对于非血红素部位有二价金属离子的模型体系，计算得出的 Q(A)位点模型中的自旋密度表明，对自由基的 O1 和 O4 氧原子的氢键强度不同导致半醌阴离子自由基形式的自旋密度分布不对称。与近端 O1 原子的氢键明显更强。这类似于以前在 Rba sphaeroides 的细菌反应中心中存在的情况。各种耗尽的非血红素部位金属模型表明，该部位存在二价离子对 Q(A)位点半醌自旋密度分布的深远影响。需要考虑非血红素部位占据对 Q(A)位点塑料半醌自旋密度分布的计算变化，以解释实验顺磁共振数据。对于 II 型反应中心，非血红素部位 Fe(2+)的主要作用可能是提高 Q(A)/Q(A)(-)对的氧化还原电位，以确保从（细菌）叶绿素阴离子自由基的电子转移以足够快的速率发生，以与浪费的反向反应竞争。
