A quantum chemical study of hydrogen abstraction from manganese-coordinated water by a tyrosyl radical: A model for water oxidation in photosystem II
A quantum chemical study of hydrogen abstraction from manganese-coordinated water by a tyrosyl radical: A model for water oxidation in photosystem II
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DOI：
10.1021/ja9642323
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发表时间：
1997-09-03
影响因子：
15
通讯作者：
Korall, P
中科院分区：
文献类型：
--
作者：
Blomberg, MRA;Siegbahn, PEM;Korall, P
最近，在光系统II中的水氧化被认为涉及从水分子中直接提取氢原子，所述水分子通过氧化的酪氨酸自由基Tyr（Z）（.）与放氧复合物中的锰离子末端连接。这个模型在这里进行量子化学计算测试。一个经验参数化的混合密度泛函方法，单体和二聚锰模型系统进行了研究。发现通过与锰中心配位，水的第一O-H键强度从113.4 kcal/mol降低到84.3 kcal/mol。这种O-H键强度仅比酪氨酸中的O-H键强度强2.8 kcal/mol。使用扩展的基组，我们发现，这种差异进一步减少。第二个氢提取能量非常相似。由于反应中的热中性（或弱热中性）是氢提取模型的要求，本计算支持该模型。一个协调的氯化物和附近的钙络合物的可能功能的建议。五或六协调和铁或反铁磁自旋耦合锰中心进行了讨论。
Recently, water oxidation in photosystem II was proposed to involve direct abstraction of hydrogen atoms from water molecules terminally ligated to manganese ions in the oxygen-evolving complex by the oxidized tyrosine radical, Tyr(Z)(.). This model is tested here by performing quantum chemical calculations. An empirically parametrized hybrid density functional method is used, and both monomeric and dimeric manganese model systems are studied. It is found that, by coordination to a manganese center, the first O-H bond strength of water is lowered from 113.4 to 84.3 kcal/mol. This O-H bond strength is only 2.8 kcal/mol stronger than that in tyrosine. Using an extended basis set, we find that this difference decreases still further. The second hydrogen abstraction energy is quite similar. Since thermoneutrality in the reaction (or a weak exothermicity) is a requirement for the hydrogen abstraction model, the present calculations support this model. Possible functions of a coordinated chloride and a nearby calcium complex are suggested. Five-or six-coordination and ferro- or antiferromagnetic spin couplings of the manganese centers are discussed.