Webber A N, Lubitz W
Department of Plant Biology and Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe 85287-1601, USA.
Biochim Biophys Acta. 2001 Oct 30;1507(1-3):61-79. doi: 10.1016/s0005-2728(01)00198-0.
The primary electron donor of photosystem I, P700, is a chlorophyll species that in its excited state has a potential of approximately -1.2 V. The precise chemical composition and electronic structure of P700 is still unknown. Recent evidence indicates that P700 is a dimer of one chlorophyll (Chl) a and one Chl a'. The Chl a' and Chl a are axially coordinated by His residues provided by protein subunits PsaA and PsaB, respectively. The Chl a', but not the Chl a, is also H-bonded to the protein. The H-bonding is likely responsible for selective insertion of Chl a' into the reaction center. EPR studies of P700(+*) in frozen solution and single crystals indicate a large asymmetry in the electron spin and charge distribution towards one Chl of the dimer. Molecular orbital calculations indicate that H-bonding will specifically stabilize the Chl a'-side of the dimer, suggesting that the unpaired electron would predominantly reside on the Chl a. This is supported by results of specific mutagenesis of the PsaA and PsaB axial His residues, which show that only mutations of the PsaB subunit significantly alter the hyperfine coupling constants associated with a single Chl molecule. The PsaB mutants also alter the microwave induced triplet-minus-singlet spectrum indicating that the triplet state is localized on the same Chl. Excitonic coupling between the two Chl a of P700 is weak due to the distance and overlap of the porphyrin planes. Evidence of excitonic coupling is found in PsaB mutants which show a new bleaching band at 665 nm that likely represents an increased intensity of the upper exciton band of P700. Additional properties of P700 that may give rise to its unusually low potential are discussed.
光系统I的主要电子供体P700是一种叶绿素，其激发态时的电位约为-1.2V。P700的确切化学组成和电子结构仍然未知。最近的证据表明，P700是一个叶绿素（Chl）a和一个Chl a'的二聚体。Chl a'和Chl a分别由蛋白质亚基PsaA和PsaB提供的His残基进行轴向配位。Chl a'，而非Chl a，也与蛋白质形成氢键。这种氢键可能是Chl a'选择性插入反应中心的原因。对冷冻溶液和单晶中的P700(+*)进行的电子顺磁共振（EPR）研究表明，电子自旋和电荷分布向二聚体的一个Chl存在很大的不对称性。分子轨道计算表明，氢键将特异性地稳定二聚体的Chl a'-侧，这表明未配对电子将主要位于Chl a上。这得到了PsaA和PsaB轴向His残基特异性诱变结果的支持，结果表明只有PsaB亚基的突变会显著改变与单个Chl分子相关的超精细偶合常数。PsaB突变体也会改变微波诱导的三重态减去单重态光谱，表明三重态定域在同一个Chl上。由于卟啉平面的距离和重叠，P700的两个Chl a之间的激子耦合较弱。在PsaB突变体中发现了激子耦合的证据，这些突变体在665nm处显示出一个新的漂白带，这可能代表P700上激子带上带强度的增加。文中还讨论了可能导致P700异常低电位的其他特性。
