Pulsed ENDOR studies of the local structure and function of water oxidizing system in Photosynthesis.

用脉冲ENDOR研究了tris处理的PS II中酪氨酸Z （Yz）自由基的EPR线形状，发现pH值低于6.5时发生了变化。根据观察到的超精细相互作用参数，发现其电子态为阳离子自由基。该自由基介导了电子从水向反应中心P680的转移，其与其他氨基酸的氢键起着重要作用。脉冲ENDOR是研究氢键局部效应的有力手段。通过矩阵ENDOR研究Yz自由基的周围结构不仅具有柔性，而且其电子结构也与自由基的反应性有关。在PS II的受体侧，非边铁被耗尽或被锌取代。受体醌（Q_A^-）的ESEEM研究显示与该分子有2个氮偶联。一个分配给组氨酸，另一个分配给肽氮。脉冲ENDOR显示了包含两个氢键的可交换质子的存在。PS II的CN处理导致Q_A^-的ESEEM氮信号丢失，该信号被分配给组氨酸，同时氢键的结构发生变化。一种名为ELDOR的新技术已被应用于检测自由基之间的偶极相互作用。在缺钙PS II的供体侧，酪氨酸D （Yd）到捕获的Yz^+、S_3信号种和Mn簇的距离分别为29、30和27 A。

The EPR line shape of tyrosine Z (Yz) radical in Tris-treated PS II has been studied by pulsed ENDOR and was found to change at lower pH below 6.5. Based on the observed hyperfine interaction parameters, the electronic state was found to be that of cation radical. This radical mediates the electron transfer from water to the reaction center P680 and its hydrogen bonds with other amino acids play an important role. Pulsed ENDOR is a powerful mean to study the local effect of hydrogen bonding. Not only the surrounding structure of Yz radical is flexible as studied by matrix ENDOR,but also the electronic structure is responsible to the reactivity of the radical.In the acceptor side of PS II the non-hem iron was depleted or Zn-substituted. ESEEM study of the acceptor quinone (Q_A^-) showed 2 nitrogen couplings with this molecule. The one is assigned to histidine and the other to a peptide nitrogen. Pulsed ENDOR has shown the existence of exchangeable protons including two hydrogen bonds. CN treatment of PS II resulted in the loss of nitrogen signal from ESEEM of Q_A^-, that was assigned to histidine, along with structural change in the hydrogen bondings.New technique called ELDOR has been applied to detect the dipolar interaction between radicals. In the donor side of Ca-depleted PS II,the distances from tyrosine D (Yd) to the trapped Yz^+, S_3 signal species, and the Mn cluster were determined to be 29,30 and 27 A respectively.