HIGH FREQUENCY EPR OF YZ TYROSYL RADICAL OF PHOTOSYSTEM 11

光系统II含有两个对称性相关的氧化还原活性酪氨酸 残基，Yd（D2-Tyrl 6 O）和Yz（DI-Tyrl 6 I）。 Yz被认为是服务于 作为四核Mn 团簇，在那里发生水氧化，和光氧化 叶绿素部分P 680'特别地，Yz被认为起直接作用。 在水氧化中的催化作用，通过从水中提取质子， 水分子连接到金属簇活性位点。 探讨 更详细地了解Yz的功能，我们正在研究 Yz的氢键状态和Yz与金属的接近度 中心 有人提出Yz可能存在氢键 在一个实施方案中，该化合物与组氨酸残基和水分子两者相互作用。 的 低场电子g值g从自由电子的偏移 在高频率下观察到的g值已被证明对 在酪氨酰基的苯基氧上形成氢键。 一 随着氢键的增加，观察到g的位移减小。 Yz酪氨酰自由基的高频（139.5GHz）EPR谱 光系统II在锰耗尽样品中的显示菱形粉末 主g值为g1 =2.00760、92 =2.00450和93的图案 =2.00230。 观察到了沿低场转折点的沿着分裂 这可能源于Yz的不同氢键环境 自由基在这些锰耗尽的样品，其中一些分子保留 两个氢键，其他的只有一个。 的位置和宽度 Yz的g边与这种无序氢相一致 结合环境 研究也在进行中，以衡量 酪氨酰基自由基和Mn之间的交换和偶极耦合 簇在完整的光系统11样品中，其中酪氨酰基自由基具有 用醋酸盐或丙烯酸盐处理而被捕获。

Photosystem II contains two symmetry related redox active tyrosine residues, Y D (D2-Tyrl6O) and Yz(DI-Tyrl6l). Yz is thought to serve as an electron transfer intermediate between the tetranuclear Mn cluster, where water oxidation occurs, and the photo-oxidized chlorophyl moiety P 680' In particular, Yz is thought to play a direct catalytic role in the water oxidation by abstracting a proton from water molecules ligated to the metal cluster active site. To probe the function of Yz in greater detail, we are investigating the hydrogen bonding state of Yz and the proximity of Yz to the metal center. It has been proposed that Yz may have hydrogen bonding interactions with both a histidine residue and a water molecule. The shift in low field electron g value, g,, from the free electron g-value observed at high frequency has been shown to be sensitive to hydrogen bonding at the phenyl oxygen of the tyrosyl radical. A decreased shift in g, is observed with increased hydrogen bonding. The high frequency (139.5 GHz) EPR spectrum of the Yz tyrosyl radical of photosystem II in Mn depleted samples displays a rhombic powder pattern with principal g values of g,=2.00760, 92 =2.00450, and 93 =2.00230. Splittings along the low field turning point are observed that may arise from different hydrogen bonding environments forYz radicals in these Mn depleted samples, in which some molecules retain two hydrogen bonds and others only one. The position and widt h of the g, edge for Yz is consistent with such a disordered hydrogen bonding environment. Studies are also underway to measure the exchange and dipolar couplings between the tyrosyl radical and the Mn cluster in intact photosystem 11 samples where the tyrosyl radical has been trapped by treatment with acetate or acrylate.