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-Tyr16O)和YZ(Di-Tyr16l)。YZ被认为是为 作为四核锰之间的电子转移中间体 团簇，在那里发生水氧化，以及光氧化 尤其是叶绿素部分P680‘，YZ被认为起到了直接作用 从水中提取质子在水氧化中的催化作用 水分子连接到金属簇活性中心。探查 YZ的功能更详细，我们正在调查 YZ的氢键状态和YZ与金属的亲和性 中间。有人提出YZ可能存在氢键 与组氨酸残基和水分子的相互作用。这个 低场电子g值g与自由电子的移位 在高频下观察到的G值已被证明对 酪氨酸基的苯氧基处的氢键。一个 随着氢键的增加，g的移动量减小。 YZ酪氨酰自由基的高频(139.5 GHz)电子顺磁共振谱 在缺锰样品中的光系统II显示为菱形粉末 主要g值为g的图案，=2.00760、92=2.00450和93 =2.00230。观察到了沿低场转折点的分裂。 可能是由于Yz的不同氢键环境造成的 这些缺锰样品中的自由基，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边与这种无序的氢是一致的 结合环境。研究也在进行中，以衡量 酪氨酸基与锰的交换和偶极偶联 在完整的光系统11样品中聚集，其中酪氨酸自由基有 通过醋酸盐或丙烯酸酯处理而被捕获。

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.