HIGH FREQUENCY EPR OF TYROSYL RADICAL OF PHOTOSYSTEM II

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 TyrI 60）和YZ（DI-TyrI 61）。 YZ被认为是 四核Mn簇之间的电子转移中间体， 其中发生水氧化，并且光氧化的叶绿素部分 P680。 YD酪氨酰自由基的作用目前尚不清楚; 然而，据信它参与蛋白质组装， Mn簇的稳定化。 我们最近报道了 的YD酪氨酰基自由基的频率（139.5GHz）EPR谱。 光系统H具有主g值的菱形粉末图案 gl=2.00782，92=2.00450和93=2.00232，所有样品均具有超精细结构 测量了三个转折点。 高频EPR谱 在饱和条件下获得，其中分散体 信号被检测到。 光谱线形状的正确复制是 仅在 考虑到线条形状的模拟 饱和色散信号的特征。 g的变化，从 自由电子g值已经显示出对氢敏感 结合在酪氨酰基的苯基氧上。 g1的值 对于YD，与定义明确的氢键一致。 密度 功能计算表明，减少低场转移， 随着氢键的增加，自由电子g值 强度来自苯基上自旋密度的降低， 氧-在合理的氢键距离范围内为5-30% （2.0-1.1 A）-以及基态和基态之间的分裂增加， 激发态单占据分子轨道。

Photosystem II contains two symmetry related redox active tyrosine residues, YD(D2TyrI60) 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 P680. The role of the YD tyrosyl radical is currently unknown; however, it is believed to be involved in protein assembly and stabilization of the Mn cluster. We recently reported the high frequency (139.5 GHz) EPR spectrum of the YD tyrosyl radical of photosystem H. A rhombic powder pattern with principal g values gl=2.00782, 92=2.00450, and 93=2.00232 and hyperfine structure on all three turning points was measured. The high frequency EPR spectrum was acquired under conditions of saturation in which the dispersion signal is detected. Proper replication of the spectral line shapes is only achieved in simulations which account for the line shapes characteristic of saturated dispersion signals. The shift in g, from the free electron g-value has been shown to be sensitive to hydrogen bonding at the phenyl oxygen of the tyrosyl radical. The value of g 1 for YD is consistent with a well-defined hydrogen bond. Density functional calculations indicate that the decreased downfield shift in g I from the free electron g value with increasing hydrogen bond strength arises from both a decreased spin density on the phenyl oxygen -- 5-30% over a range of reasonable hydrogen bond distances (2.0-1.1 A) -- and an increased splitting between ground state and excited state singly occupied molecular orbitals.