Time-Domain EPR Studies of Tyrosine Z of Photosystem II

植物光合装置的光系统II组分利用光能氧化水并减少膜上可扩散的质体醌。光子诱导的电荷分离发生在叶绿素部分P680。提供给光生叶绿素阳离子P+680的电子传递组分是光系统II所特有的。一个被命名为YZ的酪氨酸残基作为P+680和氧演化络合物（OEC）的锰簇之间的电子转移中间体，在这里发生水氧化。将快速光激发与脉冲电子顺磁共振（EPR）光谱相结合，研究YZ在电子传递和水氧化中的作用。激光脉冲宽度为3ns，微波脉冲宽度为10ns，我们将能够测量氧演化循环中每个s态跃迁时YZ（酪氨酸基）自由基的形成和消耗动力学。动力学同位素效应将通过在富2h2o缓冲液中重复这些实验来测量。YZ EPR线形状的时间依赖性将在每个s态转变中进行检查，以寻找与Mn簇和/或酪氨酸自由基构象变化的磁相互作用的证据。将进行电子自旋回波包络调制（ESEEM）和电子自旋回波-电子核双共振（ESE-ENDOR）实验，以进一步验证最近的模型，这些模型假设YZ(直接将质子或氢原子从水配体中提取到Mn簇中。这些实验将大大提高我们对光系统II反应中心的结构和动力学的认识。这些实验代表了最先进的快速激光激发与脉冲EPR方法的结合，并为许多研究生和博士后研究人员提供了良好的培训机会。植物光合装置的光系统II组件利用光能从水分子中剥离电子，作为幸运的副产品，分子氧被释放到大气中。当光被指定为P680的叶绿素分子吸收时，它会自发地将一个电子转移到邻近的色素分子上。这种光诱导的电荷分离是将光能转化为有用化学能的第一步。所产生的带正电的叶绿素P+680分子具有很强的亲和力，可以重新获得电子，电子最初来自周围蛋白质的酪氨酸氨基酸残基。这个被命名为YZ的酪氨酸残基反过来从一小簇锰原子中拉出一个电子，人们认为锰原子是发生水分解化学反应的地方。我们的实验将检测YZ酪氨酸在快速电子转移过程中的磁性。持续时间只有30亿秒的激光脉冲将与相对较短的微波脉冲相结合，在超短的时间尺度上测量磁性的变化。将对测量结果进行解释，以了解该YZ酪氨酸在电子转移过程中的动力学。目标是测试有争议的新模型，该模型提出YZ酪氨酸从与金属团簇结合的水分子中吸引质子和电子。这些实验将大大提高我们对光系统II反应中心的结构和动力学的认识。这些实验代表了快速激光激发与脉冲磁共振方法的最新融合，并为许多研究生和博士后研究人员提供了良好的培训机会。* * *

9513648 Britt The Photosystem II component of the plant photosynthetic apparatus utilizes light energy to oxidize water and reduce membrane diffusable plastoquinone. Photon-induced charge separation occurs at the chlorophyll moiety P680 . The electron transfer components that donate to the photogenerated chlorophyll cation P+680 are unique to Photosystem II. A tyrosine residue designated as YZ serves as an electron transfer intermediate between P+680 and the manganese cluster of the oxygen-evolving complex (OEC) where water oxidation occurs. Fast optical excitation will be combined with pulsed electron paramagnetic resonance (EPR) spectroscopy to examine the role of YZ in electron transfer and water oxidation. With laser pulsewidths of (3 ns, and microwave pulsewidths of (10 ns, we will be able to measure the kinetics of formation and depletion of the YZ( tyrosyl radical at each of the "S-state" transitions of the oxygen evolving cycle. The kinetic isotope effects will be measured by repeating these experiments in 2H2O-enriched buffer. The time dependence of the YZ EPR lineshapes will be examined at each S-state transition to look for evidence of magnetic interactions with the Mn cluster and/or conformational changes of the tyrosyl radical. Electron spin echo envelope modulation (ESEEM) and electron spin echo - electron nuclear double resonance (ESE-ENDOR) experiments will be performed to further test recent models that postulate that YZ( acts directly to abstract protons or hydrogen atoms from water ligands to the Mn cluster. These experiments will greatly advance our knowledge of the structure and dynamics of the Photosystem II reaction center. The experiments represent a state-of-the-art merging of fast laser excitation with pulsed EPR methods and provide excellent training opportunities for a number of graduate students and postdoctoral researchers. %%% The Photosystem II component of the plant photosynthetic apparatus utilizes light energy to strip electrons from water mo lecules, with molecular oxygen released into the atmosphere as a fortunate byproduct. When light is absorbed by a certain chlorophyll molecule designated P680, it spontaneously transfers an electron to an adjacent pigment molecule. This light-induced charge separation is the first step in converting light energy into useful chemical energy. The resulting positively charged chlorophyll P+680 molecule has a strong affinity to regain an electron, and the electron comes initially from a tyrosine amino acid residue of the surrounding protein. This tyrosine residue, designated YZ, in turn pulls an electron from a small cluster of manganese atoms where the water-splitting chemistry is thought to occur. Our experiments will examine the magnetic properties of this YZ tyrosine during the fast electron transfer events. Laser light pulses of only 3 billions of a second duration will be combined with comparably short microwave pulses to measure the changes of magnetic properties on this ultra-short timescale. The measurements will be interpreted to understand the dynamics of this YZ tyrosine during electron transfer. The goal is to test controversial new models that propose that the YZ tyrosine pulls protons as well as electrons off water molecules bound to the metal cluster. These experiments will greatly advance our knowledge of the structure and dynamics of the Photosystem II reaction center. The experiments represent a state-of-the art merging of fast laser excitation with pulsed magnetic resonance methods and provide excellent training opportunities for a number of graduate students and postdoctoral researchers. ***