Primary Electron Transfer Processes in Photosynthetic Bacterial Reaction Centers

智力优势：本研究的目的是实现一个统一的分子水平上的理解如何通过在细菌光合反应中心（RC）的初级电子转移反应捕获和存储太阳光的能量。来自光合细菌的RC是一种蛋白质-色素复合物，包含两个独立但准对称的辅因子分支（细菌叶绿素及其衍生物和醌类）。 原则上，色素辅因子的任何一个分支都应该能够传输电子。 然而，在天然的“野生型”细菌RC中，只有“L-分支”色素被用于初级光诱导电荷分离，而替代的“M-分支”，辅因子是完全不活跃的。 在过去十年的研究过程中，实现了沿着M-分支辅因子链的电子转移，尽管产率远低于天然L-分支所达到的100%。捕获的第一个电荷分离的状态，形成的L-分支也实现了。 这种状态的寿命不到万亿分之一秒，并且在天然L分支电子转移的正常过程中只有很小的瞬态人口。 这些进展打开了大门，以全面的分子水平的理解的起源单向L-分支的电荷分离在本地RC和类似的理解电子转移沿着通常无活性的M-分支的RC辅因子。 寻求这两个目标的研究是这个项目的基础。 这些目标将通过最先进的时间分辨光谱研究来实现，时间尺度从亚皮秒（不到万亿分之一秒）到秒。将进行复杂的数据分析和计算机建模，以将各种突变体的结果与其他突变体和天然RC进行比较。 色素和周围蛋白质残基之间的特定相互作用将被作为研究的目标，以确定这种相互作用是否以及如何微调辅因子的性质，从而可以用于初始L-侧与M-侧电子转移之间的“切换”。 这些研究还将探讨颜料之间的电子耦合（区别于能量学问题）在控制方向性方面的贡献。 综合研究结果将有助于阐明细菌RC中电子转移作为一个统一整体的机制基础。 更广泛的影响在这个项目中进行的研究是相关的理解电子转移在两个光系统的植物和电荷迁移在膜结合蛋白质一般。 了解光合RC中光致电荷分离的分子水平机制对太阳能收集/转换的合成系统（包括Co-PI正在探索的途径）具有根本和深远的直接影响，从而解决了国家对下一代可再生能源的需求。 该项目将继续对本科生和代表性不足的群体参与研究和科学以及对学生进行广泛的多学科培训产生积极影响。 将研究思想纳入教学和教育发展，并向更广泛的社区传播，将继续是一个重点。 这些项目包括：（1）将进一步开发一个基于网络的教程，可针对本科生或高中生，主题是“为什么草是绿色而血红色？“（2）两项本科生物理化学实验将大幅提升，以探讨叶绿素和相关发色团的光谱学，以及光合作用的关键能量/电子转移过程，从应用的角度教授分子电子光谱学和动力学。 在一个这样的直接应用中，学生将设计，构建和表征一个简单的太阳能电池。(3)将为密苏里州植物园圣路易斯园艺大师发言人局制定一项关于光合作用一般和基本方面的广泛、基于科学的演讲，并与园艺、全球气候变化和太阳能方面的兴趣联系起来。 此外，通过与科学作家的联系，将继续利用当地和国家流行的印刷和网络出版场所，向广大受众传播光合作用研究的目标、相关性和应用。

Intellectual Merit:The objective of this research is to achieve a unified molecular-level understanding of how the energy of sunlight is captured and stored via the primary electron transfer reactions in the bacterial photosynthetic reaction center (RC). The RC from photosynthetic bacteria is a protein-pigment complex housing two separate but quasi-symmetric branches of cofactors (bacteriochlorophyll, derivatives of it, and quinones). In principle, either branch of pigment cofactors should be able to transport electrons. Yet in the native "wild-type" bacterial RC only the "L-branch" pigments are utilized for primary photo-induced charge separation, while the alternative, "M-branch?, cofactors are completely inactive. During the course of the last ten years of research, electron transfer along the chain of M-branch cofactors was achieved albeit in much less than the 100% yield attained by the native L-branch. Trapping the first charge-separated state that forms on the L-branch was also achieved. This state lives less than a trillionth of a second and has only a small transient population during the normal course of native L-branch electron transfer. These advances have opened the door to a comprehensive molecular-level understanding of the origins of the unidirectional L-branch charge separation in the native RC and analogous understanding of electron transfer along the normally inactive M-branch of RC cofactors. Studies that seek these twin goals are the basis of this project. The goals will be pursued via state-of-the art time-resolved spectroscopic investigations spanning the sub-picosecond time scale (less than a trillionth of a second) to seconds. Sophisticated data analysis and computer modeling will be undertaken to compare results from various mutants with each other and the native RC. Specific interactions between the pigments and surrounding protein residues will be targeted for investigation to determine whether and how such interactions fine-tune cofactor properties and thus may serve to "switch" between initial L-side versus M-side electron transfer. The studies will also probe the contribution of electronic couplings between the pigments (as distinguished from issues of energetics) in controlling directionality. The combined findings will help to elucidate the mechanistic underpinnings of electron transfer in the bacterial RC as a unified whole. Broader ImpactThe studies pursued in this project are relevant to understanding electron transfer in the two photosystems of plants and charge migration in membrane-bound proteins in general. Understanding the molecular-level mechanisms of photo-induced charge separation in the photosynthetic RC has fundamental and far-reaching direct impacts on synthetic systems for solar-energy harvesting/conversion (including avenues being explored by the Co-PIs), thereby addressing a national need for next-generation renewable energy sources. This project will continue to have a demonstrated positive impact on the participation of undergraduates and underrepresented groups in research and science, and in the broad multidisciplinary training of students. The integration of research ideas into teaching and educational development and dissemination to the broader community will continue to be a focus. Such projects include the following: (1) A web-based tutorial will be further developed that can be tailored for undergraduate or high school students on the topic "Why is grass green and blood red?" (2) Two undergraduate physical-chemistry laboratory experiments will be extensively upgraded to exploit the spectroscopy of chlorophyll and related chromophores, and the energy/electron transfer processes critical to photosynthesis, to teach molecular electronic spectroscopy and kinetics from application-oriented perspectives. In one such direct application, students will design, build, and characterize a simple solar cell. (3) A broad, science-based talk on general and fundamental aspects of photosynthesis with links to interests in gardening, global climate change, and solar energy will be developed as a program for the St. Louis Master Gardener Speaker's Bureau at the Missouri Botanical Garden. Additionally, local and national popular print and web-based publication venues will continue to be exploited through contacts with science writers in order to disseminate the goals, relevance, and applications of photosynthesis research to a broad audience.This project is receiving co-funding from the Chemistry of Life Processes program in the Chemistry Division