Primary Electron Transfer Processes in Photosynthetic Bacterial Reaction Centers

智力价值：这项研究的目标是在分子水平上统一理解阳光能量是如何通过细菌光合作用反应中心(RC)的初级电子转移反应捕获和储存的。来自光合作用细菌的RC是一种蛋白质-色素复合体，含有两个独立但准对称的辅因子(细菌叶绿素及其衍生物和苯二酚)分支。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”辅因子完全不起作用。在过去十年的研究过程中，实现了沿着M-支链辅助因子链的电子转移，尽管其产率远低于本地的L支链。还实现了在L分支上形成的第一个电荷分离态的俘获。这种态的寿命不到万亿分之一秒，在本征L分支电子转移的正常过程中只有很小的瞬变布居。这些进展为在分子水平上全面理解天然RC中单向L分支电荷分离的起源和类似地理解RC辅因子沿正常不活跃的M分支的电子转移打开了大门。寻求这两个目标的研究是这个项目的基础。这些目标将通过跨越亚皮秒(不到万亿分之一秒)到几秒的最先进的时间分辨光谱研究来实现。将进行复杂的数据分析和计算机建模，以比较来自不同突变体的结果彼此和天然RC。将针对色素和周围蛋白质残基之间的特定相互作用进行研究，以确定这些相互作用是否以及如何微调辅因子属性，从而可能有助于在初始L侧和M侧电子转移之间进行“切换”。这些研究还将探索颜料之间的电子耦合(与能量学问题不同)在控制方向性方面的贡献。这些联合的发现将有助于阐明细菌RC作为一个统一的整体中电子转移的机制基础。更广泛的影响本项目中进行的研究与了解植物两个光系统中的电子转移和膜结合蛋白中的电荷迁移有关。了解光合作用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-PIs正在探索的途径)具有基本和深远的直接影响，从而满足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。将研究思想融入教学和教育发展，并向更广泛的社区传播，将继续是重点。这些项目包括：(1)将进一步开发一个基于网络的教程，可以为本科生或高中生量身定做关于“为什么草是绿的，血是红的？”的主题。(2)将对两个本科物理化学实验室实验进行广泛的升级，以探索叶绿素和相关发色团的光谱以及光合作用的关键能量/电子传递过程，从面向应用的角度教授分子电子光谱和动力学。在一个这样的直接应用中，学生将设计、制造和表征一个简单的太阳能电池。(3)将为密苏里州植物园的圣路易斯园艺大师议长局开发一个广泛的、以科学为基础的、与园艺、全球气候变化和太阳能相关的光合作用一般和基本方面的演讲。此外，将继续通过与科学作家的接触，利用当地和全国受欢迎的印刷品和基于网络的出版场所，向广大受众传播光合作用研究的目标、相关性和应用。该项目由化学部生命过程化学项目共同资助

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