Bacterial Reaction Centers with New Photochemical Properties

在这个项目中，将对紫色细菌球形红杆菌的光合反应中心进行修改，以纳入涉及主要电子供体（细菌叶绿素二聚体）的原始氧化反应。这种策略已经被证明是成功地提高了反应中心的氧化能力，使附近的酪氨酸氨基酸残基氧化。各种技术，如振动、磁共振和瞬态光谱学将被用于修饰反应中心的表征。具体目标包括：(1)了解质子转移在氨基酸自由基生成中的作用。最近发展的氢提取模型提出了质子在酪基自由基功能中的关键作用。特定质子的参与将通过对可能接受质子的氨基酸残基改变的突变体进行测量来确定。(2)结合能被反应中心氧化的锰类化合物的结合位点。光系统II中的水氧化发生在锰簇中，这仍然是光合作用在分子水平上理解的一个具有挑战性的方面。高氧化反应中心在越来越复杂的水平上氧化锰的能力将被表征。(3)用二聚体中的叶绿素取代细菌叶绿素，提高高氧化反应中心的电子转移率。由于细菌叶绿素二聚体使用较低能量的近红外辐射激发，修饰反应中心的产率目前受到限制。这一限制将被叶绿素的加入所克服，叶绿素能吸收更高能量的可见辐射。在所有光合系统中，光转化为化学能的主要过程发生在色素-蛋白质复合物中。通过将这些复合体中的初级过程与次级过程相结合，光合作用装置能够产生富含能量的化合物。虽然特定的途径因生物而异，但能量转换的一般模式是非常保守的，对这一过程的理解是当前模拟光合作用的人工系统设计的基础。通过设计涉及氨基酸自由基、金属配合物和替代色素的反应来扩展反应中心的能力，不仅为研究光合作用和其他生物过程中至关重要的反应提供了一种新的策略，而且还将导致特定底物氧化控制良好的系统的发展。

In this project, modifications of the photosynthetic reaction center from the purple bacterium, Rhodobacter sphaeroides, will be designed to incorporate original oxidation reactions involving the primary electron donor, a bacteriochlorophyll dimer. This strategy has already been shown to be successful in elevating the oxidizing capacity of the reaction center, enabling the oxidation of nearby tyrosine amino acid residues. A variety of techniques such as vibrational, magnetic resonance, and transient optical spectroscopy will be employed in the characterization of the modified reaction centers. Specific goals include: (1) To understand the role of proton transfer in the generation of amino acid radicals. Recently developed hydrogen abstraction models have proposed a critical role for protons in the function of tyrosyl radicals. The involvement of specific protons will be identified by performing measurements on mutants with alterations of possible proton-accepting amino acid residues. (2) To incorporate binding sites for manganese compounds that can be oxidized by the reaction center. Water oxidation in photosystem II takes place at a manganese cluster that remains a challenging aspect of photosynthesis to understand at the molecular level. The ability of the highly oxidizing reaction centers to oxidize manganese at increasingly more complex levels will be characterized. (3) To increase the yield of electron transfer in the highly oxidizing reaction centers by replacement of bacteriochlorophyll with chlorophyll in the dimer. The yield of the modified reaction centers is presently restricted because the bacteriochlorophyll dimer uses relatively low energy near infrared radiation for excitation. This limitation will be overcome by incorporation of chlorophyll, which absorbs higher energy visible radiation, into this site. In all photosynthetic systems, the primary conversion of light into chemical energy occurs in pigment-protein complexes. By coupling the primary process in these complexes to secondary events, the photosynthetic apparatus is capable of creating energy-rich compounds. Although the specific pathways vary among organisms, the general pattern of energy conversion is remarkably conserved, and an understanding of this process serves as the basis for current designs of artificial systems that mimic the photosynthesis. The expansion of the capabilities of the reaction center by the design of reactions involving amino acid radicals, metal complexes, and alternate pigments will not only provide a novel strategy for studying reactions that are crucial in photosynthesis and other biological processes, but also will lead to the development of a well-controlled system for oxidation of specific substrates.