Biophysical studies of the mechanisms of charge separation and recombination in bacterial photosynthetic reaction centers

光合作用是地球上太阳能转化最重要的过程。能量被用来驱动重要的细胞过程，如ATP或NADPH的合成，这些过程随后被用来将二氧化碳还原为碳水化合物。光合作用的主要反应发生在反应中心（reaction center， RC），它是一种具有膜完整性的色素蛋白。RC结构的一个显著特征是存在一个近似的局部C2对称轴，该对称轴由所谓的L-和m -分支中的光合色素定义。在这种情况下，一个重要的问题是蛋白质中的组织环境如何调节结合的假基的光谱和氧化还原特性，从而产生重要的功能后果，例如几乎完全沿着l支的电子转移（ET）。该项目的目的是研究野生型细菌光合反应中心和几种缺乏细菌内生菌素辅助因子HL的突变体中电子转移过程的分子机制，这些突变体导致高产m侧ET。为此，来自球形红杆菌或荚膜红杆菌等紫色细菌的RCs是理想的选择。它们允许直接观察光诱导的电子转移过程，因为完全功能的RC可以生物化学分离，并且开发了通过位点定向诱变来操纵RC结构的协议。从所提出的研究中，期望对电荷转移单向性的机制有更深入的了解。具体来说，固体核磁共振实验如photocidnp和REDOR被设想为获得特定酪氨酸的局部结构信息，该酪氨酸被认为可以调节反应能量并触发电荷分离的关键步骤。

Photosynthesis is the most important process for solar energy conversion on earth. The energy is used to drive important cellular processes such as synthesis of ATP or NADPH, which are subsequently used to reduce CO2 to carbohydrates. The primary reactions of photosynthesis occur in the reaction center (RC), which is a membrane-integral pigment-protein. A striking feature of the RC structure is the presence of an approximate local C2 axis of symmetry defined by the photosynthetic pigmets in the so-called L- and M-branches. An important question in this context is how the organized environment in proteins modulates the spectral and redox properties of bound prosthetic groups with important functional consequences such as electron transfer (ET) almost exclusively along the L-branch. The aim of this project is to investigate the molecular mechanisms of electron transfer processes in wild-type bacterial photosynthetic reaction centers and several mutants that lack the Bacteriopheophytin cofactor HL resulting in a high yield M-side ET. For this goal RCs from purple bacteria like Rhodobacter sphaeroides or Rhodobacter capsulatus are ideally suited. They allow a direct observation of the light-induced electron transfer processes since the fully functional RC can be isolated biochemically and protocols were developed to manipulate the RC structure by site-directed mutagenesis. From the proposed studies a deeper insight into the mechanism of the unidirectionality of charge-transfer is expected. In detail, solid-state NMR experiments like photo-CIDNP and REDOR are envisaged to obtain local structural information on a particular tyrosyl that is believed to modulate reaction energetics and triggers the key step of charge separation.