STRUCTURE & FUNCTION OF BACTERIAL PHOTOSYNTHETIC UNIT

光合单位（PSU）位于胞质内， 膜的紫色细菌和由两种类型的 色素蛋白复合物：光合反应中心（RC） 和捕光复合物（LH）。 LH捕捉阳光， 将激发能量转移到RC，在RC中激发电荷 分离过程。 我们构建了一个原子模型， 细菌PSU *，由阵列包围的LH-I-RC复合物组成 通过X射线晶体学，电子 显微镜和分子建模[66 - 70]。 这一成就开启了 这是对这些途径进行光谱学和理论研究的大门， 激发转移和潜在的转移机制， 细菌的光合膜。 兴奋传递 信号通路可分为两步：分子内激发 色素-蛋白质复合物内色素之间的转移，和 不同色素蛋白分子间激发传递 配合物 在捕光复合体中，光子可以被吸收 或者是通过类胡萝卜素， 细菌叶绿素（BChls），或BChls本身。 我们 计算了各种电子激发之间的耦合， 通过Pariser-Parr-Pople自洽模型 字段/CI描述其电子状态，从而识别 激发的最可能途径和主导机制 类胡萝卜素和BChls之间的转移[71]。 激发转移 不同捕光复合体之间的相互作用是通过 BChl环的相互作用。 环状BChl聚集在 通过量子化学和有效哈密顿量的计算， 计算显示相干激发态特性， 最佳的激励转移[70，72，73]。 的基础上 有效的哈密顿描述，我们已经确定了激发 传输速率LH-II。 LH-I Rb的PSU的RC。sphaeroides [68， 70，72]，这是在良好的协议与光谱测量。 我们的量子计算也揭示了 辅助BChl作为从LH-1 BChl到LH-1 BChl的兴奋转移的介质 RC特殊对。

The photosynthetic unit (PSU) is located in the intracytoplasmic membrane of purple bacteria and consists of two types of pigment-protein complexes: the photosynthetic reaction center (RC) and light-harvesting complexes (LHs). The LHs capture sunlight and transfer the excitation energy to the RC where it initiates a charge separation process. We have constructed an atomic model of the entire bacterial PSU* consisting of an LH-I-RC complex surrounded by an array of LH-Is through a combination of x-ray crystallography, electron microscopy and molecular modeling [66-70]. This accomplishment opens the door to both spectroscopic and theoretical studies of the pathways of excitation transfer and of the underlying transfer mechanisms in the bacterial photosynthetic membrane. The excitation transfer pathway can be divided into two steps: intramolecular excitation transfer between pigments within a pigment-protein complex, and intermolecular excitation transfer between different pigment-protein complexes. Within a light-harvesting complex, photons can be absorbed either by carotenoids that instantly transfer their excitation energy to bacteriochlorophylls (BChls), or by BChls themselves. We calculated the couplings between various electronic excitations of carotenoids and BChls through the Pariser-Parr-Pople self-consistent field/CI description of their electronic states, thereby identifying the most probable pathway and dominant mechanism of the excitation transfer between carotenoids and BChls [71]. Excitation transfer between different light-harvesting complexes occurs through the interactions of rings of BChls. The ring-shaped BChl aggregates in LHs were found by quantum chemical and effective Hamiltonian calculations to display coherent excited state properties that are optimal for excitation transfer [70, 72, 73]. On the basis of the effective Hamiltonian description we have determined the excitation transfer rates LH-II . LH-I . RC for the PSU of Rb. sphaeroides [68, 70, 72], which are in good agreement with spectroscopic measurements. Our quantum calculations have also shed light on the role of the accessory BChls as mediators of excitation transfer from LH-I BChls to the RC special pair.