STRUCTURE & FUNCTION OF BACTERIAL PHOTOSYNTHETIC UNIT

光合作用单位(PSU)位于胞质内 紫色细菌的膜，由两种类型的 色素蛋白复合体：光合作用反应中心(RC) 和捕光复合体(LHS)。LHS捕捉阳光并 将激励能量转移到RC，在那里它会启动一个电荷 分离过程。我们已经构建了整个宇宙的原子模型 细菌PSU*由一个被阵列包围的L H-I-RC复合体组成 通过X-射线结晶学、电子能谱和电子能谱相结合 显微镜和分子建模[66-70]。这一成就开启了 通向光谱和理论研究途径的大门 关于激发转移和潜在的转移机制 细菌光合膜。激励传递 途径可分为两个步骤：分子内激发 色素-蛋白质复合体中的色素之间的转移，以及 不同色素蛋白之间的分子间激发转移 复合体。在捕光复合体中，光子可以被吸收。 要么是通过类胡萝卜素瞬间转移它们的激发能量 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。 类胡萝卜素和BChls通过Pariser-Parr-Pple自洽 其电子状态的字段/配置项描述，从而识别 最可能的激发途径和主导机制 类胡萝卜素和BChls之间的转移[71]。激励传递 不同的捕光复合体之间通过 BChl环的相互作用。环形的BCHL聚集在 通过量子化学和有效哈密顿算符发现了LHS 显示相干激发态属性的计算 最适合励磁传输[70，72，73]。根据 有效哈密顿描述我们已经确定了激发 转移率Lh-II。LH-I。RB的PSU的RC。Sphaeroides[68， 70，72]，与光谱测量结果吻合较好。 我们的量子计算也揭示了 辅助BChls作为Lh-I BChls向BChls的激发传递的介体 RC特殊配对。

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.