Unique Properties of the Chlorophyll in the Cytochrome b6f Complex

这项研究的目的是，由生物科学理事会分子与细胞生物科学部分子生物物理学和数学与物理科学理事会物理部生物物理项目共同支持的项目是了解氧化光合作用中细胞色素b6f复合体中叶绿素（Chl） a防止单线态氧形成的新的独特机制，并解决Chl a在该复合体中所起的不寻常作用。最近的x射线结构表明β -胡萝卜素离Chl - a太远，无法通过传统的直接三重态-三重态能量转移机制保护蛋白质免受单线态氧的形成。提出了两种非常规的保护机制：(i) Chl单线态激发态被电子转移交换机制猝灭；（ii）从Chl a到β -胡萝卜素的远程氧介导的三重态能量流。为了验证这些假设，将在氧气控制条件下使用特定位点诱变和时间分辨光学光谱进行一系列实验。此外，还将采用几种基于结构的数值方法对观测数据进行建模。这些研究将对色素激发态的猝灭机制、活性氧的猝灭和转移机制、氧通过膜内蛋白复合物的转移途径以及细胞色素b6f复合物的结构等方面产生重大的新见解。作为一个涉及生物和物理系的跨学科项目，该项目将高科技激光光谱和量子力学建模与定点诱变相结合，为物理和生物学生提供独特的体验，并将帮助他们在物理和生物科学方面打下坚实的基础。这项研究的结果还将用于各种“推广”计划和课程，旨在提高大学和高中生对现代科学跨学科性质的认识。

The objective of this research, jointly supported by Molecular Biophysics in the Division of Molecular and Cellular Biosciences in the Directorate for Biological Sciences and the Biological Physics Program in the Division of Physics in the Mathematical and Physical Sciences Directorate is to understand the new and unique mechanisms of protection against singlet oxygen formation by the chlorophyll (Chl) a in the cytochrome b6f complex of oxygenic photosynthesis and to address the unusual role the Chl a plays in this complex. The recent X-ray structures indicate that the beta-carotene is too far from the Chl a to protect the protein from singlet oxygen formation by conventional direct triplet-triplet energy transfer mechanism. It is proposed that the Chl a is protected through two unconventional mechanisms: (i) Chl singlet excited state quenching by electron transfer exchange mechanism and (ii) long-range oxygen mediated triplet energy flow from the Chl a to the beta-carotene. To test these hypotheses a series of experiments will be performed using site-specific mutagenesis and time-resolved optical spectroscopy in oxygen controlled conditions. In addition, several structure-based numerical methods will be applied to model the observed data. These studies will lead to major new insights on the mechanism of quenching of pigment excited states, the mechanism of quenching and transfer of reactive oxygen species, pathways of oxygen transfer through intramembrane protein complexes, and the structure of the cytochrome b6f complex.Being an interdisciplinary project involving Biology and Physics departments, this project offers physics and biology students a unique experience that combines high-tech laser spectroscopy and quantum mechanical modeling with site directed mutagenesis, and will help them to develop a strong foundation in physics and biosciences. The results of this research will be also used in various "outreach" programs and courses designed to increase the awareness of university and high school students of the interdisciplinary nature of modern science.