Unique Properties of the Chlorophyll in the Cytochrome b6f Complex

这项研究的目的是由生物科学局分子和细胞生物科学部的分子生物学和数学和物理科学局物理部的生物物理计划联合支持，旨在了解细胞色素b6f氧合作用中叶绿素(Chl)a防止单线态氧形成的新的独特机制，并解决Chla在该复合体中所起的特殊作用。最近的X射线结构表明，β-胡萝卜素离Chla太远，不能通过传统的直接三重态能量转移机制保护蛋白质免受单态氧形成的影响。认为Chla是通过两种非常规机制受到保护的：(I)Chl单态激发态通过电子转移交换机制猝灭；(Ii)氧介导的长程三重态能量从Chla流向β-胡萝卜素。为了验证这些假设，将在氧气控制的条件下使用定点突变和时间分辨光学光谱进行一系列实验。此外，还将使用几种基于结构的数值方法来模拟观测数据。这些研究将对色素激发态猝灭的机制、活性氧的猝灭和转移机制、膜内蛋白质复合体的氧转移途径以及细胞色素b6f复合体的结构等方面有重大的新见解。作为一个涉及生物学和物理系的交叉学科项目，该项目为物理和生物学专业的学生提供了一种将高科技激光光谱和量子力学模拟与定点突变相结合的独特体验，并将帮助他们发展坚实的物理和生物科学基础。这项研究的结果还将用于各种“外展”计划和课程，旨在提高大学生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.