Unique Properties of the Chlorophyll in the Cytochrome b6f Complex

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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.