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Electrochromic Shift of Chlorophyll Absorption in Photosystem I from Synechocystis sp. PCC 6803: A Probe of Optical and Dielectric Properties around the Secondary Electron Acceptor

Naranbaatar Dashdorj ^*, Wu Xu , Peter Martinsson , Parag R. Chitnis  and Sergei Savikhin ^*

^* Department of Physics, Purdue University, West Lafayette, Indiana 47907; Department of Biochemistry, Biophysics, and Molecular Biology, and Department of Chemistry, Iowa State University, Ames, Iowa 50011

Correspondence: Address reprint requests to Sergei Savikhin, E-mail: sergei{at}physics.purdue.edu.

Nanosecond absorption dynamics at 685 nm after excitation of photosystem I (PS I) from Synechocystis sp. PCC 6803 is consistent with electrochromic shift of absorption bands of the Chl a pigments in the vicinity of the secondary electron acceptor A₁. Based on experimental optical data and structure-based simulations, the effective local dielectric constant has been estimated to be between 3 and 20, which suggests that electron transfer in PS I is accompanied by considerable protein relaxation. Similar effective dielectric constant values have been previously observed for the bacterial photosynthetic reaction center and indicate that protein reorganization leading to effective charge screening may be a necessary structural property of proteins that facilitate the charge transfer function. The data presented here also argue against attributing redmost absorption in PS I to closely spaced antenna chlorophylls (Chls) A38 and A39, and suggest that optical transitions of these Chls, along with that of connecting chlorophyll (A40) lie in the range 680–695 nm.

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