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ABSTRACT 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 ChI a pigments in the vicinity of the secondary electron acceptor A^sub 1^. 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.
INTRODUCTION
Electrostatic forces play a crucial role in the conformation and function of biomolecules, especially in the protein complexes that carry out charge transfer functions. Due to the presence of a mixture of neutral, polar, and charged chains in proteins, the charge transfer processes involve complex reorganization of the local protein environment, leading to effective charge screening (Treutlein et al., 1992; Steffen et al. 1994; Simonson and Brooks, 1996). This local reorganization, along with the redox potential difference between electron donor and acceptor, is a key factor in defining the efficiency of electron transfer (Moser et al., 1992). The fundamental constant defining the strength of electrostatic screening is the relative static dielectric permittivity (dielectric constant) [epsilon]^sub r^. Numerous model calculations predict that a typical average dielectric permittivity for protein in water ranges from 10 to 80 (King et al., 1991; Smith et al., 1993; Simonson and Brooks, 1996; Loffler et al., 1997; Simonson, 1998; Pitera et al., 2001). Major contributors to this value are relatively flexible polar side chains on the outer surface of the protein. The dielectric constant deep within the protein is expected to drop to 2-4, which agrees with the known low polarizability of dry powders where the absence of water restricts the mobility of the peripheral side chains (Rosen, 1963;...