The specific aims of the proposed research are to characterize the structural, vibronic, and electronic properties of the functionally important cofactors in reaction center (RC) proteins. The principal investigative tool is resonance Raman (RR) spectroscopy. The first major objective is to characterize the properties of the bacteriochlorophyll/bacteriopheophytin (BCh1/BPh) in genetically modified bacterial RCs that exhibit unusual electron-transfer properties. The genetic modifications include replacements near the primary electron donor (P), both accessory BChls, and the BPh on the photophysically, active L branch of the protein (which is the primary electron acceptor). The studies focus on three general classes of genetically modified RCs: (1) Mutants in which the hydrogen bonding interactions and/or the electric fields in the vicinity of BCh1L, BCh1M, and BPhL are altered by addition/deletion of amino acid residues near ring V of the BCh1/BPh macrocycle. A particular focus of these studies concerns the effects of placing (potentially) charged residues near the photoactive cofactors. (2) Double (and higher order) mutants which incorporate the replacements characteristic of class 1 into a background in which BPhL is replaced with a BCh1 molecule (beta-type RCs). (3) Mutants in which the histidine axial ligands to the BChls of P are replaced by non-ligating glycine residues (cavity mutants). In all cases the RR studies will be conducted on RCs whose detailed electron-transfer kinetics have been elucidated via time-resolved optical experiments. The second major objective is to conduct RR studies aimed at refining the structure of oxygen- evolving complex in photosystem (PS) II RCs. The particular target of these studies is the manganese cluster, which directly mediates the water-splitting/oxygen-evolution reaction. The focus will be the low-frequency region of the spectrum where manganese-ligand vibrations are expected to occur. Toward this end, RR data will be acquired for PSII in which isotopic labels (2H, 18O, 15N, 37C1-/37C1- ) have been incorporated and/or essential ions such as Ca+2 and C1- have been exchanged (for example, Sr+2 for Ca+2 or Br- for C1-). The long-term objective of the studies on bacterial and PSII RCs is to determine how the physical properties of the cofactors (structure, conformation, electron-density distribution) govern and/or reflect their functional characteristics (electron transfer/charge separation across the biological membrane; water splitting/oxygen evolution).
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