Energy Transfer and Photo-induced Electron Transfer in Photofunctional Molecule-Copolypeptide Complexes

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Spectroscopic studies of chlorophyll a (Ch1) and/or beta-carotene (beta-Car) /copolypeptide/surfactant systems have been carried out to model the early events in photosynthesis. The copolypeptides were composed of charged amino acid residues (L-Glu, L-Lys or L-Orn) and uncharged aromatic amino acid residues (L-Tyr, L-Trp or L-Phe) at around pH 6.5. We used cetyltrimethylammonium chloride (CTAC) as a cationic surfactant or sodium dodecyl sulfate (SDS) as anionic Surfactant. The following remarkable results were obtained.1. The surfactant ion bound cooperatively onto the copolypeptide chain to give a micelle-like cluster. Ch1 and/or beta-car were incorporated into such clusters to from Ch1 and/or beta-Car-copolypeptide complexes with the ordered structure (alpha-helix or beta-structure).2. The efficiencies of energy transfer from the aromatic amino acid residue or beta-Car to Ch1 in the complexes were determined as a function of surfactant concentration. We also determined energy-transfer parameters such as effective Ch1 concentrations and effective donor-acceptor mean distances on the basis of the Forster theoty.3. Laser flash photolysis studies of the Ch1 copolypeptide complexes revealed that photo-induced electron transfer took place from the aromatic amino acid residue to Ch1. The electron-transfer pathway mechanisms were discussed.4. Resonance Raman spectra of Ch1 in the complexes strongly suggested that Ch1 interacted with L-Tyr and L-Trp, most possibly through hydrogen bond, but not L-Phe.