Research Sector of Structural Biology for Photosynthesis
We study the mechanisms of light-energy absorption, transfer, conversion and water oxidation in natural photosynthesis in various algae and plants, by means of structural biology in combination with a variety of functional approaches. The main targets of our research are photosystem II (PSII) and photosystem I (PSI), two large membrane-protein complexes involved in light energy absorption and conversion, and various light-harvesting pigment antenna systems. PSII utilizes the light-energy to split water, leading to the production of molecular oxygen indispensible for sustaining aerobic life on the earth. We have solved the structure of PSII from a thermophilic cyanobacterium at 1.9 Å resolution (Umena et al. Nature 473, 55-60, 2011), and further obtained its damage-free structure at 1.95 Å resolution (Suga et al. Nature 517, 99-103, 2015) using femtosecond X-ray free electron laser (XFEL) at SACLA, Japan. These studies provided important clues to elucidating the mechanism of light-induced water oxidation. On the other hand, PSI utilizes electrons derived from water via PSII to reduce NADP+, which is then utilized to reduce carbon dioxide into sugars. In higher plants, the core of PSI is surrounded by 4 subunits of light-harvesting complex I (LHCI), forming a PSI-LHCI supercomlex. We have solved the structure of the PSI-LHCI supercomplex from pea at 2.8 Å resolution (Qin et al. Science 348, 989-995, 2015), which provided important information for elucidating the mechanism of highly efficient energy transfer within this supercomlpex.
Research Sector of Functional Biology for Photosynthesis
The Takahashi Group works on the structure, function, and assembly of photosynthetic reaction center complexes (photosystem I and photosystem II) to elucidate how light energy is efficiently converted into redox energy using a green alga Chlamydomonas reinhardtii as a model organism. Our main projects are; 1) elucidation how photosystem I complex is assembled through integrating a number of subunits and cofactors by focusing assembly factors that assist the assembly process, 2) characterization of structure and dynamics of antenna complexes that harvest light energy, 3) identification of amino acid residues that are essential for photochemical reaction or electron and proton transfers, and 4) production of devices to generate electric power and hydrogen by combing photosystem protein complexes and inorganic electrodes using affinity peptide tags.
Research Sector of Artificial Photosynthesis System
New ruthenium(II or III) complexes with the general formula [Ru(O–N)(bpy)2]n+ (O–N = unsymmetrical bidentate phenolate ligand; bpy = 2,2'-bipyridine) were synthesized and their crystal structures and electrochemical properties characterized. The RuII complexes with 2-(2-imidazolinyl)phenolate (Himn–) or 2-(1,4,5,6-tetrahydropyrimidin-2-yl)phenolate (Hthp–) could be deprotonated by addition of excess KOtBu, although the deprotonated species were easily reprotonated by exposure to air. Unlike these RuII complexes, their RuIII analogues showed interesting ligand oxidation reactions upon the addition of bases. With [RuIII(Himn)(bpy)2]2+, two-electron oxidation of Himn– and reduction of the RuIII center resulted in conversion of the 2-imidazolinyl group to a 2-imidazolyl group. On the other hand, the corresponding Hthp– complex exhibited four-electron oxidation of the ligand to form 2-(2-pyrimidyl)phenolate (pym–) (Fig. 1). These aromatization reactions of imidazolinyl and 1,4,5,6-tetrahydropyrimidyl groups were also achieved by the electrochemically generated RuIII complexes.
Fig. 1 Four-electron oxidation of the tetrahydropyrimidyl group induced by oxidation of the Ru center and addition of bases.