Hitoshi Tamiaki

Synthetic Zinc and Magnesium Chlorin Aggregates as Models for Supramolecular Antenna Complexes in Chlorosomes of Green Photosynthetic Bacteria

Abstract— A comparison of the spectra of in vitro (3‐hydroxymethyl‐131‐oxometallochlorin) and in vivo chlorosomal (bacterio‐chlorophyll‐c) aggregates suggests a similar supramolecular structure for the artificial oligomers and the bacte‐riochlorophyll‐c aggregates in the extramembranous antenna complexes (chlorosomes) of green photosynthetic bacteria. Synthetic zinc and magnesium chlorins have been found to aggregate in 1 % (vol/vol) tetrahydrofuran and hexane solutions and in thin films to form oligomers with the Qy absorption bands shifted to longer wavelengths by about 1900 (Zn chlorins) and 2100 cm−1 (Mg) relative to the corresponding monomer bands. Visible absorption and circular dichroism spectra of various zinc chlorins establish that a central metal, a 31‐hydroxy and a 131‐keto group are functional prerequisites for the aggregation. Vibrational bands measured by IR spectroscopy of solid films reveal two characteristic structural features of the oligomers: (1) a five‐coordinated metallochlorin macrocycle with an axial ligand (bands at 1500‐1630 cm−1), and (2) a hydrogen bond between the keto oxygen of one chlorin and the hydroxy group of a second chlorin, the oxygen of which is chelated to the metal atom of a third molecule, i.e. C=O…H‐O…M (=Zn or Mg).

SUPRAMOLECULAR STRUCTURE IN EXTRAMEMBRANEOUS ANTENNAE OF GREEN PHOTOSYNTHETIC BACTERIA

Abstract Supramolecular structures in extramembraneous antennae of green photosynthetic bacteria (so-called “chlorosomes”) are reviewed. In chlorosomes, bacteriochlorophylls- c, d and e (magnesium complexes of chlorins with a 1-hydroxyethyl group) self-aggregate to form the main light-harvesting components. This arrangement is unique and different from that in other antennae where the pigments bond with some proteins. Model studies of artificial aggregates and comparison of in-vitro aggregates with in-vivo aggregates provide useful insights in the elucidation of the supramolecular structures. The pigments in natural chlorosomes self-aggregate with the assistance of a special hydrogen-bond, C  O … H(X)O … Mg and π-π interaction of the cyclic tetrapyrroles. Magnesium chlorins thus form a two-dimensional sheet and subsequent rolling of the sheet makes a rod of the main components in chlorosomes.

The 17-propionate function of (Bacterio)chlorophylls : Biological implication of their long esterifying chains in photosynthetic systems

Molecular structures of (bacterio)chlorophylls [=(B)Chls] in photosynthetic apparatus are surveyed, and a diversity of the ester groups of the 17‐propionate substituent is particularly focused on in this review. In oxygenic photosynthetic species including green plants and algae, the ester of Chl molecules is limited to a phytyl group. Geranylgeranyl and farnesyl groups in addition to phytyl are observed in (B)Chl molecules inside photosynthetic proteins of anoxygenic bacteria. In main light‐harvesting antennas of green bacteria (chlorosomes), a greater variety of ester groups including long straight chains are used in the composite BChl molecules. This diversity is ascribable to the fact that chlorosomal BChls self‐aggregate to form a core part of chlorosomes without any specific interaction of oligopeptides. Biological significance of the long chains is discussed in photosynthetic apparatus, especially in chlorosomes.

X-ray crystal structure of the light-independent protochlorophyllide reductase

Photosynthetic organisms adopt two different strategies for the reduction of the C17 = C18 double bond of protochlorophyllide (Pchlide) to form chlorophyllide a, the direct precursor of chlorophyll a (refs 1–4). The first involves the activity of the light-dependent Pchlide oxidoreductase, and the second involves the light-independent (dark-operative) Pchlide oxidoreductase (DPOR). DPOR is a nitrogenase-like enzyme consisting of two components, L-protein (a BchL dimer) and NB-protein (a BchN–BchB heterotetramer), which are structurally related to nitrogenase Fe protein and MoFe protein, respectively. Here we report the crystal structure of the NB-protein of DPOR from Rhodobacter capsulatus at a resolution of 2.3 Å. As expected, the overall structure is similar to that of nitrogenase MoFe protein: each catalytic BchN–BchB unit contains one Pchlide and one iron–sulphur cluster (NB-cluster) coordinated uniquely by one aspartate and three cysteines. Unique aspartate ligation is not necessarily needed for the cluster assembly but is essential for the catalytic activity. Specific Pchlide-binding accompanies the partial unwinding of an α-helix that belongs to the next catalytic BchN–BchB unit. We propose a unique trans-specific reduction mechanism in which the distorted C17-propionate of Pchlide and an aspartate from BchB serve as proton donors for C18 and C17 of Pchlide, respectively. Intriguingly, the spatial arrangement of the NB-cluster and Pchlide is almost identical to that of the P-cluster and FeMo-cofactor in nitrogenase MoFe-protein, illustrating that a common architecture exists to reduce chemically stable multibonds of porphyrin and dinitrogen.

Cyclic tetrapyrrole based molecules for dye-sensitized solar cells

In this Perspective, recent progress on dye-sensitized solar cells (DSSCs) based on cyclic tetrapyrrole type sensitizers including porphyrins, (bacterio)chlorins, and phthalocyanines has been reviewed. Cyclic tetrapyrrole type molecules have been studied extensively with respect to their photochemical and photophysical properties as well as to various applications. The photophysical properties of tetrapyrrole molecules can be readily controlled upon molecularly structural modification. Their low-lying singlet states are also suitable for studying excited state dynamics in the electron donor–acceptor systems. Here, we selected the most representative porphyrin, (bacterio)chlorin, and phthalocyanine sensitizers to discuss how those structural modifications on the cyclic tetrapyrrole rings affect the performance of DSSCs. The most important factors that strictly determine the power conversion efficiencies of DSSCs based on tetrapyrrole type sensitizers are also discussed in detail.

Self-aggregation of synthetic zinc chlorins with a chiral 1-hydroxyethyl group as a model for in vivo epimeric bacteriochlorophyll-c and d aggregates

Abstract 31-Epimerically pure zinc 3-(1-hydroxyethyl)-131-oxochlorins possessing several substituents at the 20-position were prepared. In non-polar organic solvents, the synthetic zinc complexes self-aggregated to form oligomers with >700-nm absorption and giant CD peaks, which were dependent upon the 31-absolute configuration as well as the 20-substituents. The in vitro self-aggregates of each epimeric zinc chlorin with a 20-methyl group showed similar visible and CD spectra with the in vivo bacteriochlorophyll-c ( 3 1 R S = 2 1 ) aggregates in extramembranous antennae of a green photosynthetic bacterium. The spontaneous in vitro self-aggregates of 3 1 R S (= 2 1 )- epimeric mixture of the zinc 20-methylchlorins were different from the natural supramolecules, indicating that in vivo slow oligomerization of 3 1 R S (= 2 1 )- bacteriochlorophylls -c induced the regular supramolecular structures and/or the epimerically separated assemblies.

Chlorophyll-a Derivatives with Various Hydrocarbon Ester Groups for Efficient Dye-Sensitized Solar Cells: Static and Ultrafast Evaluations on Electron Injection and Charge Collection Processes

Five chlorophyll-a derivatives, chlorins-1-5 possessing C3(2)-carboxy and O17(4)-esterified hydrocarbon groups including methyl, hexyl, dodecyl, 2-butyloctyl, and cholesteryl were synthesized. Their performance as sensitizers in dye-sensitized solar cells (DSSCs) was compared. These sensitizers have similar surface coverage on the unit surface of TiO(2) film and their absorption spectra on transparent TiO(2) films were identical. On the basis of DFT and TD-DFT calculations of these sensitizers in ethanol, a major difference between them was the geometry of the hydrocarbon ester group, to affect their electron injection and charge recombination with the TiO(2) electrode rather than the energy level of their molecular orbitals. DSSC based on chlorin-3 with a dodecyl ester group gave a solar energy-to-electricity conversion efficiency of 8%, which was the highest among all the chlorophyllous sensitizers. The large photocurrent in the chlorin-3 sensitized solar cell can be explained by the least impedance in the electrolyte-dye-TiO(2) interface in electrical impedance spectroscopy measurements. Subpicosecond time-resolved absorption spectroscopic studies have also been carried out to evaluate the electron injection and charge recombination dynamics in the dye-TiO(2) interface. For the electron injection and charge recombination processes, a charge separated state of the dye-TiO(2) complex has been found to be free from the type and concentration of dye sensitizer, reflecting the same type of electron transfer process for all the five chlorin sensitizers. A new quenching pathway of the dye excitation, which is probably from the exciton annihilation, in addition of the charge recombination has been observed for chlorin-1 and chlorin-5, but not for chlorin-3. The higher open-circuit photocurrent observed in the present dyes with larger ester groups can be attributed to the reduced leaking of charges in the TiO(2)-electrolyte interface, which was supported by the longer electron lifetimes.

Synthetic zinc tetrapyrroles complexing with pyridine as a single axial ligand

Zinc chlorins were prepared from chlorophyll-a. Visible spectra in benzene showed that synthetic zinc chlorins complexed with pyridine as an axial ligand to form the monopyridine adducts. The equilibrium constants for the complexation were dependent upon the chlorin structure: substitution of electron-withdrawing groups at the peripheral position enhanced the coordinated ability of the central zinc. 1H NMR spectra in benzene-d6 also indicated that single pyridine coordinated to the central zinc. Comparison of the equilibrium constant in a zinc chlorin with those of the corresponding zinc bacteriochlorin (7,8-dihydrochlorin) and porphyrin (17,18-dedihydrochlorin) led to an increase in the saturation and flexibility of the tetrapyrrole pi-plane ligands making the central zinc more axial-ligated. All the zinc tetrapyrroles in benzene complexed with pyridine to form 5-coordinated (1:1) complexes, not 6-coordinated bisadducts. The observed equilibrium constants were consistent with the energy changes of the complexation calculated from molecular modeling.

Aggregation of synthetic zinc chlorins with several esterified alkyl chains as models of bacteriochlorophyll-c homologs

Zinc complexes of 3-hydroxymethyl-13l-oxochlorin possessing several branched alkyl chains as an esterified group at the 17-position were systematically prepared. In non-polar organic solvents, these compounds aggregated to form oligomers absorbing up to 800-nm light with around a 740-nm peak, which are good models for bacteriochlorophylls-c and d, extramembranous antenna pigments of photosynthetic green bacteria. All the visible spectra of the in-vitro oligomers are the same and the esterified alcohols induced no effect on the local structure of the oligomers. The circular dichroism spectra changed with elongation of esterified alkyl chains, which resulted in the similar spectra to the in-vivo oligomers. Esterified alcohols should subtly affect the supramolecular structure and/or stability of the self-aggregates.

A Second Nitrogenase-like Enzyme for Bacteriochlorophyll Biosynthesis RECONSTITUTION OF CHLOROPHYLLIDE a REDUCTASE WITH PURIFIED X-PROTEIN (BchX) AND YZ-PROTEIN (BchY-BchZ) FROM RHODOBACTER CAPSULATUS

In most photosynthetic organisms, the chlorin ring structure of chlorophyll a is formed by the reduction of the porphyrin D-ring by the dark-operative nitrogenase-like enzyme, protochlorophyllide reductase (DPOR). Subsequently, the chlorin B-ring is reduced in bacteriochlorophyll biosynthesis to form a bacteriochlorin ring structure. Phenotypic analysis of mutants lacking one of three genes, bchX, bchY, or bchZ, which show significant sequence similarity to the structural genes of nitrogenase, suggests that a second nitrogenase-like enzyme is involved in the chlorin B-ring reduction. However, there is no biochemical evidence for this. Here, we report the reconstitution of chlorophyllide a reductase (COR) with purified proteins. Two Rhodobacter capsulatus strains that overexpressed Strep-tagged BchX and BchY were isolated. Strep-tagged BchX was purified as a single polypeptide, and BchZ was co-purified with Strep-tagged BchY. When BchX and BchY-BchZ components were incubated with chlorophyllide a, ATP, and dithionite under anaerobic conditions, chlorophyllide a was converted to a new pigment with a Qy band of longer wavelength at 734 nm (P734) in 80% acetone. The formation of P734 was dependent on ATP and dithionite. High performance liquid chromatography and mass spectroscopic analysis indicated that P734 is 3-vinyl bacteriochlorophyllide a, which is formed by the B-ring reduction of chlorophyllide a. These results demonstrate that the B-ring of chlorin is reduced by a second nitrogenase-like enzyme and that the sequential actions of two nitrogenase-like enzymes, DPOR and COR, convert porphyrin to bacteriochlorin. The evolutionary implications of nitrogenase-like enzymes to determine the ring structure of (bacterio)chlorophyll pigments are discussed.