Michio Kunieda

Synthesis of zinc 3-hydroxymethyl-porphyrins possessing carbonyl groups at the 13- and/or 15-positions for models of self-aggregative chlorophylls in green photosynthetic bacteria.

Zinc 3-hydroxymethyl-protopyropheophorbide-d derivatives 1, 2, and 3 possessing carbonyl group(s) at the 13/15-, 13-, and 15-positions, respectively, and 4 lacking the C=O group were prepared from naturally occurring chlorophyll-a. Zinc porphyrins 1 and 2 possessing the 13-C=O self-aggregated in an aqueous micellar solution, similarly to the self-aggregates of BChls-c/d/e in the main light-harvesting antenna systems of green photosynthetic bacteria, whereas 3 and 4 lacking the 13-C=O failed to make such well-ordered aggregates. These results indicated that both the presence of the C=O group and its situation at the 13-position were preferable for formation of the well-ordered J-aggregates.

Diastereoselective Self-aggregation of Synthetic 3-(1-Hydroxyethyl)-bacteriopyrochlophyll-a as a Novel Photosynthetic Antenna Model Absorbing Near the Infrared Region¶

Abstract 3-Deacetyl-3-(1-hydroxyethyl)bacteriopyrochlorophyll-a (1), 7,8-dihydrobacteriochlorophyll-d possessing 8-ethyl, 12-methyl and 174-phytyl groups, was prepared by modifying naturally occurring bacteriochlorophyll-a. The synthetic 31-epimers were separated by high-performance liquid chromotagraphy, and the absolute configuration at the 31-position was determined by derivatization of 1 to a structurally determined chlorin. A dichloromethane solution of 31R-1 or 31S-1 was diluted with a 1000-fold volume of cyclohexane to give self-aggregation species absorbing light at a near-infrared (NIR) region (<910 nm). The resulting Qy maximum in 31R-1 was 860 nm and redshifted by 2170 cm−1 from the monomeric one, whereas epimeric 31S-1 showed a less redshifted peak at ca 800 nm, with a small dimeric band around 740 nm. Such visible spectra indicated that 31R/S-1 formed different supramolecular structures in the self-aggregates. In contrast, self-aggregation of the 7,8-dehydro-compound 2, bacteriochlorophyll-dP, found in natural antennas of photosynthetic green bacteria showed much smaller diastereomeric control. The self-aggregates of 31R-1 absorbing light in the NIR region would be models of intrinsic membranous light-harvesting complexes 1 in photosynthetic purple bacteria as well as extramembranous antennas in green bacteria.

Stereochemical determination of chlorophyll-d molecule from Acaryochloris marina and its modification to a self-aggregative chlorophyll as a model of green photosynthetic bacterial antennae

Acaryochloris marina is a unique photosynthetic prokaryote containing chlorophyll(Chl)-d as a major photoactive pigment (over 95%). The molecular structure of Chl-d is proposed as the 3-formyl analog of Chl-a. However, the stereochemistry of Chl-d at the 13(2)-, 17- and 18-positions has not yet been established unambiguously. In the first part of this paper, we describe the determination of their stereochemistries to be 13(2)-(R)-, 17-(S)- and 18-(S)-configurations by using 1H-1H NOE correlations in 1H-NMR and circular dichroism spectra as well as chemical modification of Chl-a to produce stereochemically defined Chl derivatives. In the second part of the paper, we report a facile synthesis of a self-aggregative Chl by modifying isolated Chl-d. Since Chl-d was characterized by its reactive 3-formyl group, the formyl group was reduced with t-BuNH2BH3 to afford the desirable Chl, 3-deformyl-3-hydroxymethyl-pyrochlorophyll-d (3(1)-OH-pyroChl-d). The synthetic 3(1)-OH-pyroChl-d molecules spontaneously self-organized to form well-ordered aggregates in a non-polar organic solvent. The self-aggregates are a good model of major light-harvesting antenna systems of green photosynthetic bacteria, chlorosomes, in terms of the following three findings. (1) Both the red-shifted electronic absorption band above 750 nm and its induced reverse S-shape CD signal around 750 nm were observed in 0.5% (v/v) THF-cyclohexane. (2) The stretching mode of the 13-carbonyl group was downshifted by about 35 cm(-1) from the wavenumber of its free carbonyl. (3) The self-aggregates were quite stable on titration of pyridine to the suspension, in comparison with those of natural chlorosomal bacteriochlorophyll-d possessing the 3-(1-hydroxyethyl) group.

In vitro synthesis and characterization of bacteriochlorophyll-f and its absence in bacteriochlorophyll-e producing organisms.

Bacteriochlorophyll(BChl)-f which has not yet been found in natural phototrophs was prepared by chemically modifying chlorophyll-b. The retention time of reverse-phase high-performance liquid chromatography of the synthetic monomeric BChl-f as well as its visible absorption and fluorescence emission spectra in a solution were identified and compared with other naturally occurring chlorophyll pigments obtained from the main light-harvesting antenna systems of green sulfur bacteria, BChls-c/d/e. Based on the above data, BChl-f was below the level of detection in three strains of green photosynthetic bacteria producing BChl-e.

Covalently linked zinc chlorophyll dimers as a model of a chlorophyllous pair in photosynthetic reaction centers

A heterodimer, where zinc pyropheophorbide-a was linked with zinc pyropheophorbide-d through ethylene glycol diester, was prepared, as well as the corresponding homodimers. The synthetic dimers were complexed with methanol in benzene to give folded dimers by mutual Zn...O(Me)-H...O=C13(1) bonding. Such complexes had furthest red (Qy) absorption bands at longer wavelengths than the monomeric species. These red-shifts were ascribable to excitonic coupling of the Qy transition states in the chlorin pi-pi stacking conformer. In the heterodimeric system, a minor band was observed at the shorter wavelength side of the main Qy band. This observation can be explained by an additional contribution of Qy vibronic state to the exciton-coupled states. Based on the experimental results, a pair of chlorophyll(Chl)-d with Chl-a as well as a Chl-d homopair were proposed as dimers in reaction centers of Chl-d dominating cyanobacteria.

Self-aggregation of synthetic bacteriochlorophyll-d analogues possessing a B-ring reduced chlorin pi-system.

Zinc 3(1)-hydroxy-13(1)-oxo-chlorophyll derivatives 3 and 4 having a B-ring reduced chlorin pi-system (C7-C8, C17=C18) were prepared as models of self-aggregative bacteriochlorophyll-d, which are regioisomers of 1 and 2 possessing a natural-type D-ring reduced chlorin pi-system (C7=C8, C17-C18). 3(1)-Epimerically pure forms of secondary alcohol 3 (3-CH(OH)CH(3)) as well as primary alcohol 4 (3-CH(2)OH) were effectively synthesized by modifying naturally available bacteriochlorophyll-a. Self-aggregation of 3 and 4 in an aqueous micellar solution was examined by UV-vis and CD spectroscopies and compared with that of their regioisomeric 1 and 2.

Synthesis of Regioisomeric Bacteriochlorophyll-d Analogues Possessing a Hydroxy Group at the 21- or 31-Position: Their Chlorosome-Like Self-Aggregation in Solid Films

Zinc 2(1)- and 3(1)-hydroxy-13-carbonylhexaethylporphyrins 2 and 3 were synthesized by modifying easily available octaethylporphyrin, and the HPLC-separated regioisomers were fully characterized. The latter compound 3(1)-OH-13-C=O-3, which structurally resembled naturally self-aggregative bacteriochlorophyll-d (3(1)-OH/13-C=O), was preferable to make ordered J-aggregates in the solid film, compared to its regioisomeric 2(1)-OH-13-C=O-2. In contrast, more ordered oligomers were obtained by self-aggregation of zinc porphyrin 1 possessing the interactive moieties (3(1)-OH, Zn, 13-C=O) at the same positions, which was prepared by modifying natural chlorophyll-a molecule. The difference between self-aggregation of 1 and 3 was ascribable to the methyl and ethyl groups attached at the 2-position neighboring the 3(1)-OH group which is requisite for self-aggregation; the steric environment around the interactive OH group is important to make highly ordered supramolecules through strong molecular packings.

Self-aggregation behavior of synthetic zinc 3-hydroxymethyl-13/15-carbonyl-chlorins as models of main light-harvesting components in photosynthetic green bacteria

Zinc complexes of 3-hydroxymethyl-13/15-carbonyl-chlorins having a six-membered lactone as the E-ring were prepared by modifying purpurin-18 as models of bacteriochlorophyll-d, one of the chlorophyllous pigments in the main light-harvesting antenna systems (chlorosomes) of green photosynthetic bacteria. The synthetic 13-carbonylated compound self-aggregated in 1%(v/v) tetrahydrofuran and hexane to give large oligomers possessing red-shifted and broadened electronic absorption bands and intense circular dichroism bands at the shifted Qy region, indicating that the supramolecular structure of the resulting self-aggregate was similar to those of natural and artificial chlorosomal aggregates. The red-shift value observed here was smaller than the reported values in chlorosomal pigments having a five-membered keto-ring, which was ascribable to a weaker intermolecular hydrogen-bonding of 13-C=O with 31-OH in a supramolecule of the former self-aggregate and suppression of the π–π interaction among the composite chlorins. On the other hand, the isomeric 15-carbonylated molecule was monomeric even in the nonpolar organic solvent, confirming the reported proposal that the linear orientation of three interactive moieties, OH, C=O and Zn, in a molecule is requisite for its chlorosomal self-aggregation.

Self-aggregation of synthetic zinc 31-hydroxy-131-oxo-17,18-cis-chlorin in a non-polar organic solvent

Zinc methyl 31-demethyl-17,18-cis-bacteriopheophorbide-d was prepared as a model of naturally occurring bacteriochlorophyll-d. From the absorption spectral analysis, the synthetic cis-chlorin self-aggregated in a non-polar organic solvent to give an oligomer possessing red-shifted and broadened peaks, compared to the non-aggregated form. The red-shifted values by self-aggregation were smaller than those observed in the corresponding trans-isomer, indicating that self-aggregates of the cis-isomer took on more disordered supramolecular structures than those of the trans-isomer.

Synthesis of meso-tetraarylporphyrins possessing amino and carboxy groups and their peptides

meso-Tris(3,5-di-tert-butylphenyl)porphyrin (P-H) is bonded with L-phenylalanine (H-Phe-OH) directly at the unsubstituted meso-position of the former and the p-position of phenyl group of the latter to afford chiral porphyrin-amino acid conjugate H-Phe(p-P)-OH. The N-(9-fluorenyl)-methyloxycarbonyl compound, Fmoc-Phe(p-P)-OH, was synthesized without any loss of enantiomeric purity (based on chiral HPLC analysis) from commercially available L-tyrosine and was useful for preparation of the peptides in both liquid and solid phases. Other meso-tetraarylporphyrins possessing multi-amino acid moieties are reported, as well as achiral porphyrin-amino acids readily prepared and their dipeptidyl porphyrin dyads.