Rikuhei Tanikaga

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).

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.

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.

Asymmetric synthesis of methyl bacteriopheophorbide-d and analogues by stereoselective reduction of the 3-acetyl to the 3-(1-hydroxyethyl) group

Abstract Asymmetric borane-reduction of 3-acetyl-131-oxo-tetrapyrroles (=bacteriochlorin, chlorin and porphyrin) in the presence of a chiral source gave selectively chiral 3-(1-hydroxyethyl)-131-oxo-tetrapyrroles. Oxazaborolidines were effective as chiral auxiliaries. Reduction with (S)-oxazaborolidines led to the (31S)-alcohol as the major product, whose stereoselectivity was the opposite of that in the same asymmetric reduction of usual prochiral ketones ArCOR. The asymmetric reduction of the 3-acetyl group in methyl bacteriopyropheophorbide-a and the 7,8-oxidation afforded 31-epimeric methyl (31S)-bacteriopheophorbide-d (89% de).

Self-aggregation of synthetic zinc chlorins possessing “inverse” keto and hydroxyl groups

Abstract Zinc 3-acetyl-13 1 -hydroxychlorin Zn- 4 was efficiently prepared as a model for naturally occurring bacteriochlorophyll- d possessing 3 1 -hydroxyl and 13-keto groups. Synthetic model Zn- 4 self-aggregates to form oligomers in non-polar organic solvents as well as the isomeric zinc 3 1 -hydroxy-13 1 -oxochlorin Zn- 1 . Both the in-vitro aggregates are similar with the in-vivo aggregates of bacteriochlorophyll- d in the main light-harvesting antennae of photosynthetic green bacteria.

Selective synthesis of α-sulphenyl-, α-sulphinyl-, and α-sulphonyl-α,β-unsaturated carbonyl compounds by the knoevenagel reaction

Simple treatment of aldehydes with the carbonyl compounds (1)[R2COCH2S(O)nAr; n= 0,1,2] and piperidine stereoselectively produces the condensation products (7), the stereochemistry of which is controlled by the steric requirements of two functional groups COR2 and S(O)nAr in a sulphur-stabilized carbanion intermediate.

Synthesis of isotopically labeled zinc methyl bacteriopheophorbide-d as a model for light-harvesting antenna pigments

Abstract Grignard reaction of methyl pyropheophorbide-d (1) possessing a 3-formyl group with methyl magnesium iodide selectively gave methyl bacteriopheophorbide-d (2) possessing a 3-(1-hydroxyethyl) group. When 13CH3MgI and CD3MgI were used as the Grignard reagents, specifically labeled 2 were afforded, of which zinc complexes 3 serve as model compounds for natural bacteriochlorophylls-d.

Acylations and aldol-type reactions of cyclopropyl α-sulfonyl carbanions

Abstract Acylations and aldol-type reactions of cyclopropyl α-sulfonyl carbanions formed from (1R ∗ ,2R ∗ )-2- alkyl-1-(phenylsulfonyl)cyclopropanes (1R ∗ ,2R ∗ )-3 gave (1R ∗ ,2R ∗ )-1- acyl-, and (1R ∗ ,2R ∗ )-1-(1- hydroxyalkyl)-2-alkyl-1-(phenylsulfonyl)cyclopropanes [ (1R ∗ ,2R ∗ )-6 and (1R ∗ ,2R ∗ )-8 ] in high yields, respectively, but no diastereoisomeric (1S ∗ ,2R ∗ )- products were formed. The carbanion generated from (1S ∗ ,2R ∗ )-2- methyl-1-(phenylsulfonyl)cyclopropane 1S ∗ ,2R ∗ -3 b led to rapid isomerization to provide a single diastereoisomeric (1R ∗ ,2R ∗ )- product .

Conformational effects on lipase-mediated acylations of 2-substituted cyclohexanols

Lipase-mediated acetylations of trans- and cis-2-substituted cyclohexanols gave the corresponding (1R)-cyclohexyl acetates and (1S)-cyclohexanols in high yields and ee, but c-4-tert-butyl-c-2-ethenyl-r-1-cyclohexanol was unreactive owing to the steric interaction between the axial OH group and the axial H atoms at the 3- and 5-positions. In the cis-isomer the OH group occupies an equatorial position to bind to the lipase, and less bulky axial alkenyl and alkynyl groups might not so much prevent acetylations than an alkyl group.

Lipase-mediated diastereoselective and enantioselective acetylations of 3-substituted cyclohexanols

Abstract Lipase-mediated acetylation of four diastereomeric and enatiomeric isomers of 3-substituted cyclohexanols 2 has led to an efficient resolution to provide a single stereoisomer, (1 R ,3 S )-cyclohexyl acetate (1 R ,3 S )- 3 in a high enantiomeric excess.