Tomohiro Miyatake

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.

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.

Artificial Light‐Harvesting Antennae: Singlet Excitation Energy Transfer from Zinc Chlorin Aggregate to Bacteriochlorin in Homogeneous Hexane Solution

Abstract— Zinc chlorins possessing 31‐hydroxyl and 131‐carbonyl groups self‐assemble in nonpolar solvents, such as hexane, in a manner similar to bacteriochlorophyll c in the chlorosomes of green photosynthetic bacteria. Visible absorption and steady‐state fluorescence measurements of zinc chlorin aggregates containing a small amount of the bacteriochlorin‐zinc chlorin dyad molecules showed that singlet excitation energy transfer from the zinc chlorin aggregate to the bacteriochlorin moiety of the coaggre‐gated dyad occurs in the homogeneous solution. In the coaggregated dyad, the bacteriochlorin moiety plays the role of an efficient energy trap and the chlorin moiety the role of an anchor to the donor aggregate. The artificial assembly thus mimics the structure and function of natural chlorosomes and can be considered as the first in vitro supramolecular light‐harvesting antenna.

Self‐Assembly of Synthetic Zinc Chlorins in Aqueous Microheterogeneous Media to an Artificial Supramolecular Light‐Harvesting Device

Self-assembled aggregates of a synthetic zinc chlorin in an aqueous suspension with either α-lecithin or Triton X-100 exhibit unique structural and functional properties. Absorption, circular dichroism, fluorescence, and resonance Raman spectra indicate that the supramolecular structure in an aqueous microheterogeneous medium is very similar to that of the bacteriochlorophyll c aggregates in non-polar organic solvents and in chlorosomes, the main light-harvesting antennae of green photosynthetic bacteria. The nature of the aggregates is controlled by structure and/or concentration of the added surfactants. When a small amount of metal-free bacteriochlorin is present it acts as an efficient energy acceptor from the aggregated zinc chlorins. Thus, self-assembly of synthetic zinc chlorins, doped with appropriate energy acceptors and surrounded with surfactants, affords an artificial supramolecular light-harvesting device in aqueous environment.

Synthesis and self-assembly of amphiphilic zinc chlorins possessing a 31-hydroxy group

Abstract Amphiphilic zinc chlorins possessing a 3 1 -hydroxy group were prepared. Non-ionic (oligo)oxyethylene group, anionic sulfonate and cationic quaternary ammonium groups were introduced on the 17-position of a zinc chlorin moiety. All the three kinds of amphiphilic chlorophyll derivatives self-assembled in an aqueous medium to favor a formation of the anti-parallel dimeric structure. In contrast, the amphiphilic zinc chlorins formed large aggregates in non-polar organic solvent. These studies showed self-assemblies of amphiphilic zinc chlorin chromophores were controlled in environments.

Supramolecular Structure of Self-assembled Synthetic Zinc-131-oxo-chlorins Possessing a Primary, Secondary or Tertiary Alcoholic 31-Hydroxyl Group: Visible Spectroscopic and Molecular Modeling Studies¶

Abstract Zinc-chlorin 3 (see Fig. 2 in text) possessing a tertiary 31-hydroxyl group and a 13-keto group was synthesized as a model for the antenna chlorophylls of green bacteria. Self-aggregation of 3 in nonpolar organic media was examined and compared to 1 and 2 possessing a primary and secondary 31-hydroxyl group, respectively. Zinc-chlorin 3 self-aggregated in 1 vol% CH2Cl2–hexane to form oligomers and showed a red-shifted Qy maximum at 704 nm compared to the monomer (648 nm in CH2Cl2). This red-shift is larger than that of 2S (648 → 697 nm) and comparable to that of 2R (648 → 705 nm), but smaller than that of 1 (648 → 740 nm), indicating that while a single 31-methyl group (prim-OH → sec-OH) suppressed close and/or higher aggregation, the additional 31-methyl group (sec-OH → tert-OH) did not further suppress aggregation. The relative stability of the aggregates was in the order 1 > 2R ∼ 3 > 2S as determined by visible spectral analyses. Molecular modeling calculations on dodecamers of zinc-chlorins 1, 2R and 3 gave similar well-ordered energy-minimized structures, while 1 stacked more tightly than 2R and 3. In contrast, 2S gave a relatively disordered (twisted) structure. The calculated dodecameric structures could explain the visible spectral data of 1–3 in nonpolar organic media.

Pure and scrambled self-aggregates prepared with zinc analogues of bacteriochlorophylls c and d.

Zinc analogues of bacteriochlorophylls c and d self‐assembled in aqueous media with phospholipids. A methanol solution of zinc chlorin and α‐lecithin was put in a cellulose tube and the inner methanol solvent was gradually replaced with water by dialysis to form the self‐assembled oligomers. Visible absorption spectra of the aqueous solution showed that zinc chlorins formed J‐aggregates within the hydrophobic core of α‐lecithin assemblies and that the supramolecular structure of the aggregates depended upon the stereochemistry at the 31‐position and the alkyl substituents at the 8‐, 12‐, and 174‐positions of the zinc chlorin. When the aqueous aggregates were prepared with a mixture of 31‐epimers and/or 8‐, 12‐, or 174‐homologues of zinc 31‐hydroxy‐131‐oxochlorins, the structurally distinct components coaggregated to make scrambled oligomers. However, during the dialysis, zinc 31‐hydroxy‐ and 71‐hydroxy‐131‐oxochlorins slowly individually aggregated to give two structurally different oligomer units in the cellulose tube. In contrast, if the two zinc chlorin components rapidly self‐assembled in an aqueous medium, these components coaggregated to form scrambled oligomers. The present study shows that both the molecular structure of the pigments and the speed of the oligomerization determine the molecular arrangement in chlorosome‐type self‐assembled oligomers.

Amplifier-Mediated Activation of Cell-Penetrating Peptides with Steroids: Multifunctional Anion Transporters for Fluorogenic Cholesterol Sensing in Eggs and Blood†

Off-the-shelf, pret-a-porter multianalyte sensing systems are based on the ability of cell-penetrating peptides (CPPs; see picture, blue) to mediate the export of hydrophilic anions (yellowgreen) from lipid bilayer vesicles (gray). Covalent capture of hydrophobic analytes (black) with hydrophilic anions (red) produces amphiphilic anions that can activate CPPs.

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.

Facile Synthesis of 131-Oxo-porphyrins Possessing Reactive 3-Vinyl or 3-Formyl Group, Protochlorophyll-a/d Derivatives by 17,18-Dehydrogenation of Chlorins

Methyl protopyropheophorbides-a/d were prepared by modification of readily available chlorophyll-a. The key step was oxidation of 131-oxo-chlorin to the corresponding porphyrin, and 2,3-dichloro-5,6-dicyano-p-benzoquinone was effective for 17,18-dehydrogenation. A reduction in oxidation potential of the chlorin was necessary for the oxidation. Facile oxidation was achieved by insertion of zinc as the central metal, reduction of the 13-carbonyl group and/or aggregation by high concentration. The synthetic 131-oxo-porphyrins had vinyl and formyl groups at the 3-position. These groups were very reactive in the molecule and selectively converted to other functional groups.