Synthetic Chlorophyll Derivatives Research Articles

Direct Assembly in Aqueous Solutions of Stable Chlorophyllide Complexes with Type II Water-soluble Chlorophyll Proteins.

Water-soluble chlorophyll-binding proteins (WSCPs) from Brassicaceae constitute a small family of non-photosynthetic proteins that may provide a useful benchmark and model system for studying molecular aspects of chlorophyll-protein interactions such as the tuning of absorption and emission spectra, and binding selectivity. WSCP apo-proteins are readily expressed by recombinant DNA techniques and can be assembled in vitro with natural and synthetic chlorophyll derivatives. The complexes with native chlorophylls are exceptionally stable toward thermal dissociation and protein denaturation due to hydrophobic interactions with the chlorophyll's phytyl chains that stabilize the core of the WSCP tetrameric complexes. However, assembly requires the use of detergents or water-in-oil emulsions to introduce the hydrophobic pigments into the water-soluble apo-proteins. Here, we explore the direct assembly of recombinant WSCPs with the water-soluble phytyl-free chlorophyll analogue chlorophyllide a in aqueous solutions. We show that the complexes formed by mixing chlorophyllide and WSCP apo-proteins are exclusively tetrameric, and while they lack the extreme thermostability of the respective chlorophyll complexes, they are still thermostable up to around 60°C. Their absorption and CD spectra are very similar to the chlorophyll complexes albeit slight peak shifts and broadening of the bands indicate variations in pigment and protein conformations, and less rigid structures. Simplifying the assembly process of WSCPs opens new possibilities for their use in modelling natural chlorophyll-protein complexes, and as templates for designing novel artificial protein-pigment complexes.

Covalent heterodyads of synthetic chlorophyll derivatives linked with linear rigid substituents at the 20-positions constructing photoexcited energy transfer systems

Chlorophyll and bacteriochlorophyll pigments are important porphyrinoids, which play key roles in light absorption, energy migration/transfer, and electron transfer in natural photosynthesis. Dyads of such pigments have attracted much attention from the viewpoints of mimicking photosynthetic light-harvesting and charge-separating systems. In this study, we report the synthesis and optical properties of heterodyads of zinc chlorin and free-base bacteriochlorin covalently linked by linear rigid substituents at the 20-position, which were prepared from naturally occurring chlorophyll-a. A free-base bacteriochlorin moiety accepted the singlet excitation energy from the zinc chlorin in the heterodyads. In the co-aggregate systems of heterodyads with chlorosomal self-assemblies, the latter transfers the singlet excitation energy to the bacteriochlorin moiety of the former, which are useful for artificial chlorosomal supramolecular light-harvesting systems.

Fabrication and performance of all-solid-state dye-sensitized solar cells using synthetic carboxylated and pyridylated chlorophyll derivatives

All-solid-state dye-sensitized solar cells (DSSCs) based on a mesoporous TiO2 electrode were fabricated using synthetic chlorophyll derivatives as a photosensitizer and 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene as a hole-transport material. Methyl pyropheophorbides-a possessing a carboxy group in the C3-substituent as the anchoring moiety to the TiO2 surface were effective DSSCs. Direct conjugation of the COOH group with the terminus of the C3-vinyl group gave the highest solar energy-to-electricity conversion efficiency (η) of 2.25%. Insertion of a 1,3-phenylene group in the connection partially suppressed the efficiency by 25% (η = 1.68%), whereas insertion of an isomeric 1,4-phenylene moiety decreased to one third (0.75%). A pyridyl group was not useful as the sensitizer anchor due to its low binding affinity to the TiO2 electrode.

Rotational isomerization of 3-substituents in synthetic chlorophyll derivatives

Methyl pyropheophorbides-a possessing a (pseudo)planar substituent at the 3-position were prepared from naturally occurring chlorophyll-a. Some of the semisynthetic π-conjugates with the chlorin skeleton took two atropisomeric conformations for the sterically demanding 3-substituents, CONMe2, NHCOMe, C[CHC(CN)2]C(CN)2, and Ph(2,3,4,5-Ph4). Their rotational isomerization in a solution was analyzed by 1H NMR and HPLC. Zinc complex of the 3-aryl-chlorin gave a large energy barrier for the rotation of the C3C31 single bond (estimated ΔG‡=108 kJ mol−1 at 20 °C) and the atropisomerically pure conformers were separated at room temperature.

Inside Cover: Enhancement of Light Absorption Ability of Synthetic Chlorophyll Derivatives by Conjugation with a Difluoroboron Diketonate Group (Chem. Eur. J. 29/2016)

Inside Cover: Enhancement of Light Absorption Ability of Synthetic Chlorophyll Derivatives by Conjugation with a Difluoroboron Diketonate Group (Chem. Eur. J. 29/2016)

Enhancement of Light Absorption Ability of Synthetic Chlorophyll Derivatives by Conjugation with a Difluoroboron Diketonate Group.

The enhancement of the light absorption ability of synthetic chlorophyll derivatives is demonstrated. Chlorophyll derivatives directly conjugated with a difluoroboron 1,3-diketonate group at the C3 position were synthesized from methyl pyropheophorbide-d through Barbier acylmethylation of the C3-formyl moiety, oxidation of the C3-carbinol, and difluoroboron complexation of the diketonate. Electronic absorption spectra in a diluted solution showed that the synthetic conjugates gave an absorption band at λ=400-500 nm, with a Qy band shifted to a longer wavelength of λ≈700 nm. DFT calculations demonstrated that the absorption bands and redshifts were ascribable to the coupling of the LUMO of chlorin with that of the difluoroboron diketonate moiety. The introduction of a pyrenyl group at the C3(3) -position of the conjugate afforded an additional charge-transfer band over λ=500 nm, producing a pigment that bridged the green gap in standard chlorophylls.

Back Cover: Construction of Chlorosomal Rod Self-Aggregates in the Solid State on Any Substrates from Synthetic Chlorophyll Derivatives Possessing an Oligomethylene Chain at the 17-Propionate Residue (Chem. Eur. J. 42/2012)

Back Cover: Construction of Chlorosomal Rod Self-Aggregates in the Solid State on Any Substrates from Synthetic Chlorophyll Derivatives Possessing an Oligomethylene Chain at the 17-Propionate Residue (Chem. Eur. J. 42/2012)

Construction of Chlorosomal Rod Self‐Aggregates in the Solid State on Any Substrates from Synthetic Chlorophyll Derivatives Possessing an Oligomethylene Chain at the 17‐Propionate Residue

Chlorosomes are one of the most unique natural light-harvesting antennas and their supramolecular nanostructures are still under debate. Chlorosomes contain bacteriochlorophyll (BChl)-c, d and e molecules and these pigments self-aggregate under a hydrophobic environment inside a chlorosome. The self-aggregates are mainly constructed by the following three interactions: hydrogen bonding, coordination bonding and π-π stacking. Supramolecular nanostructures of self-aggregated BChls have been widely investigated by spectroscopic and microscopic techniques. Model compounds of such chlorosomal BChl molecules have been synthesized and the effects of esterified long alkyl chains at the 17-propionate residue for their self-aggregation have been studied. Structurally simple zinc chlorophyll derivatives possessing an oligomethylene chain as the esterifying group at the 17-propionate residue were prepared as chlorosomal BChl models. The synthetic zinc BChls self-aggregated in nonpolar organic solvents to give precipitates. The resulting insoluble self-aggregated solids were investigated on a variety of substrates, including hydrophobic, neutral and hydrophilic substrates, by visible absorption, circular dichroism and polarized light absorption spectroscopies, as well as atomic force, transmission electron and scanning electron microscopies. The self-aggregates of synthetic Zn-BChls formed rods with an approximately 5 nm diameter and wires with further elongated growth of the rods (aspect ratio >200). The diameter size was consistent with that estimated for natural chlorosomal rods in a filamentous anoxygenic phototroph, Chloroflexus aurantiacus. The supramolecular formation and stability of the rod on the examined substrates depended on the length of an oligomethylene chain at the 17-propionate residue as well as on the surface properties. Especially, the number of the 5 nm rods on the substrates increased with an elongation of the chain.