Optimizing LiMn1.5M0.5O4 cathode materials for aqueous photo-rechargeable batteries.

Spinel oxides are promising for high-potential cathode materials of photo-rechargeable batteries. However, LiMn1.5M0.5O4 (M = Mn) shows a rapid degradation during charge/discharge under the illumination of UV-visible light. Here, we investigate various spinel-oxide materials by modifying the composition (M = Fe, Co, Ni, Zn) to demonstrate photocharging in a water-in-salt aqueous electrolyte. LiMn1.5Fe0.5O4 exhibited a substantially higher discharge capacity compared to that of LiMn2O4 after long-term photocharging owing to enhanced stability under illumination. This work provides fundamental design guidelines of spinel-oxide cathode materials for the development of photo-rechargeable batteries.

Ring-shaped self-assembly of a naphthalene-linked chlorophyll dimer.

Light-harvesting antennas, for example the LH2 complex in purple bacteria, sophisticatedly align chlorophyll molecules in a cyclic fashion by using protein scaffolds. However, artificial preparation of the circular LH antenna model without any templates has not been reported. We demonstrated the construction of ring-shaped supramolecules by self-assembly of a semisynthetic chlorophyll dimer through a transformation from wavy fiber-like aggregates.

Light-induced Li extraction from LiMn2O4/TiO2 in a water-in-salt electrolyte for photo-rechargeable batteries.

Photocharging of high-potential spinel LiMn2O4 is demonstrated by using a water-in-salt electrolyte and TiO2 nanoparticles. In a developed half-cell system with an electron acceptor, Li extraction from LiMn2O4 proceeds under the illumination of UV-visible light at an estimated rate of ∼23 mA g-1. This work paves the way for high-potential cathode materials in photo-rechargeable batteries.

A Peptide Nanocage Constructed by Self-Assembly of Oligoproline Conjugates.

Cage-like supramolecular assemblies called molecular cages, which possess attractive functions, have been prepared. Although biomolecule-based nanocages are required for biological/medical applications such as drug delivery systems, the majority of nanocages are constructed using aromatic compounds with lower biocompatibility and biodegradability. In this study, the construction of a peptide nanocage consisting of an oligoproline conjugate is demonstrated. The conjugate was easy to prepare and had high biocompatibility. The oligoproline moiety of the conjugate had a rigid, rod-like structure suitable for the backbone of the supramolecular nanocage. The conjugates self-assembled to form peptide nanocages with a huge inner cavity.

Fabrication of CaCO3-Coated Vesicles by Biomineralization and Their Application as Carriers of Drug Delivery Systems.

We fabricated CaCO3-coated vesicles as drug carriers that release their cargo under a weakly acidic condition. We designed and synthesized a peptide lipid containing the Val-His-Val-Glu-Val-Ser sequence as the hydrophilic part, and with two palmitoyl groups at the N-terminal as the anchor groups of the lipid bilayer membrane. Vesicles embedded with the peptide lipids were prepared. The CaCO3 coating of the vesicle surface was performed by the mineralization induced by the embedded peptide lipid. The peptide lipid produced the mineral source, CO32-, for CaCO3 mineralization through the hydrolysis of urea. We investigated the structure of the obtained CaCO3-coated vesicles using transmission electron microscopy (TEM). The vesicles retained the spherical shapes, even in vacuo. Furthermore, the vesicles had inner spaces that acted as the drug cargo, as observed by the TEM tomographic analysis. The thickness of the CaCO3 shell was estimated as ca. 20 nm. CaCO3-coated vesicles containing hydrophobic or hydrophilic drugs were prepared, and the drug release properties were examined under various pH conditions. The mineralized CaCO3 shell of the vesicle surface was dissolved under a weakly acidic condition, pH 6.0, such as in the neighborhood of cancer tissues. The degradation of the CaCO3 shell induced an effective release of the drugs. Such behavior suggests potential of the CaCO3-coated vesicles as carriers for cancer therapies.

Growth model of chlorosome antenna by the environment-dependent stepwise assembly of a zinc chlorophyll derivative.

A zinc chlorophyll derivative possessing an oligoethylene glycol ester at the 17-propionate residue was prepared as a model of specific pigments in chlorosomes, such as bacteriochlorophylls-c, d, and e, by chemical modification of naturally occurring chlorophyll-a. The zinc chlorophyll derivative aggregated in aqueous or hexane solutions containing 1% (v/v) ethanol to give red-shifted and broadened Soret/Qy absorption bands with intense circular dichroism signals, indicating the formation of its chlorosome-like J-type self-aggregates. The atomic force microscope images of the self-aggregates prepared in aqueous or hexane solutions showed thin tube-like (ca. 3 nm diameter) or thick rod-like aggregates (> 20 nm diameter), respectively. After standing these solutions for several days, the nanotubes or nanorods further assembled to give ribbon- or bundle-like aggregates, respectively. The latter transformation (tube to ribbon) was triggered by hydrogen bonding between oligoethylene glycol esters located outside of the tubes via water or ethanol molecules. These dynamic supramolecular transformations may also be useful for revealing the growth process of bacteriochlorophyll self-aggregates in a chlorosome.

Bioinspired supramolecular nanosheets of zinc chlorophyll assemblies.

Two-dimensional sheet-like supramolecules have attracted much attention from the viewpoints of their potential application as functional (nano)materials due to unique physical and chemical properties. One of the supramolecular sheet-like nanostructures in nature is visible in the self-assemblies of bacteriochlorophyll-c-f pigments inside chlorosomes, which are major components in the antenna systems of photosynthetic green bacteria. Herein, we report artificial chlorosomal supramolecular nanosheets prepared by the self-assembly of a synthetic zinc 31-methoxy-chlorophyll derivative having amide and urea groups in the substituent at the 17-position. The semi-synthetic zinc chlorophyll derivative kinetically formed dimeric species and transformed into more thermodynamically stable chlorosomal J-aggregates in the solid state. The kinetically and thermodynamically formed self-assemblies had particle-like and sheet-like supramolecular nanostructures, respectively. The resulting nanosheets of biomimetic chlorosomal J-aggregates had flat surfaces and well-ordered supramolecular structures. The artificial sheet-like nanomaterial mimicking chlorosomal bacteriochlorophyll-c-f J-aggregates was first constructed by the model molecule, and is potentially useful for various applications including artificial light-harvesting antennas and photosyntheses.

Phototriggered Dynamic and Biomimetic Growth of Chlorosomal Self-Aggregates.

Supramolecular polymerizations mimicking native systems, which are step-by-step constructions to form self-aggregates, were recently developed. However, a general system to successively and spontaneously form self-aggregates from monomeric species remains challenging. Here, we report a photoinduced supramolecular polymerization system as a biomimetic formation of chlorophyll aggregates which are the main light-harvesting antennas in photosynthetic green bacteria, called "chlorosomes". In this system, inert chlorophyll derivatives were UV-irradiated to gradually produce active species through deprotection. Such active monomers spontaneously assembled to form fiberlike chlorosomal self-aggregates in a similar manner as a dynamic growth of natural chlorosomal self-aggregates. The study would be useful for elucidation of the formation process of the chlorosomal aggregates and construction of other supramolecular structures in nature.

Synthesis and Self-Aggregation of π-Expanded Chlorophyll Derivatives to Construct Light-Harvesting Antenna Models.

Chlorosomes are one of the elegant light-harvesting antenna systems in anoxygenic photosynthetic bacteria, whose core is constructed from J-type self-aggregation of bacteriochlorophyll- c, bacteriochlorophyll- d, bacteriochlorophyll- e, and bacteriochlorophyll- f molecules without the influence of polypeptides. Chlorosomal supramolecular models were built up using synthetic porphyrin-type bacteriochlorophyll- d analogues with a methoxycarbonylethenyl, formyl, vinyl, or ethyl group at the 8-position. Their chlorosomal self-aggregates in an aqueous micelle solution showed relatively intense absorption bands around 500-600 nm where antennas of natural oxygenic phototrophs, as well as green sulfur bacteria possessing bacteriochlorophylls- c/ d, absorb light less efficiently; this observation is called the "green gap". Furthermore, the functional chlorosomal models were constructed by simple addition of a small amount of an energy acceptor model bearing a bacteriochlorin moiety to the pigment self-assemblies in an aqueous micelle. The resulting excited energy donor-acceptor supramolecules played the roles of chlorosomal light-harvesting and energy-transfer antenna systems and were efficient at light absorption in the "green gap" region.

Synthesis of chlorophyll-c derivatives by modifying natural chlorophyll-a.

Chlorophyll-a (Chl-a) was extracted from cyanobacterial cells and modified to methyl pyropheophorbide-a. The 3-vinyl-chlorin was transformed to zinc complex of the corresponding 3-acetyl-porphyrin. The zinc porphyrin was oxidized to give cis-7,8- and 17,18-dihydroxy-chlorins as well cis-7,8-cis-17,18-tetrahydroxybacteriochlorin. After zinc-demetallation, the isolated cis-7,8- and 17,18-diols were reduced at the 3-acetyl group and triply dehydrated under acidic conditions to afford two regioisomeric 3-vinyl-porphyrins, methyl divinyl-pyroprotopheophorbide-a possessing the 8-vinyl group and 17-propionate residue (one of the divinyl-protoChl-a derivatives) and methyl pyropheophorbide-c 1 possessing the 8-ethyl group and 17-acrylate residue (one of the Chl-c 1 derivatives), respectively. The resulting 7,8,17,18-tetrol was reduced and then acidically treated, giving five-fold dehydrated free base porphyrin, methyl pyropheophorbide-c 2 possessing the 3,8-divinyl groups and 17-acrylate residue (one of the Chl-c 2 derivatives). The visible absorption and fluorescence emission spectra of the three semi-synthetic 3-vinyl-porphyrins in dichloromethane were compared with those of the corresponding 8-ethyl-porphyrin bearing the 17-propionate residue, methyl pyroprotopheophorbide-a (one of the protoChl-a derivatives). The Soret and Qy absorption maxima were shifted to longer wavelengths with an increase of π-conjugation in a molecule: protoChl-a (8-CH2CH3/17-CH2CH2COOCH3) < divinyl-protoChl-a (8-CH=CH2/17-CH2CH2COOCH3) < Chl-c 1 (8-CH2CH3/17-CH=CHCOOCH3) < Chl-c 2 derivatives (8-CH=CH2/17-CH=CHCOOCH3). The 17(1),17(2)-dehydrogenation broadened the absorption bands. The emission maxima were bathochromically shifted in the same order. The reaction mechanism of the present dehydration indicates that the biosynthetic pathway of Chls-c would include the hydroxylation of the 17-propionate reside at the 17(1)-position and successive dehydration to the 17-acrylate residue.