Kouichi Asakura

Generation of an optically active octahedral cobalt complex by a chiral autocatalysis

Abstract Optically active cis-[CoBr(NH3)(en)2]Br2 could be readily obtained by the reaction of optically inactive [Co(H2O)2{(OH)2Co(en)2}2](SO4)2 with ammonium bromide in a water suspension. The optically active cis-[CoBr(NH3)(en)2]Br2 was not readily racemized in its aqueous solution, indicating it not to be produced by total spontaneous resolution but by asymmetric synthesis. This synthesis was exclusively observed when the weight ratio of[Co(H2O)2{(OH)2Co(en)2}2](SO4)2 to water was above 0.31. Since the crystal of each enantiometer of cis-[CoBr(NH3)(en)2]Br2 introduced into the reaction system lead to the preferential production of the same enantiomer, chiral autocatalysis was shown to be performed during the reaction. In the concentrated suspension, crystallization of cis-[CoBr(NH3)(en)2]Br2 that was produced by the reaction would be started during the relatively early stage of the reaction. If several crystals to be initially produced are enantiomerically unequivalent by statistical fluctuation, it may cause the spontaneous generation of the enantiomeric predominance.

Mechanism of chiral asymmetry generation by chiral autocatalysis in the preparation of chiral octahedral cobalt complex

Chiral asymmetry generation, the predominant production of one enantiomer in a non-chiral environment, could occur in the production of the chiral complex cis-[CoBr(NH3)(en)2]Br2 by the reaction of [Co(H2O)2{(OH)2Co(en)2}2](SO4)2 with ammonium bromide in an aqueous medium. The main kinetic steps in the reaction system have been determined. During the reaction, the product crystallizes at an early stage. When a very small amount of crystalline enantiomer was added to the reaction system at an early stage, the same enantiomer was produced preferentially; in addition, the enantiomeric excess of the product increased with increasing the stirring rate. Thus, it seems that each enantiomer generates chiral crystals that could self-replicate through secondary nucleation when the solution is stirred; these crystals in turn enhance the production of the same enantiomer. With a computer code that simulates such a kinetic mechanism, it is shown that enantiomeric excess observed in the experiments could be reproduced. Chirality 10:343–348, 1998.

Fabrication of spatially periodic double roughness structures by directional viscous fingering and spinodal dewetting for water-repellent surfaces

Water-repellent and self-cleaning properties of lotus leaves are considered to be due to its double roughness structure, protrusion structure (approximately 20 microm) and hairy structure (0.2-1.0 microm). In this study, attempts to fabricate a spatially periodic double roughness structure by two far-from-equilibrium self-organization phenomena, a directional viscous fingering and a spinodal dewetting, were made. A mixture of an octylsilyl titanium dioxide particle having an average diameter of 35 nm suspended in volatile silicone, decamethyl cyclopentasiloxane, and octyl p-methoxycinnamate was spread on a glass plate by dragging an applicator across the top. Formation of a stripe pattern parallel to the direction of dragging, called directional viscous fingering, was sometimes observed. Influences of spreading conditions on the pattern formation were analyzed. In addition, attempts were made to apply the stripe pattern formation to the preparation of a water repellent surface. We have succeeded in preparing a highly water-repellent surface by immersing a glass plate, on which a spatially periodic stripe pattern having a characteristic wavelength of 200-700 microm was formed, in water, after the completion of evaporation of decamethyl cyclopentasiloxane. In this case, dewetting patterns having a characteristic wavelength at around 5 microm were formed at the bottom part of the stripe patterns. Neither the surface on which only the mesoscopic spatially periodic stripe pattern was formed nor the one on which only the microscopic dewetting pattern was formed showed high water-repellent properties, indicating that the coexistence of the two different scales of patterns increased the water-repellent properties of the hydrophobic surface.

Turing pattern formation by the CIMA reaction in a chemical system consisting of quaternary alkyl ammonium cationic groups

For the spontaneous generation of a Turing pattern, two intermediate species, an activator and an inhibitor, should be generated with the diffusion coefficient of the activator smaller than that of the inhibitor. The chlorite-iodide-malonic acid (CIMA) reaction that generates the activator, I(-), and inhibitor, ClO(2-), was performed in an open gel reactor. In order to lower the effective diffusivity of I(-), micelles of quaternary alkyl ammonium cationic amphiphiles and polymers having a quaternary alkyl ammonium cationic side chain were combined in the CIMA reaction system in an open gel reactor. A Turing pattern formation was observed with the addition of n-dodecyltrimethylammonium bromide. Employing the gel reactor prepared by the polymerization of a monomer having quaternary alkyl ammonium cationic side chains also leads to the generation of a Turing pattern. The micelles and polymers are believed to trap I(-) in their vicinity as a counterion to lower the effective diffusivity.

Deformable Self-Propelled Micro-Object Comprising Underwater Oil Droplets.

The self-propelled motion with deformation of micrometer-sized soft matter in water has potential application not only for underwater carriers or probes in very narrow spaces but also for understanding cell locomotion in terms of non-equilibrium physics. As far as we know, there have been no reports about micrometer-sized self-propelled soft matter mimicking amoeboid motion underwater. Here, we report an artificial molecular system of underwater oil droplets exhibiting self-propelled motion with deformation as an initial experimental model. We describe the heterogeneity in a deformable self-propelled oil droplet system in aqueous and oil phases and at their interface based on the behavior and interaction of surfactant and oil molecules. The current results have great importance for scientific frontiers such as developing deformable micro-swimmers and exploring the emergence of self-locomotion of oil droplet-type protocells.

FORMATION OF THE INTERMEDIATE, AQUOHYDROXOBIS (ETHYLENEDIAMINE)COBALT(III) COMPLEX, IN THE PREPARATION OF CIS-BROMOAMMINEBIS (ETHYLENEDIAMINE)COBALT(III) BROMIDE BY THE REACTION OF BIS[BIS(ETHYLENEDIAMINE)COBALT(III)-μ-DIHYDROXO]DIAQUOCOBALT(II) SULFATE WITH AMMONIUM BROMIDE IN WATER

Abstract The production of cis-[CoBr(NH3)(en)2]Br2 by reaction of [Co(H2O)2{(OH)2Co(en)2}2](SO4)2 with ammonium bromide in water was investigated. The reaction seems to proceed through the intermediate, [Co(H2O)(OH)(en)2]2+, and then convert to the product, cis-[CoBr(NH3)(en)2]2+. This is supported by the observation that the absorption spectrum of the reaction solution immediately after the disappearance of the starting complex was different from that of cis-[CoBr(NH3)(en)2]2+ but was quite similar to that of [Co(H2O)(OH)(en)2]2+. In order to confirm this reaction pathway, cis-[Co(H2O)(OH)(en)2]Br2 was prepared and reacted with ammonium bromide producing the expected complex, cis-[CoBr(NH3)(en)2]Br2.

(+)589-Λ-Amminebromobis(ethylenediamine-N,N')cobalt(III) Dibromide

The absolute configuration of (+)589-cis-[CoBr(C2H8N2)2(NH3)]2+ was confirmed to be Λ. The ethylenediamine five-membered chelate rings take the lel2 conformation. There are N—H⋯Br intermolecular hydrogen bonds with N⋯Br distances from 3.267 (9) to 3.715 (8) A.

Glass transition behavior of octyl β-d-glucoside and octyl β-d-thioglucoside/water binary mixtures

The lyotropic behavior and glass-forming properties of octyl β-D-glucoside (C8Glu) and octyl β-D-thioglucoside (C8SGlu)/water binary mixtures were evaluated using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The results clearly indicate that the mixture forms a glass in the supercooling state of liquid crystalline phases such as cubic, lamellar, and smectic. The glass transition temperature (T(g)) of the mixture was strongly dependent on solute concentration, with a higher concentration correlating with a higher T(g). The experimental T(g) was consistent with the predicted value calculated using the Couchman-Karasz equation in both the C8Glu and C8SGlu/water mixtures. The change of heat capacity at T(g) showed the two bending points under variation of concentrations. And the highest temperature of phase transition from lamellar to isotropic solution was observed at around 50% molar concentration. It was expected that non-percolated state of water existed in extremely higher concentration ranges.

Antioxidant effect and antimicrobial activity of phenolic sulfides

Some phenolic compounds containing both phenolic hydroxyl groups and sulfide groups in the molecules were prepared by Michael addition and radical addition reactions of ethanethiol and ethanedithiol witho-, m- andp-vinylphenol. The antioxidant effect of these products on the autoxidation of lard was examined by an oven test at 60°C. The antimicrobial activities of these products were evaluated by an agar dilution method. Three kinds of gram-positive and three kinds of gram-negative bacterial strains, as well as six kinds of fungal strains were used for the test. It was found that the Michael addition products showed considerably better antioxidant effects than BHT, whereas the radical addition products did not show any antioxidant effect. The antioxidant effects of these compounds were influenced by the positions of the sulfide groups in their molecules. The phenolic sulfides tested in this report showed good antimicrobial activities against bacteria and fungi.

Reactivity of Alanylalanine Diastereoisomers in Neutral and Acid Aqueous Solutions: a Versatile Stereoselectivity

A good comprehension of the reactivity of peptides in aqueous solution is fundamental in prebiotic chemistry, namely for understanding their stability and behavior in primitive oceans. Relying on the stereoselectivity of the involved reactions, there is a huge interest in amino acid derivatives for explaining the spontaneous emergence of homochirality on primitive Earth. The corresponding kinetic and thermodynamic parameters are however still poorly known in the literature. We studied the reactivity of alanylalanine in acidic to neutral conditions as a model system. The hydrolysis into amino acids, the epimerization of the N-terminal residue, and the cyclization into diketopiperazine could be successfully identified and studied. This kinetic investigation highlighted interesting behaviors. Complex mechanisms were observed in very acidic conditions. The relative kinetic stability of the diastereoisomers of the dipeptide is highly dependent of the pH, with the possibility to dynamically destabilize the thermodynamically more stable diastereoisomers. The existence of the cyclization of dipeptides adds complexity to the system. On one hand it brings additional stereoselectivities; on the other hand fast racemization of heterochiral dipeptides is obtained.