Masa Aki Morikawa

Ionic liquids induced structural changes of bovine serum albumin in aqueous media: a detailed physicochemical and spectroscopic study.

Structural changes of a globular protein, bovine serum albumin (BSA), as a consequence of interaction with the surface active ionic liquids (ILs)-3-methyl-1-octylimidazolium chloride, [C(8)mim][Cl], and 1-butyl-3-methylimidazolium octylsulfate, [C(4)mim][C(8)OSO(3)]-have been investigated using various physicochemical and spectroscopic techniques such as tensiometry, conductometry, steady-state fluorescence, far-UV circular dichroism spectroscopy (CD), and dynamic light scattering (DLS). The interactional behavior of ILs (monomers and self-assembled structures) toward BSA in different IL concentration regimes at the air/solution interface as well as in the bulk is investigated and discussed depending upon the nature of ions of ILs. CD combined with the steady state fluorescence spectroscopy provided valuable insights into the unfolding of BSA as a consequence of IL binding. The complementary results obtained from the multitechnique approach proved very useful in drawing out the mechanism of interaction between ILs and BSA in different IL concentration regimes.

Conversion of Molecular Information by Luminescent Nanointerface Self-Assembled from Amphiphilic Tb(III) Complexes

A novel amphiphilic Tb(3+) complex (TbL(+)) having anionic bis(pyridine) arms and a hydrophobic alkyl chain is developed. It spontaneously self-assembles in water and gives stable vesicles that show sensitized luminescence of Tb(3+) ions at neutral pH. This TbL(+) complex is designed to show coordinative unsaturation, i.e., water molecules occupy some of the first coordination spheres and are replaceable upon binding of phosphate ions. These features render TbL(+) self-assembling receptor molecules which show increase in the luminescence intensity upon binding of nucleotides. Upon addition of adenosine triphosphate (ATP), significant amplification of luminescent intensity was observed. On the other hand, ADP showed moderately increased luminescence and almost no enhancement was observed for AMP. Very interestingly, the increase in luminescence intensity observed for ATP and ADP showed sigmoidal dependence on the concentration of added nucleotides. It indicates positive cooperative binding of these nucleotides to TbL(+) complexes preorganized on the vesicle surface. Self-assembly of amphiphilic Tb(3+) receptor complexes provides nanointerfaces which selectively convert and amplify molecular information of high energy phosphates linked by phosphoanhydride bonds into luminescence intensity changes.

New colorimetric detection of glucose by means of electron-accepting indicators: ligand substitution of [Fe(acac)3-n(phen)n]n+ complexes triggered by electron transfer from glucose oxidase.

A new colorimetric detection technique for glucose, based on electron transfer from glucose oxidase (GODred) to iron(III) acetylacetonate(acac)/phenanthroline(phen) mixed complexes, is developed. When GOD is added to an aqueous mixture that contains tris(acetylacetonato)iron(III) complex (FeIII-(acac)3), 1,10-phenanthroline (phen), and glucose, the color immediately changes from pale yellow to red. The red color originates from formation of tris(1,10-phenanthroline)iron(II) complex ([FeII(phen)3]2+). Differential pulse voltammetry indicates that cationic, mixed-ligand complexes of [Fe(acac)3-n-(phen)n]n+ are formed upon mixing the labile FeIII(acac)3 complex and phenanthroline. The cationic mixed-ligand complexes electrostatically bind to GOD (pI 4.2), and are easily reduced by electron transfer from GODred. This electron transfer is not affected by the presence of oxygen. The reduced complex [FeII(acac)3-n(phen)n](n-1)+ then undergoes rapid ligand exchange to FeII (phen)3. Formation of the colored FeII complex is repressed when the salt concentration in the mixture is increased, or when anionic bathophenanthroline disulfonate (BPS) is employed in place of phenanthroline. The use of labile metal complexes as electron acceptors would be widely applicable to the design of new biochromic detection systems.

Photoliquefiable Ionic Crystals: A Phase Crossover Approach for Photon Energy Storage Materials with Functional Multiplicity

Ionic crystals (ICs) of the azobenzene derivatives show photoinduced IC-ionic liquid (IL) phase transition (photoliquefaction) upon UV-irradiation, and the resulting cis-azobenzene ILs are reversibly photocrystallized by illumination with visible light. The photoliquefaction of ICs is accompanied by a significant increase in ionic conductivity at ambient temperature. The photoliquefaction also brings the azobenzene ICs further significance as photon energy storage materials. The cis-IL shows thermally induced crystallization to the trans-IC phase. This transition is accompanied by exothermic peaks with a total ΔH of 97.1 kJ mol(-1) , which is almost double the conformational energy stored in cis-azobenzene chromophores. Thus, the integration of photoresponsive ILs and self-assembly pushes the limit of solar thermal batteries.

A liquid azobenzene derivative as a solvent-free solar thermal fuel

A liquid solar thermal fuel is developed; a low-molecular weight liquid trans-azobenzene derivative shows facile photoisomerization to the higher-energy cis-isomer in neat condition so that a high volumetric energy density is achieved. Shear viscosity measurements for each isomer liquid unveiled transitions from non-Newtonian to Newtonian fluids.

Biopolymer-Encapsulated Protein Microcapsules Spontaneously Formed at the Ionic Liquid–Water Interface

Aqueous microdroplets introduced in ionic liquids (ILs) provide unique interfaces where surface-modified protein microcapsules are spontaneously formed at systemic temperature. The susceptibility of proteins to form microcapsules at the water-IL microinterface depends on protein species and is related to the number of charged residues exhibited on protein surfaces. When both of the capsule-forming (host) proteins and guests biopolymers such as nucleic acids or enzymes are introduced in the aqueous microdroplets, microcapsules are formed selectively from host proteins while the guest biopolymers remain encapsulated in the aqueous pool. Microcapsules formed in the IL phase are facilely extracted to aqueous phase after consecutive cross-linking and surface modification reactions, and the whole processes can be done in one pot. Enzymes confined in the inner water phase of aqueous protein microcapsules showed innate activity, as visualized by site-specific fluorescence detection using confocal laser scanning microscopy (CLSM). The present IL-water interfacial synthesis of protein microcapsules eliminates the use of volatile organic solvents or solid colloid templates, which creates a much-coveted solution to existing technologies.

Photoresponsive Nanosheets of Polyoxometalates Formed by Controlled Self-Assembly Pathways

Anionic Keggin polyoxometalates (POMs) and ether linkage-enriched ammonium ions spontaneously self-assemble into rectangular ultrathin nanosheets in aqueous media. The structural flexibility of the cation is essential to form oriented nanosheets; as demonstrated by single-crystal X-ray diffraction measurements. The difference in initial conditions exerts significant influence on selecting for self-assembly pathways in the energy landscape. Photoillumination of the POM sheets in pure water causes dissolution of reduced POMs, which allowed site-specific etching of nanosheets using laser scanning microscopy. By contrast, photoetching was suppressed in aqueous AgNO3 and site-selective deposition of silver nanoparticles occurred as a consequence of electron transfer from the photoreduced POMs to Ag+ ions on the nanosheet surface.

Controlled Self-Assembly and Luminescence Characteristics of Eu(III) Complexes in Binary Aqueous/Organic Media

Luminescence of sodium tetrakis(naphthoyl trifluoroacetonato) europium(III) (Na[Eu(nta)4]) in binary aqueous-ethanol media is quenched continuously with an increase in the water content, which is ascribed to commonly observed relaxation of photoexcited lanthanide complexes through vibrational coupling with coordinating water. Meanwhile, replacement of sodium ion with an ammonium amphiphile 1 gives a lipid complex 1[Eu(nta)4] which shows distinct changes: its luminescence quantum yield Φ is remarkably increased to ~0.6 above the water content of ~60 vol. %. This unusual enhancement in luminescence intensity occurs in response to self-assembly of 1[Eu(nta)4] into nanoparticles. The lipid counterions provide a hydrophobic atmosphere inside nanoparticles, and they simultaneously form monolayers on the nanoparticle surface that enhance dispersion stability. The size of nanoparticles is tunable depending on the volume fraction of water in the binary media. The lipid-assisted self-assembly of lanthanide complexes provides a unique means to fabricate luminescent nanomaterials, and this approach will be widely applied to fabricate functional coordination nanomaterials.

Interaction between polylysine monolayer and DNA at the air–water interface

The interaction of a polylysine amphiphile, which consists of a poly-L- or -D-lysine (1L or 1D) segment and two long alkyl chains at the C-terminus, with polynucleotides was studied with respect to the highly organized structure of polylysine assemblies on water. The results of surface pressure-area isotherm measurement showed that both of 1L and 1D formed stable monolayers on water in a neutral pH region. The secondary structure of polylysine segment for the surface monolayer was examined by means of circular dichroism and Fourier transform infrared spectroscopies. The helical structure was retained even at neutral pH, at which polylysine has been known to form a complete random coiled conformation in bulk solution. Protonated, positively charged and coiled 1L monolayer could interact electrostatically with guest polyanions including DNA in the subphase, and at the same time the conformation of the polylysine segment was converted from a random coil to an alpha-helix. Deprotonated, helical monolayers did not interact with single stranded polyadenylic acid, but with double stranded DNA. Double stranded DNA was found to interact more strongly with right-handed 1L monolayer than left-handed 1D monolayer. An obvious difference in the melting temperatures for these complexes was observed and discussed on the basis of difference in the interaction mode.

Interlocked dimerization of C3-Symmetrical boron difluoride complex: designing non-cooperative supramolecular materials for luminescent thin films

A lipophilic complex with radially-connected three β-diketonate boron difluoride (BF2dk) units to a central benzene ring was newly developed. The C3-symmetrical BF2dk complex (1) formed a self-complementary interlocked dimer (1)2 with increasing concentration in CHCl3 as revealed by NMR spectroscopy and quantum chemical calculations. A remarkable luminescence color change from blue to yellow was observed in response to the formation of interlocked dimers. Electrostatic interactions, hydrogen bonding between negative convexes (BF2 moiety) and positive concaves (three protons aligned on each arm) principally contribute to the dimerization, whereas the formation of interlocked dimers was accompanied by conformational changes of constituent molecules which interrupted further association. Consequently, casting of the chloroform solution of interlocked dimers on solid supports gave uniform thin films without uneven crystallization. It provides a new perspective for designing anti-cooperative systems for homogeneous molecular coatings.