Katsuhiko Ariga

Preparation and surface modification of novel vesicular nano-particle Cerasome with liposomal bilayer and silicate surface

A novel class of nano-hybrid materials, “Cerasome,” bearing a liposomal bilayer structure and a ceramic surface, has been developed by a sol-gel reaction of a double-chain proamphiphile having an trialkoxysilane in the head moiety and followed self-assembling. In the present work, we developed new methods for preparation of the Cerasomes forming under various pH conditions and their surface modification by employing appropriate alkoxysilane compounds. When the proamphiphilic alkoxysilane was hydrolyzed in an acidic ethanol and then injected into an aqueous media at various pH, the morphologically stable Cerasomes were obtained in a wide pH range. On the other hand, an acid treatment of the proamphiphile in ethanol in the presence of tetraethoxysilane gave the Cerasome with highly developed siloxane networks on its vesicular surface. Surface modification of the Cerasome with amino groups was readily achieved in the similar manner by replacing tetraethoxysilane to 3-aminopropyltriethoxysilane. The Cerasomes thus prepared were characterized by means of dynamic light scattering and zeta potential measurements.

Dynamic behavior of a transmembrane molecular switch as an artificial cell-surface receptor

Abstract Supramolecular functions of a novel synthetic amphiphile with an azobenzene core interposed between two aminoalkyl chains as an artificial transmembrane receptor were investigated. Hybrid vesicular membrane was prepared in combination with the receptor and a bilayer-forming lipid dimyristoylphosphatidylcholine (DMPC). Aggregation behavior of the receptor embedded in the bilayer membrane was evaluated spectrophotometrically by monitoring stacking of the azobenzene moiety. Upon addition of 1-hydroxy-2-naphthaldehyde as an external signal ligand, it was effectively recognized at both ends of the receptor by forming the corresponding Schiffs base, and followed by phase reorganization of the receptor in the membrane. The resulting receptor–ligand complex provided an appropriate binding site for copper(II) ions, which may act as a second messenger signal. Signal transduction behavior of the present supramolecular system was examined by employing NADH-dependent lactate dehydrogenase (LDH) as an effector. We found that the transmembrane receptor is capable of performing as a molecular switch for the enzymatic reaction.

Control of enzymic activity by artificial cell-surface receptors

Abstract A novel type of artificial cell-surface receptor capable of affecting enzymic activity was designed by employing steroidal moiety. Bile acid derivatives having an amino group, effectively recognized both the aromatic aldehydes and copper(II) ions in bilayer membrane formed with a synthetic peptide lipid. On the other hand, pig heart l -lactate dehydrogenase (LDH) was bound to the bilayer membrane surface mainly through electrostatic interactions, and the LDH activity was specifically inhibited by copper(II) ions. On these grounds, we constituted artificial signaling systems in combinations, with a steroidal receptor, a bilayer-forming synthetic lipid, and LDH. The present supramolecular vesicles acted as nano-reactors in which the artificial receptor was able to switch on the enzymic activity via accompanying double signal recognition and phase reorganization.

Hydrophobic vitamin B12. Part 18. Preparation of a sol–gel modified electrode trapped with a vitamin B12 derivative and its photoelectrochemical reactivity

A vitamin B12 derivative, heptapropyl cobyrinate perchlorate, was readily trapped onto an indium tin oxide (ITO) electrode by a sol–gel reaction. The complex was physically retained in a silica gel film which is formed on an ITO electrode. The thickness of the film could be controlled by the withdrawing speed of the dip coating process. Formation of a sol–gel film was confirmed by SEM measurements, and the total amount of the complex in the film was determined by UV–VIS absorption spectra. The complex exhibits the CoII/CoI redox couple at −0.42 V vs. Ag–AgCl. The amount of the electroactive complex in the sol–gel film deduced from electrochemical measurements is 3.0 × 10−11 and 6.2 × 10−11 mol cm−2 for thicknesses of 170 and 330 nm, respectively. This electroactive complex shows a high reactivity towards organic halides, and the controlled-potential electrolysis of benzyl bromide using the sol–gel modified electrode at −1.20 V vs. Ag–AgCl in aqueous solution containing 0.1 M KCl afforded dehalogenated products, bibenzyl and toluene, with a total turnover number of >1000 for 1 h.

Langmuir monolayer of organoalkoxysilane for vitamin B12-modified electrode

In n order to develop a novel type of vitamin B12-functionalized electrode, the monolayer properties of the vitamin n B12 mimics, which have a core structurally identical with the naturally occurring vitamin B12 n and seven ester chains on the outside, have been investigated as a mixed monolayer with an organosilane n amphiphile. Vitamin B12 n derivatives having seven C8 chains were stably incorporated into the matrix n organosilane monolayer, while the short C3 chain derivatives were easily squeezed out from the matrix monolayer n upon compression. Analyses of the molecular areas revealed that the long chain derivatives had a face-on orientation in the matrix monolayer. The mixed monolayers of the vitamin B12 with the long chains n and the organosilane amphiphile were transferred as an X-type LB film on a quartz plate that showed reasonable n UV absorbance. The monolayer containing the vitamin B12 derivative was similarly immobilized n on an ITO electrode. A cyclic voltammetric measurement of this electrode in aqueous solution showed a Co(II)/Co(I) redox couple at − 0.65 V s. Ag/AgCl which is in good agreement with the corresponding value of the vitamin n B12 derivative dissolved in methanol. It indicates that n this modified electrode n can be used n as a reactive electrode with a vitamin B12 function.

Size-Selective Organization of Silica and Silica-Like Particles on Solid Interfaces through Layer-by-Layer Assembly

Employing a layer-by-layer assembly technique, we created three-dimensional architectures of silica and silica-like particles on solid interfaces. Atomic force and scanning electron microscopy confirmed a size-selection effect for assembling a mixture of two kinds of monodispersed silica particles prepared through the sol-gel process. Size-selective assembly was also applied for layer-by-layer organization of Cerasomes, which are organic-inorganic vesicular hybrids with a silica-like structure on the lipid bilayer surface. This study obtained an alternating layer-by-layer assembly of Cerasomes with a relatively uniform size on solid interfaces from polydispersed aqueous colloids of a surface-modified cationic Cerasome with an unmodified anionic Cerasome or an anionic poly(vinyl sulfate). Quartz crystal microbalance measurements and atomic force microscopy were used to evaluate this assembly process.

Molecular Recognition by Cyclophane/Guanidinium Supramolecular Receptor Embedded at the Air-Water Interface

In order to develop a supramolecular receptor through a self-assembling process, a site-specific host and an inclusion-type host were mixed as a Langmuir monolayer, and guest binding and pressure-induced fluorescence emission were investigated. A guanidinium amphiphile and several cyclophanes carrying hydrophobic moieties were used as the host molecules; molecular recognition of an aqueous fluorescent guest, 6-p-toluidino-2-naphthalenesulfonic acid (TNS) by binary mixed receptor monolayers was evaluated by a surface pressure-molecular area (π-A) isotherm and a surface fluorescence measurement. An apparent increase in fluorescence intensity was observed when the mixed monolayers of the guanidinium and cyclophane amphiphiles were compressed on an aqueous TNS solution. In contrast, single-component monolayers of the guanidinium or the cyclophane did not show a significant increase in fluorescence emission. In the mixed monolayers, the guest TNS would be bound to the interface by strong electrostatic interaction with the guanidinium, and inclusion of the formed complex probably suppresses the quenching effect in polar medium and/or self-quenching. Experiments with various mixing ratios of these components suggest selective formation of an equimolar cooperative receptor of the guanidinium and the cyclophane. Investigation of the cyclophane structures by fluorescence emission and a competitive binding experiment with another guest were also carried out.

Lyotropic aggregate of tripeptide derivatives within organic solvents: study on dynamic property of molecular assembling

Abstract Tripeptide derivatives possessing two hydrophobic chains aggregated in organic solvents, which were accompanied by changes in the Fourier transform infrared (FT-IR) spectra. A marked shift from a nonbonding to a bonding amide band was used for the diagnostics of the aggregation in a light and transparent nonaqueous solution in which it is difficult to determine aggregate formation based on macroscopic, rheologic and thermodynamic observations. The diagnostics was supported by the band shift in a CH 2 stretching mode. Using the spectral changes, we demonstrated that the tripeptide derivatives are divided into two types. One was observed for amphiphilic tripeptide derivatives such as N -[ N -11-trimethylammonioundecanoyl)-di- l -diphenylalanyl]- O , O ′-didodecyl- l -glutamate bromide. The other was observed for hydrophobic tripeptide derivatives lacking a hydrophilic ammonium head, which were the intermediates of the amphiphiles. The tripeptide-containing amphiphiles formed a gel at about 10 −3 M, whereas the later hydrophobic tripeptide derivatives generally only formed a viscoelastic solution with increase in the concentration. However, aggregate formation of the molecules was completed prior to the formation of a gel. The tripeptide-containing amphiphiles formed a stable aggregate at a very low concentration ( −5 M). On the other hand, the hydrophobic tripeptide derivatives stepwise aggregated at a higher concentration. In this case, the monomeric species and the aggregate coexisted at equilibrium over the wide concentration range (10 −2 –10 −3 M). We pointed out that the concentration-induced assemblage formed an artifact during preparation of the air-dried sample, which was provided for the structural analysis such as AFM observation.

Syntheses and monolayer properties of vitamin B12 derivatives with seven alkyl chains

Abstract The heptapropyl and heptaoctyl esters of vitamin B12 derivatives with a Co(II) or Co(III) center have been synthesized and their monolayer properties have been investigated. The effect of the esters chain length on stability and orientation of the vitamin B12 in a lipid monolayer is discussed on the basis of their surface pressure molecular area (π-A) isotherms. Their isotherms significantly depend on the length of the side chains regardless of the oxidation state of the center cobalt. The isotherm of the heptaoctyl derivatives has a relatively steep shape with a molecular area of ca. 3 nm2 which is fairly close to the molecular area estimated by molecular modeling for face-on orientation. In contrast, the heptapropyl derivatives showed unstable characteristics with collapsing even at low pressures. The heptaoctyl derivatives can also be stably incorporated in the DPPC matrix monolayer, while the heptapropyl derivatives were squeezed out from the same matrix upon compression at high pressures. The effect of coordination of the center cobalt to the matrix lipid was also investigated. Mixing of the heptapropyl Co(II) derivatives with a lysine-functionalized lipid significantly improved the preservation of the vitamin B12 function in the monolayer due to coordination of the lysine amino group to the open axial position of Co(II). The obtained results in this study indicate that stable accommodation of the vitamin B12 functions in the lipid assembly can be achieved by the introduction of a long chain and/or coordination to the matrix lipid. The present study is the first example of a monolayer system of vitamin B12 mimics with a core structurally identical to the naturally occurring one.

Information conversion on molecular assemblies containing steroid cyclophanes

Abstract We have developed steroid cyclophanes which are unique hosts with a cyclic core of a 1,6,20,25-tetraaza[6.1.6.1]paracyclophane connected to four steroid moieties through a flexible l -lysine spacer. The polarity of the cavity interior can be dynamically controlled by mixing artificial lipids, resulting in a change in fluorescence of the bound probe [6-(p-toluidino)naphthalene-2-sulfonate, TNS]. Dynamic light scattering and circular dichroism measurements revealed that the assembling structures of the mixtures of the steroid cyclophane and the lipid were altered depending on the mixing ratio. Electrostatic interaction and self-aggregation of the steroid cyclophanes would influence their mixing state. A fluorescence maximum of TNS bound to the steroid cyclophane cavity biphasically changed between 433 and 409 nm. This observation can be explained by a hydrophilic–hydrophobic cavity conversion. This system provides lipid-dependent photo-signal modulation as a novel type of information conversion.