Introduction of pseudo-base benzimidazole derivatives into nucleosides via base exchange by a nucleoside metabolic enzyme.

In alternate organic synthesis, biocatalysis using enzymes provides a more stereoselective and cost-effective approach. Synthesis of unnatural nucleosides by nucleoside base exchange reactions using nucleoside-metabolizing enzymes has previously shown that the 5-position recognition of pyrimidine bases on nucleoside substrates is loose and can be used to introduce functional molecules into pyrimidine nucleosides. Here we explored the incorporation of purine pseudo bases into nucleosides by the base exchange reaction of pyrimidine nucleoside phosphorylase (PyNP), demonstrating that an imidazole five-membered ring is an essential structure for the reaction. In the case of benzimidazole, the base exchange proceeded to give the deoxyribose form in 96 % yield, and the ribose form in 23 % yield. The reaction also proceeded with 1H-imidazo[4,5-b]phenazine, a benzimidazole analogue with an additional ring, although the yield of nucleoside was only 31 %. Docking simulations between 1H and imidazo[4,5-b]phenazine nucleoside and the active site of PyNP (PDB 1BRW) supported our observation that 1H-imidazo[4,5-b]phenazine can be used as a substrate by PyNP. Thus, the enzymatic substitution reaction using PyNP can be used to incorporate many purine pseudo bases and benzimidazole derivatives with various functional groups into nucleoside structures, which have potential utility as diagnostic or therapeutic agents.

Applicability of a particularly simple model to nonlinear elasticity of slide-ring gels with movable cross-links as revealed by unequal biaxial deformation.

The strain energy density function (F) of the polyrotaxane-based slide-ring (SR) gels with movable cross-links along the network strands is characterized by unequal biaxial stretching which can achieve various types of deformation. The SR gels as prepared without any post-preparation complication exhibit considerably smaller values of the ratio of the stresses (σy/σx) in the stretched (x) and constrained (y) directions in planar extension than classical chemical gels with heterogeneous and nearly homogeneous network structures do. This feature of the SR gels leads to the peculiar characteristic that the strain energy density function (F) has no explicit cross term of strains in different directions, which is in contrast to F with explicit strain cross terms for most chemical gels and elastomers. The biaxial stress-strain data of the SR gels are successfully described by F of the Gent model with only two parameters (small-strain shear modulus and a parameter representing ultimate elongation), which introduces the finite extensibility effect into the neo-Hookean model with no explicit cross term of strain. The biaxial data of the deswollen SR gels examined in previous study, which underwent a considerable reduction in volume from the preparation state, are also well described by the Gent model, which is in contrast to the case of the classical chemical gels that the stress-strain relations before and after large deswelling are not described by a common type of F due to a significant degree of collapse of the network strands in the deswollen state. These intriguing features of nonlinear elasticity of the SR gels originate from a novel function of the slidable cross-links that can maximize the arrangement entropy of cross-linked and non-cross-linked cyclic molecules in the deformed networks.

Pressure-responsive polymer membranes of slide-ring gels with movable cross-links.

Polymer membranes comprising slide-ring gels with movable cross-links exhibit a nonlinear pressure-dependence in the fluidic flow rate. The proportional constant between the flow rate and pressure significantly changes at a critical pressure. The slide-ring gels are promising polymer membrane materials, which would allow for the on-off control of fluid permeability using an imposed pressure.

Fabrication of mechanically improved hydrogels using a movable cross-linker based on vinyl modified polyrotaxane.

This manuscript describes the preparation of new slide-ring gels by using a polyrotaxane as a cross-linker.

Thermal properties and microstructures of methylated polyrotaxane solutions.

Aqueous solutions of polyrotaxanes consisting of poly(ethylene glycol) and methylated alpha-cyclodextrins (alpha-CD) were studied by means of differential scanning calorimetry (DSC), dynamic light scattering, and X-ray diffraction in order to investigate the effect of the degree of methylation on thermoresponsive behavior. Polyrotaxanes with a degree of methylation higher than 50% had a lower critical solution temperature (LCST) and showed reversible associations and dissociations in water. In the transmittance measurements, the cloud point of methylated polyrotaxanes (MePR) shifted to a lower temperature with an increase in the degree of methylation. The heating curve obtained by DSC for the nearly permethylated polyrotaxane showed one broad endothermic peak that was associated with the microcrystallization of methylated CDs by hydrophobic interactions. On the other hand, the DSC profiles for partially methylated polyrotaxanes had several endothermic peaks, indicating multiple phase transitions of the MePR solutions. The results imply that the thermal properties of the MePR-water system are significantly affected by not only the methyl groups on alpha-CDs but also by the remaining hydroxyl groups.

Liquid-liquid equilibria of polyrotaxane and poly(vinyl alcohol).

Liquid-liquid equillibria (LLE) of the tertiary system of hydroxypropylated polyrotaxane (HPPR)-poly(vinyl alcohol) (PVA)-solvent have been investigated by focusing on the internal structures of HPPR-PVA blend gels. The phase diagrams of the HPPR-PVA aqueous systems displayed two liquid phases at a high concentration and molecular weight of PVA. This result was consistent with the prediction of the Flory-Huggins lattice model. On the contrary, the HPPR-PVA-DMSO system exhibited only a single phase. The HPPR-PVA blend gels crosslinked in dimethylsulfoxide (DMSO) were highly transparent over a wide concentration range, while the gels prepared in water were opaque at high polymer concentrations. Spherical domains were observed in the opaque gels by laser scanning confocal microscopy, and the sizes of the domains were significantly dependent on the amount of cross-linking reagent utilized. These results indicated that the transparency of the HPPR-PVA blend gels was strongly affected by the competition between the liquid-liquid two-phase separation and the crosslinking HPPR and PVA polymers during the preparation of the blend gels.

Local and network structure of thermoreversible polyrotaxane hydrogels based on poly(ethylene glycol) and methylated alpha-cyclodextrins.

Thermoreversible gelation and microphase formation of aqueous solutions of a methylated polyrotaxane (MePR) were investigated by means of differential scanning microcalorimetry, rheometry, and X-ray diffractometry (XRD). The aqueous solutions of MePR show a lower critical solution temperature (LCST) and form an elastic gel with increasing temperature. The sol-gel transition of the MePR solutions was induced by formation and deformation of aggregates of methylated alpha-cyclodextrins (alpha-CDs) of polyrotaxane due to hydrophobic dehydration and hydration, respectively. The XRD investigation revealed localization and highly ordered arrangement of methylated alpha-CDs along the PEG chain in the gel. The arrangement of CDs was also reflected by the changes in elasticity and long relaxation behavior of the solution around the sol-gel transition. The quasiequilibrium shear modulus of MePR solutions showed the critical phenomena against temperature. The scaling exponents measured at two different concentrations were almost equal to the values predicted by a gel percolation theory. Therefore, the heat-induced gelation of aqueous MePR solutions is well explained by a model in which clusters assembled with methylated alpha-CDs are gradually connected to the network as the temperature increases.

Thermoreversible sol-gel transition of an aqueous solution of polyrotaxane composed of highly methylated alpha-cyclodextrin and polyethylene glycol.

Novel thermo-gelling aqueous solution systems are developed with a polyrotaxane, consisting of methylated alpha-cyclodextrin and polyethylene glycol, based on the slide-ring properties of the macrocycles along the linear chain.

Surface-mediated photoalignment of discotic liquid crystals on azobenzene polymer films.

This paper describes a simple strategy for the formation of photoaligned and micropatterned discotic liquid crystal (DLC) film on the surface of photoirradiated azobenzene-containing polymer thin film. The key material for the surface-mediated photoalignment of the DLCs was poly[4-(4-cyanophenylazo)phenyl methacrylate] (pMAzCN). Optical anisotropy was generated in a pMAzCN film by oblique exposure to nonpolarized light which resulted in angle-selective photoisomerization and reorientation of the azobenzenes. Subsequent annealing of the film at 240 degrees C enhanced the photoaligned state of the p-cyanoazobenzenes due to strong intermolecular dipole-dipole interaction and semicrystalline nature of the pMAzCN. This combination of photoirradiation and subsequent annealing of the pMAzCN film made it possible to realize the surface-assisted orientation control of a DLC molecule, which displays both columnar (Col) and discotic nematic (N(D)) phases over 152 degrees C. When the pMAzCN film was exposed to linearly polarized light from the surface normal, the DLC molecules showed homeotropic orientation with the director perpendicular to the substrate surface. In the contrast, oblique irradiation of the pMAzCN film with nonpolarized light gave rise to tilted DLC orientation with well-ordered optical birefringence at the N(D) phase. Rapid cooling from the N(D) phase produced a well-aligned glassy N(D) state at room temperature, which was adequately stable for 10 months even though no covalent cross-linking among the DLCs was performed. The spatial orientation of photoaligned DLCs in both their bulk film and in their interface region was characterized by means of optical birefringence, X-ray diffraction, and fluorescence measurements. At the N(D) phase, the DLC molecules were aligned in a hybrid manner such that their tilt angles varied throughout the thickness of DLC film. The direction of tilted DLCs was opposite to the propagation of the actinic nonpolarized light. The photoaligned DLC films exhibited polarized fluorescence emission with an s-polarized/p-polarized intensity ratio of 4.1, despite the nonpolarized excitation of only DLC at outmost surface. These results indicate that the three-dimensionally aligned azobenzene moieties of the pMAzCN thin film were transferred to the tilted DLC molecules at air/DLC interface. Finally, we demonstrated micrometer-scale photopatterned orientation of DLC molecules on the pMAzCN surface by oblique nonpolarized irradiation of the film through a photomask.

Unique crystal structures of donor-acceptor complexes: crossed arrangement of two charge-transfer columns.

Unique crystal structures containing two kinds of crossed charge-transfer (CT) columns are demonstrated by cocrystals consisting of ion pairs of acceptors of methyl viologen and anthraquinone disulfonate derivatives and a donor, hydroquinone, mediated by electrostatic and donor-acceptor (DA) interactions.