Thermoreversible gelation and phase separation in aqueous methyl cellulose solutions

We derived typical phase diagrams for aqueous solutions of methyl cellulose (MC) of different molecular weights via micro‐differential scanning calorimetry, small‐angle X‐ray scattering, and visual inspection. The phase diagrams showed the cooccurrence of gelation and phase separation and qualitatively agreed with the theoretically calculated diagrams. The sol–gel transition line and phase separation line of a lower critical solution point type shifted toward lower temperatures and lower concentrations with an increase in the MC molecular weight. The sol–gel transition line intersected at a temperature higher than the critical point of the phase separation; therefore, both sol–gel phase separation and gel–gel phase separation were possible, depending on the temperature. Specifically, through visual inspection of a high molecular weight MC sample in the critical temperature region, we observed phase separation into two coexisting gels with different polymer concentrations. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 91–100, 2001     

The efficient direct synthesis of N,O-acetal compounds as key intermediates of discorhabdin A: oxidative fragmentation reaction of alpha-amino acids or beta-amino alcohols by using hypervalent iodine(III) reagents

Hypervalent iodine(III) reagents are readily available, easy to handle, and have a low toxicity and similar reactivities to those of heavy metal reagents, and hence they are used for various oxidative reactions. The oxidative cleavage of alkynes or carbonyl compounds by using bis(trifluoroacetoxy)iodo(III) pentafluorobenzene (C(6)F(5)I(OCOCF(3))(2)) has been reported. Herein, the efficient direct synthesis of N,O-acetal compounds as key intermediates of discorhabdin A, by the oxidative fragmentation reaction of alpha-amino acids or beta-amino alcohols by using C(6)F(5)I(OCOCF(3))(2), is described.     

Conformational changes during apoplastocyanin folding observed by photocleavable modification and transient grating

A new method to investigate the initial protein folding dynamics is developed based on a pulsed laser light triggering method and a unique transient grating method. The side chain of the cysteine residue of apoplastocyanin (apoPC) was site-specifically modified with a 4,5-dimethoxy-2-nitrobenzyl derivative, where the CD and 2D NMR spectra showed that the modified apoPC was unfolded. The substituent was cleaved with a rate of about 400 ns by photoirradiation, which was monitored by the disappearance of the absorption band at 355 nm and the increase in the transient grating signal. After a sufficient time from the photocleavage reaction, the CD and NMR spectra showed that the native beta-sheet structure was recovered. Protein folding dynamics was monitored in the time domain with the transient grating method from a viewpoint of the molecular volume change and the diffusion coefficient, both of which reflect the global structural change, including the protein-water interaction. The observed volume decrease of apoPC with a time scale of 270 micros is ascribed to the initial hydrophobic collapse. The increase in the diffusion coefficient (23 ms) is considered to indicate a change from an intermolecular to an intramolecular hydrogen bonding network. The initial folding process of apoPC is discussed based on these observations.     

Systematic structural control of multichromic platinum(II)-diimine complexes ranging from ionic solid to coordination polymer

Reactions of a Pt(II)-diimine-based metalloligand Na(2)[Pt(CN)(2)(4,4'-dcbpy)] (4,4'-H(2)dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) with alkaline-earth metal salts Mg(NO(3))(2)·6H(2)O, CaCl(2), SrCl(2)·6H(2)O, and BaBr(2)·2H(2)O in aqueous solution gave luminescent complexes formulated as [Mg(H(2)O)(5)][Pt(CN)(2)(4,4'-dcbpy)]·4H(2)O (MgPt-4·9H(2)O), {[Ca(H(2)O)(3)][Pt(CN)(2)(4,4'-dcbpy)]·3H(2)O}(∞) (CaPt-4·6H(2)O), {[Sr(H(2)O)(2)][Pt(CN)(2)(4,4'-dcbpy)]·H(2)O}(∞) (SrPt-4·3H(2)O), and {[Ba(H(2)O)(2)][Pt(CN)(2)(4,4'-dcbpy)]·3H(2)O}(∞) (BaPt-4·5H(2)O), respectively. The crystal structures of all MPt-4 complexes were determined by X-ray crystallography. In these structures, the alkaline-earth metal ions are commonly coordinated to the carboxyl groups of the [Pt(CN)(2)(4,4'-dcbpy)](2-) metalloligand. In the case of MgPt-4·9H(2)O, the Mg(II) ion is bound by five water molecules and one oxygen atom of a carboxyl group to form a neutral complex molecule [Mg(H(2)O)(5)][Pt(CN)(2)(4,4'-dcbpy)]. In contrast, the alkaline-earth metal ion and metalloligand form two-dimensional (CaPt-4·6H(2)O) and three-dimensional (SrPt-4·3H(2)O and BaPt-4·5H(2)O) coordination networks, respectively. All fully hydrated complexes exhibited a strong phosphorescence from the triplet π-π* transition state. Luminescence spectroscopy revealed that MgPt-4·9H(2)O exhibited interesting multichromic (i.e., thermo-, mechano-, and vapochromic) luminescence, whereas CaPt-4·6H(2)O showed only thermochromic luminescence. The other two complexes did not exhibit any chromic behaviour. Combination analysis of powder X-ray diffraction, thermogravimetry, and IR spectroscopy suggests that the dimensionality of the coordination network contributes considerably to both the structural flexibility and luminescence properties; that is, the low-dimensional flexible coordination network formed in MPt-4 complexes with smaller alkaline-earth metal ions enables a structural rearrangement induced by thermal and mechanical stimuli and vapour adsorption, resulting in the observed multichromic behaviour.     

Terpyridine platinum(ii) complexes containing triazine di- or tri-thiolate bridges: structures, luminescence, electrochemistry, and aggregation

Dinuclear complexes [{Pt(trpy)}(2)(L)](PF(6))(2) (trpy = 2,2':6',2'-terpyridine, L = 2-octylthio-1,3,5-triazine-4,6-dithiolate ion (1), L = 2-octadecylthio-1,3,5-triazine-4,6-dithiolate ion (2), L = 2-di-n-butylamino-1,3,5-triazine-4,6-dithiolate ion (3)) and a trinuclear complex [{Pt(trpy)}(3)(L)](PF(6))(3) (L = 1,3,5-triazine-2,4,6-trithiolate ion (4)) have been synthesized and characterized. The single crystal X-ray analysis revealed that the two {Pt(trpy)}(2+) fragments in 1 and 3 adopt a syn-configuration. The PtPt distances are around 4.3 ?, suggesting no intramolecular PtPt interactions. Complexes 1-4 in acetonitrile show broad absorption bands at around 470 nm, assigned to mainly the ligand-to-ligand charge transfer ((1)LLCT) from triazine thiolates to trpy based on the comparison to the related complexes and the density functional theory (DFT) calculations. The red luminescence of 1-4 in acetonitrile is attributable to emission predominantly from (3)LLCT. Cyclic voltammograms of 1-3 exhibit four redox couples from -2.0 V to 0 V vs. Ag/AgCl. The two consecutive processes at around -0.70 V are assigned to the sequential reduction of two trpy ligands. This assignment was further supported by the observation of the anion radical of trpy in spectroelectrochemical experiments. The splitting of the redox potentials of two trpy ligands evidences the moderate electronic coupling interactions mediated by the triazine dithiolate bridges. Complex 2 formed a transparent red gel in CH(3)CN, whereas 4 produced a gel-like solid in the mixtures of CH(3)CN and other solvents. The interactions dominating the aggregative behaviours have been discussed based on the results of electronic absorption and emission spectroscopy.     

Gravitational compression dynamics of charged colloidal crystals

We examine the compression of charged colloidal crystals under the influence of gravitational force by monitoring the spatiotemporal variations of Bragg diffraction from the crystal lattice. We use the dilute aqueous dispersions of colloidal silica particles (diameter=216 nm, charge number=733, a particle volume fraction φ=0.06) in the presence of 5-15 μM sodium chloride. The sedimentation profiles of the colloidal crystals along the crystal height are determined by in situ fiber optics reflection spectroscopy. The time evolutions of the sedimentation profiles are calculated by numerical simulations based on a phenomenological continuum model that explicitly incorporates the electrostatic interparticle interactions. The simulation results correctly describe the experiments at sufficiently high ionic strength.     

Gas transport properties of hyperbranched polyimide/hydroxy polyimide blend membranes

Gas transport properties of novel hyperbranched polyimide/hydroxy polyimide blends and their silica hybrid membranes were investigated. Gas permeability coefficients of the blend membranes showed positive deviation from a semilogarithmic additive rule. The enhanced gas permeability were resulted from the increase in free volume elements caused by the intermolecular interaction between terminal amine groups of the hyperbranched polyimide and hydroxyl groups of the hydroxy polyimide backbone. Additionally, CO₂/CH₄ separation ability of the blend membranes was markedly promoted by hybridization with silica. The remarkable CO₂/CH₄ separation behavior was considered to be due to characteristic distribution and interconnectivity of free volume elements created by the incorporation of silica. For the hyperbranched polyimide/hydroxy polyimide blend system, polymer blending and hybridization techniques synergistically provided the excellent CO₂/CH₄ separation ability.     

Benzoin reaction in water as an aqueous medium catalyzed by benzimidazolium salt

Benzoin reactions are catalyzed by N,N-dialkylbenzimidazole to yield α-hydroxy ketones; the reaction proceeds in water as an aqueous medium under mild conditions. The utility of these salts as pre-catalysts in these reactions has been demonstrated.