Chemoenzymatic Synthesis of Ganglioside Gm4 Analogs as Potential Immunosuppressive Agents

ABSTRACT

An efficient, chemoenzymatic synthesis of ganglioside GM4 analogs having a potent immunosuppressive activity is described. One-step and highly regìoselective 6-O-acetylation of long-chain alkyl, 2-(trimethysilyl)ethyl and phenyl 1-thio β-D-galactopyranosides was performed by using vinyl acetate and lipase PS. The resulting 6-O-acetates (70-93%) were sialylated with methyl (phenyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-D-galacto-2-nonulopyranosid)onate promoted by N-iodosuccinimide (NIS) and trifluoromethanesulfonic acid (TfOH). The 2-(trimethylsilyl)ethyl glycoside derivative was converted to the imidate which was then coupled with dodecan-1-ol, hexadecan-1-ol, and 2-(tetradecyl)hexadecan-1-ol, respectively, to give the protected GM4 derivatives (90-96%). O-Deacylation and saponification of the methyl ester gave the target ganglioside GM4 analogs in high yields.     

Experimental demonstration of excitation and propagation of intrinsic localized modes in a mass–spring chain

Excitation and propagation of nonlinear localized oscillations are demonstrated experimentally by using a mass–spring chain to emulate those in the celebrated Fermi–Pasta–Ulam chain of β type. Two types of experiments are performed, one corresponding to a boundary-value problem and the other to an initial-value one. In the former experiment, one end of the chain is driven sinusoidally in the direction of the chain above a cut-off frequency. In the latter experiment, only a number of weights centered around the weight in the middle of the chain are held displaced initially and released suddenly. The experimental results show quantitatively good agreements with the numerical ones. Two videos linked also show visually mobile and stationary localized oscillations.     

Synthesis and structures of 15- and 18-membered unsaturated selenacrown ethers and their complexation with silver trifluoroacetate

The reaction of Na(2)Se with cis-dichloroethene yielded (Z,Z,Z,Z,Z)-1,4,7,10,13-pentaselenacyclopentadeca-2,5,8,11,14-pentaene (15-US-5) and (Z,Z,Z,Z,Z,Z)-1,4,7,10,13,16-hexaselenacyclooctadeca-2,5,8,11,14,17-hexaene (18-US-6). X-ray crystallographic analysis of 15-US-5 and 18-US-6 revealed an all-cis geometry around the carbon-carbon double bonds, with all of the selenium atoms located on one side of the respective ring planes. 15-US-5 and 18-US-6 were also found to be oxidized more easily than the corresponding sulfur analogues. The complexation of 15-US-5 and 18-US-6 with silver trifluoroacetate afforded Ag(I)(15-US-5)(CF(3)COO) and Ag(I)(5)(18-US-6)(3)(CF(3)COO)(5), respectively. [reaction: see text]     

Optical resolution and racemization mechanism of a tellurinic acid

[reaction: see text] Optically active tellurinic acid was obtained for the first time by chromatographic resolution of racemic 2,4,6-triisopropylbenzenetellurinic acid (1) using a chiral column. Optically active tellurinic acid (+)-1 was stable toward racemization in hexane, although racemization occurred in hexane/2-propanol. The kinetic studies for the racemization, oxygen exchange reaction using H(2)(18)O, and theoretical studies clarified that the racemization of the optically active tellurinic acid in solution proceeds via a hypervalent tellurane formed by addition of water remaining in solvent.     

Use of functional microgels with vinyl groups to accelerate photopolymerization reaction

Three types of functional microgels with vinyl groups on their surface were prepared. For the first type, the counter anion from clorin was exchanged with β-methacryloylethyl sulfonic acid, styrene sulfonic acid or allyl sulfonic acid in a microgel with ammonium anions. For the second and third types, a quaternization with N,N-dimethylaminoethyl methacrylate of 3-chloro-2-hydroxypropyl methacrylate in the presence of microgel particles was prepared by emulsion copolymerization of styrene, chloromethylstyrene or N,N-dimethylamino-methylbenzene, and divinylbenzene. The resulting samples show good dispersibility in organic solvents without an emulsifier. A functional microgel-based photopolymer combined with an acrylate monomer and ultraviolet (UV) or visible (VIS) light-absorbing photoinitiators provides oleophilic images when exposed to UV or VIS light and developed in tap water. This photopolymer has a higher sensitivity than those of photopolymers based on microgels with an analogous composition but without vinyl groups. Photopolymers pepared by using functional microgels with a methacryloyl group exhibited a higher rate of polymerization (R_p) than that of photopolymers based on microgels without a vinyl group. The R_p of photopolymers prepared by using a functional microgel with either an allyl group or vinylphenyl group was nearly equal to that of photopolymers based on microgels with ammonium ions. Their high sensitivities are attributed to the rapid photopolymerization in the methacryloyl group. To determine how the photoreaction mechanism enhances sensitivity, the photoreaction products were investigated using a model photopolymerization system. It was found that the gelation reactions enhancing sensitivity are predominantly the polymerization and crosslinking ones when a microgel with the methacryloyl group is used, and the graft copolymerization with acrylate monomers when a microgel with either the allyl group or vinylphenyl group is used.     

Phosphorescence from pyrimidine vapor

The phosphorescence from pyrimidine vapor has been observed by a method of time-resolved laser spectroscopy. The phosphorescence spectrum commences at 350.5 nm and consists mainly of three totally symmetric vibrations in the ground state, v_6a, v₁₂ and v_9a as in the case of the zero-point vibrational level fluorescence of pyrimidine vapor. The phosphorescence quantum yield and lifetime are found to be about 1 × 10^?4 and 50 μs.     

Coprecipitation Behavior of Some Metals with Chitosan and Their Determination by Graphite Furnace Atomic Absorption Spectrometry

Chitosan is derived from chitin by deacetylation. Chitosan forms a complex with metal ions and is soluble an organic acid. The metal ions were concentrated in the precipitates of chitosan when the sample solution containing metal ions was adjusted to neutral after the chitosan solution was added. These properties are applied to the preconcentration of metal ions in water samples and their determination by graphite furnace atomic absorption spectrometry. Metal ions,such as ruthenium, indium, vanadium, strontium,rhodium were collected in the chitosan precipitates.     

Determination of indium by graphite furnace atomic absorption spectrometry after coprecipitation with chitosan.

A sensitive and simple method for the determination of trace amounts of indium in water samples by graphite furnace atomic absorption spectrometry (GFAAS) after coprecipitation with chitosan was investigated. Indium was quantitatively preconcentrated from water samples by coprecipitation with chitosan at pH 7.0-9.0. The coprecipitant was easily dissolved with acetic acid, and indium in the resulting solution was determined by GFAAS. The addition of lanthanum as a chemical modifier was more effective for the atomic absorbance of indium. The detection limit (S/N > or = 3) for indium was 0.04 microg dm(-3), and the relative standard deviations (n = 5) were 3.5-4.5% at 1.0 microg/100 cm3. The results obtained in this study indicate that the proposed method can be successfully applied to the determination of trace indium in water samples.     

Supramolecular complexes of La@C82 with unsaturated thiacrown ethers

The paramagnetic La@C82-A(C2v) with unsaturated thiacrown ethers forms 1 : 1 host-guest complexes of [La@C82-A(C2v)]-[D]+ in solution as a result of electron transfer.