Intramolecular charge-transfer process of jet-cooled (p-cyanophenyl)pentamethyldisilane: roles of the torsional motion and the Si-Si bond change

To investigate the intramolecular charge-transfer (ICT) process of (p-cyanophenyl)pentamethyldisilane (CPDS), laser-induced fluorescence, dispersed fluorescence, and two-color resonance enhanced two-photon ionization spectra were measured in a jet-cooled isolated condition. Dual fluorescence of CPDS was observed from a ground vibrational level in the locally excited pipi state. Similar to an emission from the charge-transfer (CT) state in solution, one of the dual emissions of the isolated molecule in the jet was assigned as the CT emission. A significant vibrational dependence on the ICT process was found as exciting vibronic levels of the molecule. It was identified that the promoting mode of the ICT process is a torsional motion of the disilanyl group with respect to the phenyl ring. It was also revealed that an effective appearance energy of the CPDS cation via the CT state is much lower than that via the locally excited pipi state suggesting that the electronic configuration of the CT state is similar to that of the cation. On the basis of an electronic configuration of the cationic state, that of the CT state was suggested to be of the (sigma(Si)(-)(Si), 2ppi) type.     

Prediction of membrane protein structures by replica-exchange Monte Carlo simulations: case of two helices

We test our prediction method of membrane protein structures with glycophorin A transmembrane dimer and analyze the predicted structures in detail. Our method consists of two parts. In the first part, we obtain the amino-acid sequences of the transmembrane helix regions from one of existing WWW servers and use them as an input for the second part of our method. In the second part, we perform a replica-exchange Monte Carlo simulation of these transmembrane helices with some constraints that indirectly represent surrounding lipid and water effects and identify the predicted structure as the global-minimum-energy state. The structure obtained in the case for the dielectric constant epsilon=1.0 is very close to that from the nuclear magnetic resonance experiments, while that for epsilon=4.0 is more packed than the native one. Our results imply that the helix-helix interaction is the main driving force for the native structure formation and that the stability of the native structure is determined by the balance of the electrostatic term, van der Waals term, and torsion term, and the contribution of electrostatic energy is indeed important for correct predictions. The inclusion of atomistic details of side chains is essential for estimating this balance accurately because helices are tightly packed.     

The Solid-State and Solution Conformations of (+)-Chelidonine

The solid-state and solution conformations of (+)-chelidonine ( 1 ), a biologically active alkaloid, were determined by X-ray diffraction and ¹H-NMR spectroscopy, X-Ray diffraction analysis revealed a conformer with B/C ‘anti-type’ cis conjunction, a half-chair of ring B , and a twist half-chair of ring C. One H₂O molecule per one alkaloid molecule was cocrystallized and stabilized by H-bonding with OH? C(11). Analysis of the thermal behavior of the crystal showed more thermal stability in the monohydrate than the anhydrate. The NMR measurement of concentration and temperature dependences in CDCl₃ and in (CD₃)₂SO suggested that the OH group of 1 was intramolecularly H -bonded to N(5) in (CD₃)₂SO and intermolecularly H-bonded to the solvent in CDCl₃. Conformational-energy calculations by the MNDO method showed that the intramolecular H -bond was little affected by the conformational stabilization of 1 .     

An influence of a basic side chain moiety on the tautomer ratio between the enamine and methylene imine forms in azole condensed quinoxalines

The reaction of 7-chloro-4-(2-cyano-2-hydroxyvinyl)tetrazolo[1,5-α]quinoxaline 2a with 4-aminopyridine, p-toluidine or p-aminophenol gave 7-chloro-4-(4-pyridylcarbamoylmethylene)-4,5-dihydrotetrazolo[1,5-α]-quinoxaline 7a , 7-chloro-4-(p-tolylcarbamoylmethylene)4, 5-dihydrotetrazolo[1,5-α]quinoxaline 8a or 7-chloro-4-(p-hydroxyphenylcarbamoylmethylene)-4,5-dihydrotetrazolo[1,5-α]quinoxaline 9a , respectively. The reaction of 7-chloro-4-(2-cyano-2-hydroxyvinyl)-1,2,4-triazolo[4,3-α]quinoxaline 2b with 4-aminopyridine, p-toluidine or p-aminophenol afforded 7-chloro4-(4-pyridylcarbamoylmethylene)-4,5-dihydro-1,2,4-triazolo-[4,3-α]quinoxaline 7b , 7-chloro-4-(p-tolylcarbamoylmethylene)-4,5-dihydro-1,2,4-triazolo[4,3-α]quinoxaline 8b or 7-chloro-4-(p-hydroxyphenylcarbamoylmethylene)-4,5-dihydro-1,2,4-triazolo[4,3-α]quinoxaline 9b , respectively. The reaction of compound 2a with 2-aminopyridine or 3-aminopyridine provided 7-chloro-4-(2-pyridyl-carbamoylmethylene)-4,5-dihydrotetrazolo[1,5-α]quinoxaline 10 or 7-chloro-4-(3-pyridyl-carbamoylmethylene)-4,5-dihydrotetrazolo[1,5-α]quinoxaline 11 , respectively. Compounds 7a,b (4-pyridylcarbamoyl) predominated as the enamine tautomer A in a trifluoroacetic acid solution, while compounds 8a,b (p-tolylcarbamoyl) and compounds 9a,b (p-hydroxyphenylcarbamoyl) coexisted as the enamine A and methylene imine B tautomers in a trifluoroacetic acid solution. Moreover, the ratio of the enamine tautomer A elevated in an order of compound 11 (3-pyridylcarbamoyl), compound 10 (2-pyridylcarbamoyl) and compound 7a (4-pyridylcarbamoyl), reflecting an order of the increase in the pKa values of the aminopyridine side chain moieties. In general, the ratio of the enamine tautomer A was higher in the basic carbamoyl derivatives 7–11 than in the neutral ester derivatives 3a,b . From these results, the basic side chain moiety of the tetrazolo[1,5-α]quinoxalines 7a-11 or 1,2,4-triazolo[4,3-α]quinoxalines 7b-9b was found to increase the ratio of the enamine tautomer A in trifluoroacetic acid media.     

Development of chiral stationary phases consisting of low-molecular-weight cellulose derivatives covalently bonded to silica gel

We prepared chiral supports whose chiral stationary phase (CSP), consisting of a low-molecular-weight cellulose derivative (degree ofpolymerization: 15), is covalently bonded to silica gel. The cellulose used asthe base material of the CSP was pre-hydrolysed with phosphoric acid before thecoupling reaction to unite a reducing terminal in the cellulose and anaminopropyl group on the surface of silanized silica. After substitutinghydroxyl groups in cellulose by using 3,5-dichlorophenyl isocyanate or phenylisocyanate, we tested the CSP thus obtained for its performance in chiralrecognition and found a wide range of chiral discrimination ability. We alsoconfirmed that an elution using strong solvents as a mobile phasecould be achieved, which is difficult for the coated-type CSPs because themobile phase may dissolve the CSP. However, more enantiomeric mixtures showedlarger selectivity factors () when eluted on the coated-type CSPs thanonthe covalently bonded CSPs. A coated-type CSP consisting of thelow-molecular-weight cellulose phenylcarbamate, prepared as a control CSPsample, showed comparable performance with the commercial coated-type column(CHIRALCEL® OC), so the slightly poorer performance ofthe chemically bonded CSPs may be explained by the difficulty of the polymerconsisting of the CSPs in taking an optimal supermolecular structure requiredfor chiral recognition due to the fixation to the silica gel. The lowdegreeof polymerization may have an additional effect.     

The high throughput analysis of N-methyl carbamate pesticides in wine and juice by electrospray ionization liquid chromatography tandem mass spectrometry with direct sample injection into a short column

We developed a new analysis method for the N-methyl carbamate pesticides in juice and wine. The juice and wine were diluted with ultra pure water, and determined by electrospray ionization tandem mass spectrometry (ESI LC/MS/MS) with direct sample injection into a short column. The new method, including sample preparation and determination, is simple and rapid, and allows simultaneous determination of nine N-methyl carbamate pesticides in juice and wine within analysis time that is much shorter as compared with the traditional method. The average recoveries from juice and wine fortified at the level of 0.1 ppm ranged from 59.6 to 126.7% with the coefficients of variation ranging from 0.4 to 5.1% for intra-day (n = 5 × 3 days) and from 0.5 to 22.6% for inter-day (n = 15). At the fortified level of 0.5 ppm, the recoveries ranged from 69.3 to 127.2% with the coefficients of variation ranging from 0.4 to 6.9% for intra-day (n = 5 × 3 days) and from 0.5 to 22.6% for inter-day (n = 15). The method is considered to be satisfactory for the monitoring of the carbamate pesticides residues in juice and wine, suggesting that the present method is applicable to other pesticide residues in foods.     

Selective substitution of alkali cations in mixed alkali glass by solid-state electrochemistry

Electrosubstitution of alkali cations in mixed-alkali glass containing both Na₂O and K₂O for other monovalent metal cations (M⁺=Li⁺, Ag⁺, and Cs⁺) was investigated using a solid-state electrochemical method. The fundamental electrolysis system consists of anode/M⁺-conducting microelectrode/glass/Na-β″-Al₂O₃/cathode, where M⁺ is substituted for the alkali metal ions in the glass under an applied electric field. Li⁺ ions attacked only Na⁺ sites, and Ag⁺ ions replaced Na⁺ sites more readily than K⁺. In contrast, Cs⁺ ions simultaneously substituted for both Na⁺ and K⁺ sites. The substitution behavior appears to depend on the difference in ionic conductivity between K⁺ and Na⁺ and the radius of the dopant. This mechanism was discussed qualitatively.     

Thermotropic liquid-crystalline peptide derivatives: oligo(glutamic acid)s forming hydrogen-bonded columns

We report on new thermotropic liquid-crystalline oligo(amino acid) derivatives forming columnar structures. These are based on branched oligo(glutamic acid)s and 2-(3,4-dialkyloxyphenyl)ethyl moieties. An oligo(glutamic acid) derivative, alpha,gamma-bis(L-glutamoyl) L-glutamic acid tetra[2-(3,4-dioctadecyloxyphenyl)ethyl]ester, shows a hexagonal columnar phase, whilst a glutamic acid derivative, alpha,gamma-bis[2-(3,4-dioctadecyloxyphenyl)ethyl] L-glutamate, does not show a mesophase. Hydrogen bonds formed by the oligo(glutamic acid) moieties should contribute to the induction of the columnar liquid-crystalline properties. In addition, we have examined the effects of the molecular chirality of the oligo(glutamic acid) parts and the functionalisation at the focal position of the taper shaped molecules on the liquid-crystalline properties of the compounds.     

Chemical modification of butyl rubber. I. Synthesis and properties of poly(ethylene oxide)-grafted butyl rubber

A new amphiphilic polymer, i.e., poly(ethylene oxide) (PEO)-grafted butyl rubber (IIR-g-PEO), was synthesized by chemical modification of chlorinated butyl rubber (CIIR). This synthesis was based on the reaction between chlorine in CIIR and potassium salt of polyethylene glycol monomethyl ether (PEGM). PEGM's with molecular weights of 750 and 2000 were used. The maximum grafting percent of the resulted copolymers after the purification was ca. 45%, regardless of the molecular weight of PEGM. The microphase separated structure was observed in the films of IIR-g-PEOs. IIR-g-PEO, whose PEO molecular weight was 2000 and whose PEO content was 23.8 wt %, swelled 3.1% in water, and it was six times larger than the swelling of CIIR. The hydrophilicity of grafted polymers depended on the molecular weight of PEO side chains and PEO content. Even when the PEO contents were same in the polymers which were grafted PEGM-750 and PEGM-2000, their swellings in organic solvents were not equal. It was due to the difference of the microphase-separated structures of the grafted polymers. IIR-g-PEO, whose PEO molecular weight was 750 and whose PEO content was 10.3 wt %, swelled 1.5% in water, but showed an excellent emulsifying ability, where an oil-in-water-type emulsion was formed. © 1993 John Wiley & Sons, Inc.     

An efficient synthetic methodology of chiral isoquinuclidines by the enantioselective Diels-Alder reaction of 1,2-dihydropyridines using chiral cationic palladium-phosphinooxazolidine catalyst

High purity chiral isoquinuclidines (97% ee) were obtained from the enantioselective Diels-Alder reaction of 1-phenoxycarbonyl-1,2-dihydropyridine with 1-benzyl-2-acryloylpyrazolidin-3-one using chiral cationic palladium-phosphinooxazolidine (Pd-POZ) catalyst. The obtained DA adduct was easily converted to the chiral piperidine derivative bearing three stereogenetic centers in the structure.