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 21,279 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The nature of the rate enhancements caused by gradually increasing the mole fraction of water in the solvent (from 0 to 1) for the cycloaddition reactions of pyridazinium-dicyanomethanide 1,3-dipole, 2, with the dipolarophiles ethyl vinyl ketone (a water-super dipolarophile) and methyl acrylate (a water-normal dipolarophile) in the organic solvents acetonitrile, acetone, methanol, ethanol, and tert-butyl alcohol at 37 degrees C are explored. In each case as the mole fraction of water surpassed ca. 0.9, exponential rate enhancements were triggered. When methanol replaced water, the rate enhancements were smaller, and no triggering effect was observed. The dramatic rate enhancement triggered for the water-super dipolarophile was significantly reduced as the temperature was raised in the range 29-64 degrees C. The results suggest that a dominant hydrogen-bonding effect operates in water-induced rate enhancements of 1,3-dipolar cycloaddition reactions with water-super dipolarophiles as well as the hydrophobic effect. The hydrogen-bonding effect involves secondary bridging hydrogen bonding from the primary water-solvation shell of the transition state and the growth of structured water clusters. Theoretical calculations strongly support these conclusions.
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PMID:The influence of water on the rates of 1,3-dipolar cycloaddition reactions: trigger points for exponential rate increases in water-organic solvent mixtures. Water-super versus water-normal dipolarophiles. 1538 27

Bile salt aggregates are supramolecular systems containing two different binding sites. The effect of the addition of acetonitrile on the specificity and dynamics of guest binding to the two binding sites of cholate aggregates was studied. The protection of guests included in the aggregate from interaction with ions in the aqueous phase was evaluated from quenching of the singlet and triplet excited states of guest molecules bound to the cholate aggregates. The dynamics of guest binding to the primary and secondary binding sites of the cholate aggregates were determined at increasing acetonitrile mole fractions. The structure of the aggregates was not significantly altered provided the cholate concentrations were higher than 20 mM and the acetonitrile mole fraction did not exceed 0.033 (9.1% v/v). These results show that acetonitrile can be used to modulate the solubility of guests in the aggregates and to manipulate the residence time of guests in the primary and secondary binding sites.
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PMID:Modulation with acetonitrile of the dynamics of guest binding to the two distinct binding sites of cholate aggregates. 1551 84

The reaction of [Pt3(mu-CO)3(PCy3)3](1) with one mole-equivalent of iodo-acetonitrile was quantitative at -70 degrees C giving the oxidative addition product [Pt3(mu-CO)3(PCy3)3(I)(CH2CN)](2). Fragmentation of was observed in solution giving [Pt2I(CH2CN)(CO)2(PCy3)2](3) which is the major product at room temperature if the starting cluster/reactant ratio is equal to or less than 1 to 1.5. Dimer 3 decomposes slowly in solution giving [Pt2I2(CO)2(PCy3)2](4a) and succinonitrile. Monomer [PtI(CH2CN)(CO)(PCy3)] was the final product of the reaction when using excess of iodo-acetonitrile. The reactions of with one mole-equivalent of halogens X2 gave the new 44-electron clusters [Pt3X(micro-CO)2(micro-X)(PCy3)3](X = I2(7a) or Br2(7b)) by oxidative addition followed by substitution of CO by X-. Fragmentation of and took place in solution when using one and a half mole-equivalents of X2 giving dimers 4a and [Pt2Br2(CO)2(PCy3)2](4b) as well as [Pt2X2(mu-X)2(CO)2(PCy3)2]. Monomers cis-[PtX2(CO)(PCy3)] were the final products of the reaction of with excess of halogens. Insertion of SnCl2 was observed into the Pt-Pt bond but not into the Pt-X bond, when equimolar amounts of SnCl2 x 2H2O were added to a solution of 4a or its chloro-analogue giving [Pt2X2(micro-SnCl2)(CO)2(PCy3)2]. The Pt(I) dimers have unusually small J(Pt-Pt) values as observed by 195Pt NMR and calculated by DFT. These values showed periodic changes comparing 4a and its analogues with other halides and mixed halide dimers.
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PMID:Oxidative addition of iodo-acetonitrile and of elemental halogens to [Pt3(mu-CO)3(PCy3)3]. 1561 20

The acid ionization constants of some pyrimidine bases of nucleic acids were determined pH-metrically at 25 degrees C and at the constant ionic strength I = 0.10 mol l(-1) (KNO3) in pure water as well as in aqueous media containing variable mole percentages (5-30%) of organic solvents. The organic solvents used were methanol, ethanol, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetonitrile, acetone and dioxane. The results obtained indicated that the acidity constants are generally decreased as the content of an organic solvent in the medium is increased. It was deduced that the hydrogen bonding interactions and the solvent basicity in addition to the electrostatic effect are the major effects influencing significantly the acid ionization process of pyrimidine bases in the different water-organic solvent media. Some thermodynamic parameters (deltaH, deltaG degrees, deltaS degrees) of the ionization process over the temperature range 5-45 degrees C in pure water were also determined and discussed.
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PMID:The acidity constants of some pyrimidine bases in various water-organic solvent media. 1562 45

An alcohol dehydrogenase from the hyperthermophilic archaeon Aeropyrum pernix was activated by water-miscible organic solvents. This activation was influenced by the kind and the concentration of the added organic solvents. The k(cat) was increased by a factor of over ten when the mole fraction of acetonitrile was 0.1. This effect was large when organic solvents with large log P values were added. In fact, the k(cat) showed a strong positive correlation with the log P value of the mixed solvent at a constant mole fraction of water, while it was not affected by the kind of organic solvents added. Both the activation enthalpy and the entropy decreased with an increase in log P. The contribution of the activation enthalpy to the free energy of activation was larger than that of the activation entropy, and the free energy of activation decreased with an increase in log P.
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PMID:Log P effect of organic solvents on a thermophilic alcohol dehydrogenase. 1575 97

Electric field induced switching behaviors of a series of low-density omega-carboxyalkyl modified H-Si(111) and the mixed omega-carboxyalkyl/alkyl covered H-Si(111) have been simulated by using molecular dynamics (MD) simulation techniques. The external electric fields may drive surface-confined molecules to reversibly change conformations between the all-trans (switching "on") and the mixed trans-gauche (switching "off") states. Such surfaces switch wettabilities between the hydrophilic state and the moderately hydrophobic state. It has been found in broad ranges of intensities of applied electric fields, -2.0 x 10(9) V/m < or = E(down) < or = 0 and 1.8 x 10(9) V/m < or = E(up) < or = 7.3 x 10(9) V/m, both the low-density (11.1%-33.3%) omega-carboxyalkyl and the mixed omega-carboxyalkyl/alkyl (in mole fraction of 0.4 < or = N(carboxyalkyl) : N(alkyl) < or = 3.0) monolayers covering H-Si(111) exhibit conformational switching in the aqueous medium. The critical intensity of the electric field, E(up) = 1.8 x 10(9) V/m, which is required to trigger the switches is observed by our MD simulations and further rationalized by a thermodynamical model. Some important factors in the control of switching performances, such as the steric hindrances, the formation of the electric double layer at the monolayer/electrolyte solution interface, the hydration effects of carboxylate anions, the components of surrounding electrolyte solutions, as well as the rigidity of surface-confined chains are elucidated. The lower ionic strength and additions of acetonitrile molecules in the surrounding aqueous solution can reduce the value of critical intensity of the electric field and hence facilitate the realization of switching. Some practical considerations in construction and optimum design of switching surfaces are also suggested.
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PMID:Electric field induced switching behaviors of monolayer-modified silicon surfaces: surface designs and molecular dynamics simulations. 1586 3

1H NMR spectroscopy was used to investigate the stoichiometry and stability of the drug ketamine cation complexes with some crown ethers, such as 15-crown-5 (15C5), aza-15-crown-5 (A15C5), 18-crown-6 (18C6), aza-18-crown-6 (A18C6), diaza-18-crown-6 (DA18C6), dibenzyl-diaza-18-crown-6 (DBzDA18C6) and cryptant [2,2,2] (C222) in acetonitrile (AN), dimethylsulfoxide (DMSO) and methanol (MeOH) at 27 degrees C. In order to evaluate the formation constants of the ketamine cation complexes, the CH3 protons chemical shift (on the nitrogen atom of ketamine) was measured as function of ligand/ketamine mole ratio. The formation constant of resulting complexes were calculated by the computer fitting of chemical shift versus mole ratio data to appropriate equations. A significant chemical shift variation was not observed for 15C5 and 18C6. The stoichiometry of the mono aza and diaza ligands are 1:1 and 1:2 (ligand/ketamine), respectively. In all of the solvents studied, DA18C6 formed more stable complexes than other ligands. The solvent effect on the stability of these complexes is discussed.
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PMID:The interaction between ketamine and some crown ethers in common organic solvents studied by NMR: the effect of donating atoms and ligand structure. 1597 63

A UV spectrophotometric analysis of 4-methoxy-2-(3 (4-phenyl-1-piperazinyl)) propyl-2,3-dihydro-6-methyl-1,3-dioxo-1H-pyrrolo [3,4-c] pyridine (II) in HCI (0.01 mole/L) was performed by determining the values of specific absorption coefficients at the following analytical wavelengths: 225, 285 and 350 nm. The separation by means of TLC of compound II and of its five decomposition products was also studied. Silica gel coated plates (60 F254) were used and the mobile phase consisted of butanol--acetic acid--water. A validated RP-HPLC method for the determination or purity evaluation of II, with phenacetin as an internal standard, is described. The solution of II in HCI (0.01 mole/L) was chromatographed on an octadecyl column (LiChrosorb 100 RP-18 column 250 x 4.0 mm I.D., dp = 5 microm) using an eluent composed of the mixture acetonitrile--phosphate buffer pH = 2. Ultraviolet detection was used at an operation wavelength of 239 nm. The HPLC method was validated by determination of the following parameters: selectivity, precision, accuracy, linearity, stability of the analite, LOD and LOQ. Kinetic studies of the decomposition process of II in both acidic and alkaline environments demonstrated the instability of the imide group.
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PMID:HPLC and TLC methodology for determination or purity evaluation of 4-methoxy-2-(3(4-phenyl-1-piperazinyl))propyl-2,3-dihydro-6-methyl-1,3-dioxo-1H-pyrrolo[3,4-c]pyridine. 1602 87

The influence of temperature and relative air humidity on the stability of ceftazidime in two different pharmaceutical preparations was investigated. The degradation of the substance studied was determined: (a) in dry air at 373, 378, 383, 388 and 393 K; (b) at air humidity RH = 76.4% at 323, 333, 343, 353 and 358 K; (c) in the humidity range 25.0 - 76.4% at 358 K. The decomposition of ceftazidime in these conditions is a first-order reaction relative to substance concentration. The kinetic and thermodynamic parameters of the degradation of ceftazidime were calculated and compared for the two pharmaceuticals. The study was conducted using the HPLC method on LiChrosorb RP-18 columns (250 x 4 mm) with 9% acetonitrile in 0.1 mole/L ammonium acetate as mobile phase and detection at 254 nm.
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PMID:Stability of ceftazidime pentahydrate in medicinal preparations Biotum and Ceftim. 1602 88

Counterion and organic modifier are two parameters in EKC that can be varied in order to obtain improved solubility, selectivity, and efficiency. The effect of changing surfactant counterion and/or organic modifier on the chromatographic and electrophoretic properties of cetyltrimethylammonium bromide (CTAB)/sodium octyl sulfate (SOS) vesicles is examined in EKC. The vesicles are prepared in a 1:3.66 cationic/ anionic mole ratio for a total surfactant concentration of 69 mM. The cationic CTAB is replaced by cetyltrimethylammonium chloride (CTAC) and the first use of CTAC/SOS vesicles is reported. The mean diameter of the CTAC/SOS vesicles is 96 nm while that of the CTAB/SOS vesicles is 85 nm. A class I modifier (2-amino-1-butanol) and a class II modifier (acetonitrile) have similar effects on the EOF, elution range, methylene selectivity, and the efficiency of the CTAB/SOS vesicles and the CTAC/SOS vesicles. Upon addition of 10% ACN, there is roughly a 10-fold increase in the efficiency of heptanophenone, a model hydrophobic compound, compared to the efficiency using unmodified vesicles. Linear free energy relationship (LFER) analysis using the Abraham solvation model is employed to characterize solute-vesicle interactions. The results suggest that organic modifier-vesicle interactions depend somewhat on the counterion.
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PMID:Effect of surfactant counterion and organic modifier on the properties of surfactant vesicles in electrokinetic chromatography. 1613 92


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