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51.
Robert Faure Jean-Pierre Galy Emile-Jean Vincent Jos Elguero 《Journal of heterocyclic chemistry》1977,14(8):1299-1304
The substituent effects on azido/tetrazole equilibrium on a series of 2-substituted thiazoles, benzothiazoles, thiadiazoles, benzoxazoles and isoxazoles has been studied by proton magnetic resonance in two solvents (DMSO-d6 and deuteriochloroform). For thiazole an excellent Hammett relationship was found, both for the 4 and 5 positions. 相似文献
52.
Wiskur SL Lavigne JJ Metzger A Tobey SL Lynch V Anslyn EV 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(15):3792-3804
The thermodynamics of guanidinium and boronic acid interactions with carboxylates, alpha-hydroxycarboxylates, and diols were studied by determination of the binding constants of a variety of different guests to four different hosts (7-10). Each host contains a different combination of guanidinium groups and boronic acids. The guests included molecules with carboxylate and/or diol moieties, such as citrate, tartrate, and fructose, among others. The Gibbs free energies of binding were determined by UV/Vis absorption spectroscopy, by use of indicator displacement assays. The receptor based on three guanidinium groups (7) was selective for the tricarboxylate guest. The receptors that incorporated boronic acids (8-10) had higher affinities for guests that included alpha-hydroxycarboxylate and catechol moieties over guests containing only carboxylates or alkanediols. Isothermal titration calorimetry revealed the enthalpic and entropic contributions to the Gibbs free energies of binding. The binding of citrate and tartrate was investigated with hosts 7-10, for which all the binding events were exothermic, with positive entropy. Because of the selectivity of hosts 8-10, a simple boronic acid (14) was also investigated and determined to be selective for alpha-hydroxycarboxylates and catechols over amino acids and alkanediols. Further, the cooperativity of 8 and 9 in binding tartrate was also investigated, revealing little or no cooperativity with 8, but negative cooperativity with 9. A linear entropy/enthalpy compensation relationship for all the hosts 7-10, 14, and the carboxylate-/diol-containing guests was also obtained. This relationship indicates that increasing enthalpy of binding is offset by similar losses in entropy for molecular recognition involving guanidinium and boronic acid groups. 相似文献
53.
Oscillating Chemical Reactions. III. Effects of the Temperature and Chemical Composition on the ‘Induction Period’ of the BrO /Ce4+/Cyclohexanon and BrO /Ce4+/Cyclopentanon Systems A study of the influence of the temperature and composition of the BrO/Ce4+/cyclohexanon (S1) and BrO/Ce4+/cyclopentanon (S2) systems has shown a very particular behaviour for τind. for a given ratio α of concentrations: Moreover, for 0.27 ? α ? 0.32, log1/τind. is no longer a linear function of the inverse of the temperature: a break in the line log1/τind. = f(1/T) occurs. 相似文献
54.
55.
The scarcity of structural information on carbohydrates results from combined difficulties to crystallize and the limitations in NMR analysis. Current methods for determining basic NMR parameters such as (1)H homonuclear scalar couplings are very limited, especially for large molecules such as polysaccharides, oligonucleotides, and the carbohydrate part of glycoproteins. In this paper, a NMR experiment for the determination of endocyclic (1)H homonuclear scalar couplings ((3)J(HH)) in unlabeled carbohydrates is presented. In addition to scalar couplings, cross-correlated dipole-dipole relaxation rates were measured for large polysaccharides. The measurement of all endocyclic homonuclear coupling constants within monosaccharide units is presented for lactose, a model disaccharide, and for a natural-abundance 2 MDa bacterial polysaccharide excreted by Streptococcus thermophilus Sfi39. 相似文献
56.
Bradley M Ramos J Vincent B 《Langmuir : the ACS journal of surfaces and colloids》2005,21(4):1209-1215
The use of microgels for controlled uptake and release has been an area of active research for many years. In this work copolymer microgels of N-isopropylacrylamide (NIPAM) and acrylic acid (AAc), containing different concentrations of AAc and also cross-linking monomer, have been prepared and characterized. These microgels are responsive to pH and temperature. As well as monitoring the equilibrium response to changes in these variables, the rates of swelling/de-swelling of the microgel particles, on changing either the pH or the temperature, have also been investigated. It is shown that the rate of de-swelling of the microgel particles containing AAc is much faster than the rate of swelling, on changing the pH appropriately. This is explained in terms of the relative mobilities of the H(+) and Na(+) ions, in and out of the particles. It was observed that the microgels containing AAc, at pH 8, de-swelled relatively slowly on heating to 50 degrees C from 20 degrees C. This is attributed to the resistance to collapse associated with the large increase in counterion concentration inside the microgel particles. The swelling and de-swelling properties of these copolymer microgels have also been investigated in aqueous poly(ethylene oxide) (PEO) solutions, of different MW (2000-300 000). The corresponding absorbed amounts of PEO from solution onto the microgels have also been determined using a depletion method. The results, as a function of AAc content, cross-linker concentration, PEO MW, pH, and temperature, have been rationalized in terms of the ease and depth of penetration of the PEO chains into the various microgel particles and also the H-bonding associations between PEO and either the -COOH of the AAc moeities and/or the H of the amide groups (much weaker). Finally, the adsorption and desorption of the PEO molecules in to and out of the microgel particles have been shown to be extremely slow compared to normal diffusion time scales for polymer adsorption onto rigid surfaces. 相似文献
57.
Product distributions and rate constants for the reaction of ground state C+ ions with O2, NO, HCl, CO2, H2S, H2O, HCN, NH3, CH4, H2CO, CH3OH, and CH3NH2 have been measured. Rate constants were obtained using ion cyclotron resonance trapped ion methods at JPL, and product distributions were obtained using a tandem (Dempster-ICR) mass spectrometer at the University of Utah. Rapid carbon isotope exchange has also been observed in C+-CO collisions. 相似文献
58.
It is known that the designed alpha-helical peptide family TRI [(Ac-G(LKALEEK)4G-CONH2)], containing single site substitution of a cysteine for a leucine, is capable of binding Cd(II) within a three-stranded coiled coil. The binding affinity of cadmium is dependent upon the site of substitution, with cysteine incorporated at the a site leading to cadmium complexes of higher affinity than when a d site was modified. In this work we have examined whether this differential binding affinity can be expressed in a di-cysteine-substituted peptide in order to develop site specificity within a designed system. The peptide TRI L9CL19C was used to determine whether significant differences in binding affinities at nearly proximal sites could be achieved in a short designed peptide. On the basis of 113Cd, 1H NMR, and circular dichroic spectroscopies, we have shown that 1 equiv of Cd(II) binds exclusively at the a site. Only after that position is filled does the second site become populated. Thus, the TRI system represents the first example where stoichiometrically equivalent peptides with different sequences form the framework for designing molecular assemblies that show site-specific ion recognition. We propose that the distinct metal affinities are due to the cysteine conformers at different substitution points along the peptide. Furthermore, we have shown that site selectivity in biomolecules can be encoded into relatively short peptides with helical sequences and, therefore, do not necessarily require the extensive protein scaffolds found in natural systems. 相似文献
59.
Reipa V Mayhew MP Holden MJ Vilker VL 《Chemical communications (Cambridge, England)》2002,(4):318-319
Spectroelectrochemistry measurements are used to demonstrate that active site mutation and binding of an non-natural substrate to P450cam (CYP101) reduces the shift in the redox potential caused by substrate-binding, and thereby results in slower catalytic turnover rate relative to wild-type enzyme with the natural camphor substrate. 相似文献
60.
Anicich VG Wilson P McEwan MJ 《Journal of the American Society for Mass Spectrometry》2004,15(8):1148-1155
The results of a study of the ion-molecule reactions of N(+), N(2)(+), and HCN(+) with methane, acetylene, and ethylene are reported. These studies were performed using the FA-SIFT at the University of Canterbury. The reactions studied here are important to understanding the ion chemistry in Titan's atmosphere. N(+) and N(2)(+) are the primary ions formed by photo-ionization and electron impact in Titan's ionosphere and drive Titan's ion chemistry. It is therefore very important to know how these ions react with the principal trace neutral species in Titan's atmosphere: Methane, acetylene, and ethylene. While these reactions have been studied before the product channels have been difficult to define as several potential isobaric products make a definitive answer difficult. Mass overlap causes difficulties in making unambiguous species assignments in these systems. Two discriminators have been used in this study to resolve the mass overlap problem. They are deuterium labeling and also the differences in reactivities of each isobar with various neutral reactants. Several differences have been found from the products in previous work. The HCN(+) ion is important in both Titan's atmosphere and in the laboratory. 相似文献