1,721,062 research outputs found
Enzyme activity and stability control by amphiphilic self organizing systems in aqueous solutions
No full textThe interaction of surfactants with proteins in aqueous solutions has been the subject of many investigations to understand
the interactions between membrane proteins and lipids, structurally similar to synthetic surfactants. The effect of surfactant on enzyme structure and activity is the result of chemically selective interactions that may be influenced both by the enzyme structure and by the chemistry of the surfactant. For many years, surfactants have been considered as non-specific denaturants
of proteins, even if in the literature several of them are reported to enhance activity andror stability of some enzymes: the detergent can interact with the enzyme and cause a conformational change to a more active form andror stabilize its native
folded structure. Although the surfactant head group seems to have a determining role, other structural features of the detergent are also important in influencing the catalytic properties of an enzyme, i.e. head group size and its hydrophobic hydrophilic balance. Up to now it is very difficult to predict the molecular features of the surfactant and an extensive investigation on the relationship between the surfactant chemical structure and the catalytic properties of enzyme is still required
Kinetics and stability of alpha-chymotrypsin in surfactant-rich media and in water-butyl acetate two-phase systems
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Method for the production of binary clathrate hydrates of hydrogen
No full textA method for the effective production of binary clathrate hydrates of hydrogen and a co-former is provided, said method comprising the formation of water-in-oil nanoemulsions, such as, e.g., surfactant reverse micelles in an organic solvent, and the formation of clathrate hydrate nanocrystals from the water droplets thus obtained
SN2 displacement by bromide ions in dichloromethane. The role of reverse micelles
No full textReverse micellar systems are of interest as reaction media
because they are powerful models for biological compartmentalization,
enzymatic catalysis and separation of biomolecules.
Solutions of ionic surfactants in apolar solvents may contain reverse micelles, but they may also contain ion pairs, or small clusters, with waters of hydration. We studied the
bimolecular reaction in CH2Cl2 solutions of cationic tetraalkylammonium
bromide salts (onium salts), such as cetyltrimethylammonium
bromide (CTABr), cetyltripropylammonium
bromide (CTPABr) and tetra-n-butylammonium bromide
(TBABr). Methylnaphthalene-2-sulfonate (β-MeONs),
its 6-sulfonate derivative (β-MeONsS–) as the 2,6-lutidinium salt and methyl-5-N,N,N,trimethylammonium naphthalene-1-sulfonate (α-MeONsNT+) as the trifluoromethanesulfonate
salt react with Br– in CH2Cl2. First-order rate constants, kobs, increase linearly and similarly for the three substrates with
increasing concentrations of the onium salts. Reactions are
faster with TBABr than they are with CTPABr and CTABr,
and the reactivity of the three substrates is in the order: α-
MeONsNT+ >> β-MeONsS– > β-MeONs. The reactions are
inhibited by the addition of H2O, but CTABr tolerates H2O
in large excess. At [H2O]/[CTABr] = w0 = 6, “water-pool” reverse micelles form, and kobs for all three substrates is then independent of w0
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Mercury extraction by ionic liquids: temperature and alkyl chain length effect
No full textThe preliminary results described here show the complete transfer of Hg(II) ions, in the absence of a chelating agent in
1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids; the lag time required to gain the quantitative metal ion partition turned out to be strongly dependent both on alkyl chain length on the imidazolium ring and on the working temperature
Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate as kinetic probe for piperazinium-based cationic micelles
No full textA new class of cationic surfactants containing the heterocyclic piperazinium ring in their covalent structure was prepared; cetyldialkylpiperazinium
halides, CRPX, with alkyl = Me (CMPX), Et (CEPX), n-Pr (CPPX), and with halide = Br and Cl. They were characterized by measures of critical micellar concentration, cmc, and ionization degree, α, and also by use of the decarboxylation of 6-nitrobenzisoxazole-3-
carboxylate as a kinetic probe to investigate the properties of the microinterface they provide in aqueous solutions. The pseudophase kinetic treatment fails to fit the data at high [surfactant], which show anomalies with abrupt increase in kobs, especially for CEPX and CPPX. Data can be fitted up to [surfactant] ca. 0.1 M, and 0.2 M in some cases. Compared with cetyltrialkylammonium halides, values of k'M indicate a water-richer microenvironment and less important interface property changes with increasing head group bulk. The reaction could be studied both in the presence and in the absence of NaOH; comparison shows that NaOH affects only the shape of the kinetic profile at low [surfactant], without affecting the microenvironment provided by mature micelles
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