1,720,989 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
Understanding mercury extraction mechanism in ionic liquids
No full textIn this paper the complete removal of mercury ions from aqueous solutions using hydrophobic ionic liquids
in the absence of chelating agents is reported. Several parameters were studied; in particular, the anionic component of ionic liquid, the anion of the metal salt and the nature of the aqueous phase used to dissolve the mercuric salt were varied to understand the transfer mechanism of Hg(II) ions into "classical" imidazolium-based ionic liquid. Results seem to suggest that metal ion partition into 1-octyl-3-
methylimidazolium salts involves neutral and/or anionic Hg(II) species and that the rate of the process
is strongly dependent on the experimental conditions, such as the working temperature and the nature of the anionic component of the ionic liquid, of the buffer solution and of the counterion of Hg(II). Moreover, addition of inorganic salts, such as NaCl or NaBr, to the aqueous phase increases metal ion extraction
rate, with more than 90% of metal ion being transferred into the organic phase within 30 min and total ion removal accomplished in just four hours. Salt effects could even overcome the high viscosity of the ionic liquid and then a simple method for large-scale Hg(II) extraction was developed
Comparison of Hydrogen Hydrates with existing Hydrogen Storage Technologies: Energetic and Economic Evaluations
No full textWith the development of the hydrogen economy and FCV (fuel cell vehicles), the manner of storing and delivering large quantities of hydrogen arises as a major problem, and increasing research efforts are being targeted to solve this technological issue. Nowadays several hydrogen storage methodologies are available. Technologies are being developed and/or engineered other than the classical compression and liquefaction of hydrogen, which are based on the chemical (metal hydrides, ammonia) and physical (e.g., carbon nanotubes) adsorption of H2. Also, a novel technology is in progress, which is based on clathrate hydrates of hydrogen. The object of the present work is to evaluate the features and performances of those storing systems with the aim to determine the best available technology throughout the ‘‘hydrogen chain’’. For each one of the storage solutions presented, we have compared key parameters such as: interaction energy between hydrogen and support, storage capacity, specific energy consumption (SEC). By this work, it is demonstrated that a technology based on clathrate hydrates of hydrogen, while far from being optimized, may be competitive with the other approaches
Surface charge modulation of sulfobetaine micelles by interaction with different anions: A dynamic light scattering study
No full textThis work reports a Dynamic Light Scattering study on aqueous micelles formed by tetradecyl dialkylammonium propanesulfonate surfactants (sulfobetaines; with alkyl = methyl, ethyl, n‐propyl and n‐butyl) within a range of surfactant concentrations (0.01–0.40 M) both in pure water and in the presence of various concentrations of NaBr, NaOH and NaClO4 (0.02–0.50 M NaBr; 0.10–1.00 M NaOH; 0.005–0.50 M NaClO4). From values of diffusion coefficients, D, we obtained micellar hydrodynamic radii, Rh, by application of the Stokes-Einstein relation. Plots of D vs. sulfobetaine concentrations can be qualitatively explained with a model based on a linear interaction theory, which allowed to separate thermodynamic and hydrodynamic perturbations to D. Results show that: i) formally neutral sulfobetaine micelles become negatively charged by preferential interaction with strongly interacting, “soft” anions; ii) the surface negative charge increases with the hydrophobicity of the anions; iii) bulkier alkyl substituents on the sulfobetaine head groups lead to less charged, less hydrated aggregates, which result in opposite perturbations to D; (iv) highly hydrated, high charge density hydroxide ions lead to an increase of micellar sizes through a disc-like growth pattern
Hydrate Induction Time with Temperature Steps: A Novel Method for the Determination of Kinetic Parameters
No full textGas hydrate formation usually occurs with a certain delay after a system composed of water and a hydrate forming gas is put under suitable thermodynamic conditions of pressure and temperature. This delay period is called the "induction time", and because of its large variability within a single experimental setting, hydrate formation is often referred to as a stochastic process. The evaluation of induction times, together with other measurements, is taken as an indication for the efficiency of hydrate inhibitors, and they are usually carried out by simply putting the experimental system under chosen P/T conditions and then waiting for the hydrate to form and measuring the time elapsed. In this paper, we present an improved procedure by which the variability of hydrate induction times can be remarkably reduced, while keeping a good correlation of measured induction times with the respective temperatures as obtained by a constant cooling method. In this procedure, temperatures are lowered by 0.5 degrees C after each time span of 3 h with no hydrate formation. Induction times obtained in this way show a remarkably lower coefficient of variation as compared to a standard induction time measurement
Deprotonation of indole derivatives in aqueous cationic surfactants
No full textDeprotonations of 5-nitroindole, 1a, and its 2-carboxylate ion, 2a, have been monitored in 0.01, 0.1, and 0.5 M NaOH in micellar solutions of cetyl trialkylammonium bromide, alkyl = Me, Et, nPr, nBu, CTABr, CTEABr, CTPABr, CTBABr. Extents of deprotonation (% f) have been fitted using the pseudophase model of micellar effects with interionic competition
described by ion exchange or by independent association constants. Both treatments indicate that base dissociation constants in dilute OH− are lower than in water by factors of ca. 3−11, and decrease with increasing bulk of the head groups, and that these factors increase modestly as the OH− concentration increases to 0.5 M
Effect of ethanol on micellization and on decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in acqueous cationic micelles
No full textThe effects of ethanol on the critical micellar concentration (cmc) and the rates of decarboxylation of 6-nitrobenzisoxazole-3-
carboxylate (6-NBIC) have been investigated in aqueous cationic surfactants of the cetyltrialkylammonium family with bromide [CT(R)ABr], chloride [CT(R)ACl], and nitrate [CT(R)ANO3]
counterions, and methyl (CTAX), n-propyl (CTPAX), and n-butyl (CTBAX) as the head group alkyl moieties. Effects upon cmc and reactivity are similar, featuring a break at the ethanol mole fraction,
xEtOH, of ca. 0.055; these effects have been related to changes in solvent structure, with formation of a clathrate at xEtOH = 0.055. Rate data in CTBABr were further investigated and fitting of raw kinetic data to the pseudophase model is possible up to xEtOH = 0.1, showing an unexpected decrease of rate constant values in the micellar pseudophase, k'M, as ethanol content increases: a significant variation of micellar ionization degree could account for this kinetic
effect
Role of the hydrogen bond donor component for a proper development of novel hydrophobic deep eutectic solvents
No full textThe environmental impact of chemical applications can be reduced by using novel solvents with green properties. In this field, Deep Eutectic Solvents (DESs) are promising liquids thanks to their low toxicity, high eco-compatibility and high easiness and “greenness” of preparation. DESs are mixtures of a hydrogen bond donor molecule (HBD) and a hydrogen bond acceptor molecule (HBA) at the proper molar ratio. In this paper, we present the preparation of novel hydrophobic deep eutectic solvents and the studies of their properties: density, eutectic profiles, ranges of water separation, contamination of the separated phases, extraction capabilities of phenol model polluting molecules, capabilities of extraction at acidic and basic conditions. Interesting results emerged about the role of the components of the DESs because of the use of a properly-chosen set of liquids. Their capabilities were dependent on the nature of the HBD molecule, and in particular on its hydrophobicity. Even the DESs with highly water-soluble HBA showed to be easily separable from water and really efficacious as extracting agents when prepared with hydrophobic HBDs. The results of the extractions of pollutants in acid and basic conditions showed the capability of water separation and extraction efficiency of these mixtures even with water at pH = 2 and pH = 9; therefore, the phenols could interact with these liquids without involvement of any acid/base-type of interactions. © 2019 Elsevier B.V
Activation and stabilization of α-chymotrypsin by cationic additives
No full textAlpha-chymotrypsin activity was tested with N-glutaryl-
L-phenylalanine p-nitroanilide (GPNA) in aqueous media in the presence of synthetic surfactants, which differ in the flexibility of their bulky head groups. Superactivity can be ascribed to the presence of the tributylammonium residue on
the surfactant head group, as in p-octyloxybenzyltributylammonium bromide (pOOTBABr), while in the presence of a more rigid moiety, i.e. a cyclic one, no activation was found. A nonmicellizable quaternary ammonium salt, the tetrabutylammonium bromide (TBABr), which has a head
group structure very similar to pOOTBABr, not only induces
a remarkable superactivation, at a concentration 80-fold
higher than pOOTBABr, but also allows the enzyme to
retain a high residual activity for long periods of time. The presence of a lipophilic chain, which by interacting with apolar residues on the enzyme surface, probably penetrates into hydrophobic pockets of the protein and causes a rapid
inactivation. In 0.4 M TBABr, a 20-fold increase both in kcat and Km values, with respect to buffer alone, was found. The increase of Km could be attributed either to a true decrease in affinity
between enzyme and substrate or alternatively to the
presence of TBA+ ions near the catalytic region. They could interact with protein residues around the active site and bind to negatively charged GPNA molecules, lowering the local substrate concentration. Spectroscopic experiments (CD and fluorescence) show minor changes of protein conformation in 0.4 M TBABr, while at 1 M a strong modification of both spectra was observed
- …
