Conformational changes in humic acids in aqueous solutions

Conformational changes in humic acids in two different aqueous solutions (NaCl and NaOH) are studied by means of high resolution ultrasound spectrometry. The method is based on the measurement of parameters of ultrasonic waves propagating through the sample. The attenuation describes the decay of the amplitude of the ultrasonic wave with the distance travelled. The velocity is the speed of this wave and is related to the wavelength and the frequency of oscillation of the deformation. It is determined by the density and elasticity of the sample, which is strongly influenced by the molecular arrangement. The minimal velocity of ultrasound was observed at 1 g dm^?3 for lignitic humic acids and at 0.5 g dm^?3 for IHSS Leonardite standard. The values of compressibility as computed are almost constant up to humic acids?? content corresponding to the minimum velocity of ultrasound and then decrease with the increase in concentration. This shows that the organisation of particles in diluted and concentrated humic acids sols is different. The decrease in compressibility points to the formation of a more rigid structure, which could lead to the decrease in humic acids?? binding ability. It was confirmed that the method employed was very sensitive and could be utilised as an indicator of conformational changes in humic acids in solutions with varying concentrations.     

Molecular dynamics and conformational kinetics of mono- and disaccharides in aqueous solution.

Acoustical attenuation spectra between 10 kHz and 2 GHz, complex dielectric spectra between 300 kHz and 40 GHz, and time-resolved non-equilibrium measurements are reported for aqueous solutions of various mono- and disaccharides with and without 2:1 valent salts. The spectra reveal a variety of relaxation regimes with relaxation times between 1 micros and 10 ps. In addition, the time-resolved observations enable the study of the mutarotation with relaxation times on the order of 10(3) s. Variation of the concentration and temperature as well as a careful choice of the saccharides allow a discussion of the relaxation processes in terms of a chair-chair ring inversion, two modes of pseudorotation, an exocyclic hydroxymethyl group rotation, a carbohydrate-carbohydrate association, and, in the disaccharide solutions, a rotation of the rings relative to another. Salt-containing solutions show also relaxation phenomena reflecting different steps cation-carbohydrate association and variations in the carbohydrate conformational isomerizations and associations due to interactions with cations.     

Glycoside bond cleavage in the radiolysis of aqueous solutions of methylglycosides and disaccharides

The kinetics of formation of methylglycoside and disaccharide radiolysis products resulting from the O-glycoside bond cleavage was studied, and the yields of these products were determined. It was found that oxygen inhibits these processes. The findings suggest that the fragmentation reaction of C’₂ radicals plays an important role in the formation of carbohydrate degradation products in the radiolysis of aqueous carbohydrate solutions.     

Thermal dehydration kinetics of disaccharides

The vacuum decomposition of sucrose and cellobiose has been observed in the 150–250°C temperature range. The predominant decomposition product of both sugars is H₂O with less than 5% CO, CO₂, CH₂O, CH₃CHO, CH₃OH, and C₂H₅OH formed. The detailed rates and temperature dependences suggest that with the possible exception of C₂H₅OH, the minor products are formed in secondary reactions of the dehydration products. Further it is shown that the so-called “melting with decomposition” of a sugar is in reality a high-temperature dissolution of the disaccharide in the eliminated water.     

Growth kinetics of sulfur nanoparticles in aqueous surfactant solutions

Sulfur is an important element has many practical applications when present as nanoparticles. Despite the practicable applications, limited studies are available in the literature related to synthesis of sulfur nanoparticles. Growth kinetics of colloidal sulfur particles synthesized from aqueous solutions using different surfactants have been studied here. The effects of different parameters such as reactant concentration, temperature, sonication, types of acids, types of surfactants, and even surfactant concentration are studied on the growth kinetics. Since the reaction rate is fast, particle growth depends on the parameters which affect diffusion of sulfur molecules. There is a linear relationship found among the reactant concentration and the particle coarsening rate constant. The growth kinetics was studied in the presence of different surfactants such as nonionic (poly(oxyethylene) p-tert-octylphenyl ether, TX-100), anionic (sodium dodecylbenzene sulfonate, SDBS), cationic (cetyltrimethyammonium bromide, CTAB) and results show the coarsening constant changes according to the following order: water>TX-100>SDBS>CTAB. The particle growth rate also depends on the surfactant concentration, coarsening rate constant decreases with the increase in surfactant concentration and become constant close to the critical micellar concentration (CMC). The coarsening rate constant also highly depends on the types of acid used as catalyst.     

Conformational characteristics of polyethylene glycol macromolecules in aqueous solutions according to refractometry data

The aqueous solutions of polyethylene glycols with molecular masses of 600, 1000, 1500, 3000, 6000, and 20000 were studied by refractometry. The conformational polarizabilities, mean-square distances between the ends of the macromolecular chain, segment lengths, and the number of Kuhn segments in a macromolecule were determined using the Lorentz-Lorentz equation. The polarizability of a hydrated macro-molecule was represented as the sum of polarizabilities of the nonhydrated macromolecule with retained conformation and polarizabilities of the water molecules involved in hydration of macromolecules. The size of macromolecules stabilized starting from a certain concentration. It was concluded that the initial concentration of stabilization shifts toward low concentrations as the molecular mass of polyethylene glycol increases. The dependence of the mean-square distance between the ends of the macromolecular chain on the number of Kuhn segments was expressed as the exponential function with index 0.3.     

Conformational changes in human serum albumin induced by sodium perfluorooctanoate in aqueous solutions

Conformational changes in the bulk solution and at the air-aqueous interface of human serum albumin (HSA) induced by changes in concentration of sodium perfluorooctanoate (C(7)F(15)COO(-)Na(+)) were studied by difference spectroscopy, zeta-potential data, and axisymmetric drop shape analysis. zeta-potential was used to monitor the formation of the HSA-C(7)F(15)COO(-)Na(+) complex and the surface charge of the complex. The conformational transition of HSA in the bulk solution was followed as a function of denaturant concentration by absorbance measurements at 280 nm. The data were analyzed to obtain values for the Gibbs energies of the transition in water (DeltaG(0)(W)) and in a hydrophobic environment (DeltaG(0)(hc)) pertaining to saturated protein-surfactant complexes. The conformational changes that surfactants induce in HSA molecules alter its absorption behavior at the air-water interface. Dynamic surface measurements were used to evaluate this behavior. At low [C(7)F(15)COO(-)Na(+)], proteins present three adsorption regimes: induction time, monolayer saturation, and interfacial gelation. When surfactant concentration increases and conformational changes in the bulk solution occur, the adsorption regimes disappear. HSA molecules in an intermediate conformational state migrate to the air-water interface and form a unique monolayer. At high [C(7)F(15)COO(-)Na(+)], the adsorption of denatured molecules exhibits a behavior analogous to that of dilute solutions.     

Degradation kinetics of the Alternaria mycotoxin tenuazonic acid in aqueous solutions

The degradation kinetics of the Alternaria mycotoxin tenuazonic acid (l-TA) in aqueous buffer were studied over a period of 4 months at different pH levels (3.5 and 7.0) and temperatures (4, 25 and 40 °C). l-TA and its degradation products were quantified by newly developed high-performance liquid chromatography methods with UV or electrospray multistage mass spectrometry detection. At pH 3.5, significant degradation occurred at 25 and 40 °C, the respective l-TA half-lives being 73.8 ± 0.4 and 14.0 ± 0.1 days. Two degradation processes, epimerization and hydrolysis, were evaluated kinetically. The hydrolytically formed iso-deacetyl TA (iso-DTA, epimeric mixture) was found to be the stable end product of l-TA degradation under the conditions of this study. This indicates that iso-DTA as well as the l-TA epimer u-TA are formed in aqueous beverage matrices.     

Luminescence kinetics and association equilibria of lanthanide ions in aqueous solutions

Luminescence life time measurement can be used under certain conditions to determine the thermodynamic constants of complex formation between the luminescent (central ion) and ligand. The basic equations correlating the life time and the equilibria constants were derived for two cases: the time for establishing the thermodynamic equilibrium is much shorter than the life time of the excited state of the central ion; and the time for establishing the equilibrium and the life time are of the same order of magnitude.     

Phosphorylation of disaccharides with inorganic cyclo-triphosphate in aqueous solution

The phosphorylation of disaccharides by inorganic cyclo-triphosphate (P(3m)) with a six-membered ring was examined in aqueous solution. In the phosphorylation of cellobiose, lactose, and alpha,alpha-trehalose with P(3m), beta-D-glucopyranosyl-(1-->4)-beta-D-glucopyranosyl 1-triphosphate, beta-D-galactopyranosyl-(1-->4)-beta-D-glucopyranosyl 1-triphosphate, and 3-O-triphospho-alpha-D-glucopyranosyl-(1-->1)-alpha-D-glucopyranoside were synthesized with maximum yields of 28%, 35%, and 20%, respectively. In the reactions of maltose and sucrose with P(3m), two phosphorylated products were obtained in yields of 42% and 58%, respectively. The main phosphorylated products were assigned to alpha-D-glucopyranosyl-(1-->4)-beta-D-glucopyranosyl 1-triphosphate and beta-D-fructofuranosyl-(2-->1)-2-O-triphospho-alpha-D-glucopyranoside by heteronuclear multiple bond correlation (HMBC) NMR. The phosphorylation mechanism of disaccharides with P(3m) is discussed.     

Conformational analysis of trehalose disaccharides and analogues using MM3

Energy surfaces for the relative orientations of the pyranosyl rings of α,α-, α,β-, and β,β-trehalose and analogues were generated with MM3. Sixteen starting conformations of the rotatable side groups of α,β-trehalose were considered, while only 10 conformations were needed for α,α- and β,β-trehalose because of molecular symmetry. Energies were calculated at 20° increments of the two torsional angles of the glycosidic linkage, but otherwise the molecules were fully relaxed. The structure at the overall minimum for α,α-trehalose agrees well with that found in crystal structures, and also agrees with interpretations of NMR and optical rotation data. The energy surfaces for the three trehaloses differ greatly from each other, but are each similar to those for the corresponding three 2-(6-methyltetrahydropyran-2-yloxy)6-methyltetrahydropyrans. This suggests that linkage type (axial or equatorial) is more important than exocyclic substituents in determining trehalose conformations. A comparison with surfaces from the corresponding 5a-carba trehalose analogues illustrates that the exo-anomeric effect is important in determining disaccharide conformation.     

The kinetics of the dissolution of monoclinic pyrrhotite in aqueous acidic solutions

Using the rotating disc method, the rates of dissolution of natural monoclinic pyrrhotite, FeS_1.14, in oxygen-free aqueous solutionsS([H⁺]=0.1, [Na⁺]=0.9, [ClO ₄ ^– ]=1.0 mol kg^–1) were determined. In the temperature range 40–90 °C the dissolution reaction occurs under kinetic control; the activation energy being 14±1 kcal mol^–1 (50±5 kJ mol^–1).

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Die Kinetik der Auflösung von monoklinem Pyrrhotin in sauren wäßrigen Lösungen

Zusammenfassung Die Auflösungsgeschwindigkeit von natürlichem monoklinen Pyrrhotin, FeS_1.14, wurde in sauerstofffreien LösungenS([H⁺]=0.1, [Na⁺]=0.9, [ClO ₄ ^– ]=1.0 mol kg^–1) mit Hilfe der Methode der rotierenden Scheibe bestimmt. Im Temperaturbereich von 40–90° erfolgt die Auflösungsreaktion kinetisch kontrolliert, wobei eine Aktivierungsenergie von 14±1 kcal mol^–1 (59±5 kJ mol^–1) gefunden wurde.

     

Electrochemical corrosion kinetics of tin and tin amalgams in NaCl aqueous solutions

Electrochemical techniques were used to determine the corrosion rate of pure tin metal as compared to 80 Sn/20 Hg tin amalgam. X-ray diagrams showed that this amalgam was a crystalline γ₂ phase, whereas a 50 Sn/50 Hg amalgam contained liquid alloy embedded in the same γ₂ phase. Open circuit potential measurements, combined with narrow range potential scanning voltammetry, lead to the conclusion that amalgamation resulted in enhancement of the corrosion current, mainly by increasing the cathodic electron transfer reaction kinetics both in deaerated and in oxygen-saturated NaCl solution. When maintained at zero current potential in a solution containing dissolved O₂ gas, the samples were gradually covered with an insulating oxide layer which was identified by a series of electrochemical impedance diagrams recorded at different time intervals. The oxide layer was firmly adherent to the bulk tin metal but was poor at protecting the amalgam electrode. Finally, at potential values where the anodic current reached a few mA/cm², the pure tin metal surface was suddenly deteriorated by the formation of extremely deep pinhole corrosion pits, while this effect was smoothed down by amalgamation. Electronic Publication     

The solubility and kinetics of decomposition of ozone in aqueous solutions of nitrates

The influence of the concentration of NaNO₃ on the solubility of ozone in water was studied at 20, 30, and 40°C. The solubility coefficients of ozone were calculated, and the Henry constants and Sechenov coefficients determined. The Sechenov coefficients (K _c ) were found to decrease insignificantly as the temperature increased. The kinetics of dissolved O₃ transformations was analyzed. The decomposition of ozone was described by a pseudofirst-order equation with respect to salt concentration. The rate constant (k _c ) for the decomposition of ozone in the presence of NaNO₃ was found to be 3.5 × 10^?4 l mol^?1 s^?1.     

Conformational changes in poly(vinyl alcohol)-graft-polyacrylamide in aqueous solutions vs graft content

Conformational changes have been studied in intramolecular polymer-polymer complexes (intraPC) of graft copolymers of poly(acrylamide) and poly(vinyl alcohol) (PVA-g-PAA) with various numbers of grafts (4-42) per molecule as a function of temperature and copolymer concentration. It is shown that the magnitude of conformational change depends on the grafts content while the temperature range over which the conformation changes occur is essentially determined by copolymer concentration. The conformational changes are reversible on heating and cooling.     

Conformational phase diagram of poly(l-lysine) in sodium alkanesulfonate aqueous solutions at various temperatures

The conformation of poly(l-lysine) (PLL) was investigated in sodium alkanesulfonate C_nSO₃Na (n=8, 7) at various temperatures by circular dichroism spectrum measurements. C₈SO₃Na induced a double-step conformational change from a coil, to a β-sheet, and then to an α-helix, in which C₇SO₃Na induced a single-step coil-to-helix conformational change. Binding isotherms of C₈SO₃Na by PLL were constructed from the potentiometry of equilibrium concentration of the surfactant using a surfactant ion-selective electrode. The curves indicated the cooperative binding characteristic and were analyzed by a linear lattice model using the Bethe approximation. The thermodynamic parameters obtained from the model revealed that the binding of C₈SO₃Na by PLL was an entropy-driven process. The conformational change was observed at nearly full binding, presumably due to the surfactant clustering of the ordered conformation.     

Roles of hydrophobicity and volatility of organic substrates on sonolytic kinetics in aqueous solutions

The aquasonolytic rate constants of cyclic C6H(X), aliphatic C6H(X), thioethers, thiophenes, and N-heterocyclic compounds show over a 90-fold variation under identical conditions of ultrasonic irradiations. Henry's Law constant of the substrate has a substantial effect on the aquasonolytic rate; a higher Henry's Law constant leads to a aquasonolytic rate constant, which indicating the transfer process of organic substrate between bulk liquid and cavitational bubbles is essential for aquasonolysis. The aquasonolytic rate constants, however, dramatically show an irregular variation with increasing vapor pressure among various substrates. Although the volatility of substrate has been widely regarded as a basic factor influencing aquasonolysis, it seems that vapor pressure of substrate is not a determining one that accounts for the difference of aquasonolytic rate constants. In contrast, the hydrophobic parameters of volatile substrate such as water solubility and octanol-water partition coefficient have shown obvious correlation with the aquasonolytic rate constant for the model compounds; a higher hydrophobicity of volatile substrate results in a higher aquasonolytic rate constant. It could be concluded that the transfer process from bulk liquid to cavitational bubbles and the aquasonolytic kinetics of organic substrate are jointly controlled by the hydrophobicity and volatility; therein the hydrophobicity dominates the transfer process and the aquasonolysis of volatile substrate.