Watson Loh

Thermal and photochemical nitric oxide release from S-nitrosothiols incorporated in Pluronic F127 gel: potential uses for local and controlled nitric oxide release

The local delivery of nitric oxide (nitrogen monoxide, NO) by thermal or photochemical means to target cells or organs has a great potential in several biomedical applications, especially if the NO donors are incorporated into non-toxic viscous matrices. In this work, we have shown that the NO donors S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylcysteine (SNAC) can be incorporated into F127 hydrogels, from where NO can be released thermally or photochemically (with lambda(irr)>480nm). High sensitivity differential scanning calorimetry (HSDSC) and a new spectrophotometric method, were used to characterize the micellization and the reversal thermal gelation processes of the F127 hydrogels containing NO donors, and to modulate the gelation temperatures to the range 29-32 degrees C. Spectral monitoring of the S-NO bond cleavage showed that the initial rates of thermal and photochemical NO release (ranging from 2 to 45 micromoll(-1)min(-1)) are decreased in the hydrogel matrices, relative to those obtained in aqueous solutions. This stabilization effect was assigned to a cage recombination mechanism and offers an additional advantage for the storage and handling of S-nitrosothiols. These results indicate that F127 hydrogels might be used for the thermal and photochemical delivery of NO from S-nitrosothiols to target areas in biomedical applications.

Interfacial and colloidal behavior of asphaltenes obtained from Brazilian crude oils

In this work, we present new data on the interfacial and colloidal behavior of asphaltenes in model aromatic solvents and crude oils and discuss the implications of these data on the aggregation, adsorption on solid surfaces, inhibition of deposition and emulsion stabilization capacities for two types of asphaltenes obtained from Brazilian crude oils. Surface tension measurements in solutions formed by any of these two types of asphaltenes, pentane insolubles (C5I) or heptane insolubles (C7I), in aromatic solvents suggested the occurrence of an aggregation phenomenon of asphaltenes in each of the solvents studied. Viscosity measurements in these same solutions revealed yet another type of aggregation at higher asphaltene concentrations. Both aggregation processes were found to be a function of temperature and type of asphaltenes. For the asphaltenes investigated, C7I was always more prone to aggregation, a characteristic ascribed to its higher asphaltene and lower resin content as revealed by SARA chromatographic analysis, and to its larger average molecular weight as determined by VPO. The effect of temperature on theses two processes confirmed the exothermic nature of both. A number of block copolymers, ionic and non-ionic surfactants, were tested for their effectiveness as asphaltenes stabilizers in crude oil. The results revealed different and distinct mechanisms for solubilization/dispersion of asphaltenes in aromatic (aliphatic) solvents and for the inhibition of asphaltene precipitation in crude oils. The two types of asphaltenes (C5I and C7I) were found to be effective in the stabilization of water/oil emulsions as well as water/toluene emulsions, with larger effects for C7I in water/toluene emulsions, consistent with its greater surface activity and its larger tendency to self-associate. Finally, the interaction of asphaltenes with solid surfaces was evaluated by determining the adsorption isotherms of asphaltenes on silica and activated carbon. The organic adsorbent was found to be much more efficient in removing asphaltenes due, perhaps, to the similar chemical nature of the adsorbing species and adsorbent. In all cases, evidence points to multilayer formation, in agreement with the previously detected tendency of asphaltenes to aggregate.

Polysaccharide-based hydrogels: preparation, characterization, and drug interaction behaviour.

Oxidized alginate (ADA) and oxidized alginate blended with chitosan (ADA-Chit) were prepared in the presence of borax and CaCl 2, and their interactions with an antifolate drug, pyrimethamine (PYR), have been investigated. Tablets with a mean diameter of 1.2 +/- 0.06 cm were produced and drug interactions were performed in dimethyl sulfoxide (DMSO) using isothermal titration calorimetry (ITC). From ITC responses, the enthalpy changes of interaction PYR/materials, Delta int H, have been determined and were found to be -11.73 +/- 0.517 kJ mol (-1) for ADA and -4.86 +/- 0.156 kJ mol (-1) for ADA-Chit. The PYR encapsulation of approximately 75% was achieved for both materials, as measured by UV spectrometer.

Investigations on the mechanism of aqueous solubility increase caused by some hydrotropes

The effects of nicotinamide, sodium p-toluenesulfonate and tetrapropyl and tetrabutyl ammonium bromides on the cloud points of non-ionic surfactant solutions and on the solubility of an apolar dye were determined. They all revealed a continuous effect of increased solubility as the additive concentration increases. Surface tension and heats of dilution were also measured for these solutions and the results support the view of a continuous hydrotrope self-association, which is then proposed as a key process to their ability of increasing aqueous solubility of apolar compounds.

The removal of reactive dyes from aqueous solutions using chemically modified mesoporous silica in the presence of anionic surfactant-the temperature dependence and a thermodynamic multivariate analysis.

The three-parameter Sips adsorption model was successfully employed to modeled equilibrium adsorption data of a yellow and a red dye onto a mesoporous aminopropyl-silica, in the presence of the surfactant sodium dodecylbenzenesulfonate (DBS) from 25 to 55 degrees C. The results were evaluated in relation to the previously reported surface tension measurements. The presence of curvatures of the van()t Hoff plots suggested the presence of non-zero heat capacities terms (Delta(ads)C(p)). For the yellow dye, it is observed that the values of Delta(ads)H are almost all positive and they decrease in endothermicity, in the absence and in the presence of DBS, from 25 to 55 degrees C. For the red dye, there is an increase in endothermicity in relation to the temperature increase. The negative Delta(ads)G values indicate spontaneous adsorption processes. Almost all adsorption entropy values (Delta(ads)S) were positive. This suggests that entropy is a driving force of adsorption. The adsorption thermodynamic parameters were also evaluated using a new 2(3) full factorial design analysis. The multivariate polynomial modelings indicated that the thermodynamic parameters are also affected by important interactive effects of the experimental factors and not by the temperature changes alone.

On the use of titration calorimetry to study the association of surfactants in aqueous solutions

Isothermal titration calorimetry is increasingly becoming a common tool for the investigation of surfactant association processes. This can be associated with the development of new, sensitive and easy-to-operate commercial equipment, allied with the advantage of producing simultaneous information on the main thermodynamic parameters associated with the process under investigation. However, a significant fraction of users are still unfamiliar with many aspects related with the use of these calorimetric techniques. This review intends to discuss the design of experiments to investigate surfactant self-assembly in water and in the presence of polymers (charged and uncharged), as well as on how to derive the most relevant thermodynamic parameters from these calorimetric titration curves. Some literature examples are discussed to illustrate the use of these techniques for different systems.

REVERSIBILITY AND INHIBITION OF ASPHALTENE PRECIPITATION IN BRAZILIAN CRUDE OILS

ABSTRACT A fundamental understanding of the aggregation and precipitation of asphaltenes in petroleum crudes is important for the development of preventive and curative measures for the potential problem of asphaltene deposition occurring during production, transport and refining operations. The question of reversibility of asphaltene precipitation, yet a controversial issue, is crucial for a clear and unequivocal understanding of the precipitation phenomenon, development of mathematical models that describe the behavior of asphaltenes in petroleum fluids, and the design of inhibitors. In this work, the behavior of precipitated asphaltenes in Brazilian crude tank oil samples following flocculant removal and gradual addition of fresh oil was investigated. The results obtained revealed a re-dissolution of precipitated asphaltene particles following flocculant removal and oil addition. On the inhibition of asphaltene precipitation, the capacity of a number of surfactants and block copolymers to inhibit aspha...

Polymers Containing Hydroxamate Groups: Nanoreactors for Hydrolysis of Phosphoryl Esters

A polyhydroxamicalkanoate (PHA) polymer containing the functional groups hydroxamic acid and carboxylic acid with the ability to accelerate dephosphorylation reactions is proposed. The methodology used to prepare this polymer favored the position of the two functional groups next to each other, which allows for the cooperativity between these groups. This cooperative effect has an important role when one wants to mimic enzymes. The catalytic effect promoted by the polymer was evaluated on the cleavage of the bis(2,4-dinitrophenyl) phosphate (BDNPP) and diethyl 2,4-dinitrophenyl phosphate (DEDNPP) esters. Indeed, PHA was very efficient and promiscuous because it increased the rate of both reactions by a factor of up to 10(6)-fold. Isothermal titration calorimetry (ITC) experiments showed that the presence of PHA aids the formation of cetyltrimethylammonium bromide (CTABr) micelles. Thus, the effect of the cationic surfactant CTABr on the dephosphorylation of BDNPP by PHA was also investigated, and it was observed that, when CTABr is added to PHA, the reaction is ca. 15-fold faster compared to the reaction when only PHA is present.

The effect of poly(ethylene glycol) on the activity and structure of glucose-6-phosphate dehydrogenase in solution

Abstract The effect of poly(ethylene glycol), PEG, on the enzymatic activity of glucose-6-phosphate dehydrogenase (G-6-PDH) in the oxidation of glucose-6-phosphate (G-6-P), using NADP+ as co-enzyme was investigated. The enzymatic activity was determined by means of spectrophotometry in three different media: pure Tris–HCl buffer, solution of PEG400 (20 wt.%) and of PEG4000 (20 wt.%), both in buffer. Comparing the enzymatic activity values measured in pure buffer with those in the polymer solutions, an increase in the enzymatic activity of 20% was observed in the presence of PEG400 as well as in PEG4000. Calorimetric studies indicated the absence of preferential interactions between G-6-PDH and PEG400 or PEG4000. Nevertheless, the interaction enthalpy, Δ H int , between NADP+ and PEG400 and PEG4000 amounted to −9.3 and −26.7 kJ/mol, respectively. Small angle X-ray scattering (SAXS) measurements were performed in a higher concentration range. Data analysis performed from SAXS curves by means of the intra-particle distance distribution function p ( r ) and Guinier plots yielded for G-6-PDH in pure buffer and PEG400 solutions radius of gyration, R g , of about 70 A and in PEG4000 solutions, R g of about 40 A. The latter has the same dimension as that found in the dimeric crystallographic structure of G-6-PDH, evidencing that G-6-PDH preserves its dimeric configuration in PEG4000 solution. On the contrary, different aggregates of G-6-PDH are formed in the presence of either buffer or PEG400. These findings show that the presence of PEG in solution can exert an effect on the enzyme structure and activity.

A técnica de dispersão de taylor para estudos de difusão em líquidos e suas aplicações

This paper describes the theoretical basis and the experimental requirements for the application of the Taylor dispersion technique for measurements of diffusion coefficients in liquids, emphasizing its simplicity and accuracy in comparison to other usual techniques. Some examples are discussed describing the use of this methodology on studies of solute-solvent interactions, solute aggregation, solute partitioning into macromolecular systems and on the assessment of nanoparticles sizes.