Exploring the affinity binding of alkylmaltoside surfactants to bovine serum albumin and their effect on the protein stability: A spectroscopic approach.

Steady-state and time-resolved fluorescence together with circular dichroism (CD) spectroscopic studies was performed to examine the interactions between bovine serum albumin (BSA) and two alkylmaltoside surfactants, i.e. n-decyl-ß-D-maltoside (ß-C10G2) and n-dodecyl-ß-D-maltoside (ß-C12G2), having identical structures but different tail lengths. Changes in the intrinsic fluorescence of BSA from static as well as dynamic measurements revealed a weak protein-surfactant interaction and gave the corresponding binding curves, suggesting that the binding mechanism of surfactants to protein is essentially cooperative in nature. The behavior of both surfactants is similar, so that the differences detected were attributed to the more hydrophobic nature of ß-C12G2, which favors the adsorption of micelle-like aggregates onto the protein surface. These observations were substantially demonstrated by data derived from synchronous, three-dimensional and anisotropy fluorescence experiments. Changes in the secondary structure of the protein induced by the interaction with surfactants were analyzed by CD to determine the contents of α-helix and ß-strand. It was noted that whereas the addition of ß-C10G2 appears to stabilize the secondary structure of the protein, ß-C12G2 causes a marginal denaturation of BSA for a protein:surfactant molar ratio as high as 1 to 100.

Rotational dynamics of coumarin 153 in non-ionic mixed micelles of n-octyl-ß-D-thioglucoside and Triton X-100.

Rotational diffusion of the neutral probe coumarin 153 has been examined in a mixed surfactant system containing n-octyl-ß-D-thioglucoside (OTG) and Triton X-100 (TX100), two non-ionic surfactants belonging to the alkyl glucoside and polyoxyethylene alkyl ether families, respectively. Both steady-state and time-resolved fluorescence measurements indicate that the polarity of the dye decreases slightly as the amount of ethoxylated surfactant in the mixed micelle increases. This behaviour can be attributed to migration of the probe towards the inner region of the micellar palisade layer as a result of the increasing hydration induced by the presence of TX100. The anisotropy decay curves, r(t), were well fitted to a biexponential function, characterized by two reorientational times of the probe in the micellar pseudophase. The experimental data were analyzed on the basis of the two-step and wobbling-in-a-cone model, and the results obtained correlated with the changes that occur in the palisade layer of the mixed micelles due to the different structure and nature of the head groups of both surfactants. It was found that, although the average reorientation time of the probe molecule is almost unaffected with the participation of the ethoxylated surfactant, the observed increase in the generalized order parameter suggests a certain rise in the compactness of the mixed micelles.

On the urea action mechanism: a comparative study on the self-assembly of two sugar-based surfactants.

Studies on the effect of urea on micelle formation and structure of n-octyl-beta-D-thioglucoside (OTG) and N-decanoyl-N-methylglucamide (MEGA-10) were carried out by using the steady-state and time-resolved fluorescence techniques, together with combined static and dynamic light scattering measurements. A similar increase in the critical micelle concentration with the urea addition was observed for both surfactants. This behavior was attributed to a rise in the solubility of hydrocarbon tails and the increase of solvation of the headgroup of the surfactants in the presence of urea. Structural studies mainly based on the analysis of the hydrodynamic radius and aggregation number of micelles revealed that urea induces changes much more significant on micelles of OTG. Particularly, it was found that, whereas the surface area per headgroup of OTG increases with the urea concentration, it does decrease in the case of MEGA-10. This fact suggests that different action mechanisms operate for both surfactants. Accordingly, investigations on the micellar microstructure based on the study of microenvironmental properties such as micropolarity and microviscosity also indicated a more pronounced effect in the case of OTG. Although changes were not observed in the hydrophobic inner region of both micellar systems, a significant increase of polarity and viscosity in the micellar interface of OTG suggests a direct participation of urea in the micellar solvation layer. The differences between the observed behaviors for both micellar systems were interpreted on the basis of two features: the weaker hydration and greater rigidity of the OTG headgroup as compared with MEGA-10.

Self-assembly, hydration, and structures in N-decanoyl-N-methylglucamide aqueous solutions: effect of salt addition and temperature.

The influence of NaCl addition and temperature on the self-assembly, hydration, and structures of N-decanoyl-N-methylglucamide (MEGA-10) in dilute solution has been investigated by using several experimental techniques, including tensiometry, steady-state fluorescence, density, viscosity, and static and dynamic light scattering. Tensiometry and fluorescence probe studies, by using pyrene as a probe, were used to obtain the critical micelle concentration (cmc) upon the electrolyte addition. The mean micellar aggregation numbers (N(agg)) as a function of the salt addition were obtained by both static light scattering and static quenching methods. The N(agg) values estimated by both methods were found to be in good agreement. It was found that the increase in the micelle size, produced by the addition of NaCl, is due to the increase in the aggregation number and in the amount of water non-specifically associated to the micelle. On the other hand, we have observed that the aggregation number remains invariant in the temperature range studied, whereas the hydrodynamic radius slightly decreases. The effect of electrolyte addition and temperature on the properties of MEGA-10 micelles is much less pronounced than those observed in the traditionally used POE-based surfactants.

Self-assembly of tetradecyltrimethylammonium bromide in glycerol aqueous mixtures: A thermodynamic and structural study.

In this paper, we have studied the effect of glycerol on the micelle formation of tetradecyltrimethylammonium bromide. Changes in both the critical micelle concentration and the degree of counterion binding of the surfactant upon the addition of glycerol across a temperature range (20-40 degrees C) were examined by using the conductance method. The equilibrium model of micelle formation was applied to obtain the thermodynamic parameters of micellization. An enthalpy-entropy compensation effect was observed in all the solvent systems, but whereas the micellization of the surfactant in the medium with 20% glycerol occurs under the same structural conditions as in pure water, in glycerol rich mixtures the results suggest that the lower aggregation in these media is due to the minor cohesive energy of the solvent system in relation to water. It was also observed that the micellar aggregation number, as obtained by the static quenching method, decreases with the glycerol content. This fact was attributed to an increase in the surface area per headgroup of the surfactant as a consequence of an enhanced solvation, probably induced by the incorporation of some glycerol molecules in the micellar solvation layer. Although the pyrene 1:3 ratio index does not indicate significant changes in the micropolarity at the micelle-bulk interface, the data of fluorescence anisotropy of coumarin 6 and fluorescein are compatible with the formation of a more compact solvation layer.

Interactions in binary mixed systems involving a sugar-based surfactant and different n-alkyltrimethylammonium bromides.

In this paper, mixtures of sugar-based decanoyl-N-methylglucamide with three different n-alkyltrimethylammonium bromides (n=12 (DTAB), 14 (TTAB), and 16 (CTAB)) have been studied using conductance and fluorescence spectroscopic techniques. The critical micelle concentration values of pure and mixed systems were determined by both the conductance and the pyrene 1:3 ratio methods. The experimental results were interpreted using thermodynamic mixing approaches based on the pseudophase separation model. These analyses allowed us to determine the interaction parameters and the composition of the mixed micelles through the whole composition range. Since all the ionic surfactants used in this study have the same headgroup, the differences observed between the three mixed systems were attributed to the lengths of their hydrocarbon chains. It was established that, besides interactions of electrostatic character, additional short-range interactions must be considered. By using the static quenching method, the mean micellar aggregation numbers of mixed micelles were obtained. In the cases of the mixed systems with DTAB and TTAB it was observed that the aggregation number is initially reduced with the participation of the ionic component, remaining almost constant and close to the aggregation number of the pure ionic micelle. However, in the systems involving CTAB it is observed that the size of micelles initially increases and then decreases slightly for mixtures with a high content of the ionic component. The hydrophobic index pyrene 1:3 ratio was used to examine possible changes in the micellar micropolarity; however, no definitive conclusions could be derived from these experiments. In order to study the evolution of the local viscosity of the mixed micelles upon addition of the ionic surfactant, fluorescence polarization measurements were carried out with two different probes, fluorescein and coumarin 6. It was found that the participation of the ionic component in the mixed micelle induces the formation of less ordered structure than that of pure nonionic micelles. An attempt was made to correlate these effects with the interaction parameters obtained from the theoretical mixing model and, consequently, with the alkyl chain length of the ionic components.

Thermodynamics and micellar properties of tetradecyltrimethylammonium bromide in formamide-water mixtures.

The effect of formamide on the micellization of tetradecyltrimethylammonium bromide has been investigated by conductance and fluorescence probe experiments. The critical micelle concentration and the degree of counterion dissociation of micelles were obtained from conductance measurements in the temperature range of 20 to 40 degrees C. It was found that these two parameters increase with both temperature and formamide content in the solvent system. The thermodynamic parameters of micellization were estimated using the equilibrium model of micelle formation. The standard free energy of micellization was found to be negative in all cases and becomes less negative as the formamide content in the mixed solvent increases, but it is roughly independent of temperature. Although the entropic contribution was found to be larger than the enthalpy one, in particular at lower temperatures, an enthalpy-entropy compensation effect was observed for all systems. Micellar aggregation numbers were determined by the static quenching method, using pyrene as a probe and cetylpyridinium chloride as a quencher. The observed decrease in the micelle aggregation number, which is controlled by the increase in the surface area per headgroup, was attributed to an enhanced solvation in formamide rich solvent mixtures. Changes in the pyrene 1:3 ratio index, indicating a more polar environment, are consistent with an increased micellar solvation. Fluorescence polarization of both coumarin 6 and fluorescein are indicative of a decrease in microviscosity with cosolvent addition. The data on fluorescence anisotropy of coumarin 6 were analyzed using the wobbling in cone model. Data indicated the formation of micelles with a less ordered structure as the formamide increases in the solvent system.