Maslinic acid conjugate with 7-amino-4-methylcoumarin as probe to monitor the temperature dependent conformational changes of human serum albumin by FRET.

Synthesis, characterization and spectroscopic investigation of maslinic acid labeled with fluorescent 7-amino-4-methylcoumarin is reported. It was found that the coumarin-maslinic derivative (MaCo) forms an excellent fluorescence resonance energy transfer (FRET) pair with the tryptophan (Trp) residue of human serum albumin (HSA). This feature allowed for monitoring HSA conformational alterations by measuring the distance between donor (Trp) and acceptor (MaCo) through Förster energy transfer mechanism. Displacement experiments confirmed that MaCo binds to subdomain IIA of HSA with independence of temperature. It was observed that, in the temperature range 35-45 °C, the fluorescence emission maximum of HSA-MaCo complex decreased, whereas in the range 45 °C-65 °C, an increment was detected. The concomitant change in the polarity of environment surrounding Trp was confirmed by red edge excitation shift experiments. Thermal denaturation of HSA was followed by time-resolved fluorescence spectroscopy. Average lifetime of Trp residue decreased with temperature due to the increment of solvent collisions and changes in the solvent exposure of Trp. To discriminate the importance of each effect, lifetime of N-Acetyl-L-tryptophanamide (NATA) at different temperatures was measured. Circular dichroism (CD) studies confirmed the loss of secondary structure of HSA with increasing temperature and showed a different trend in the conformational transformation below and above 45 °C, in agreement with steady-state and time-resolved fluorescence experiments.

Rotational diffusion of coumarin 153 in nanoscopic micellar environments of n-dodecyl-ß-D-maltoside and n-dodecyl-hexaethylene-glycol mixtures.

The microstructure of mixed micelles containing n-dodecyl-ß-D-maltoside and n-dodecyl-hexaethylene-glycol, two nonionic surfactants belonging to the alkyl polyglucoside and polyoxyethyelene alkyl ether families, respectively, has been investigated. With the aim of understanding how the micellar composition affects the microenvironmental properties of micelles, we have examined the photophysics and dynamics of the neutral probe coumarin 153 in the binary mixtures of the surfactants across the entire composition range. We present data on the steady-state absorption and emission spectra of the probe, as well as fluorescence lifetimes and both steady-state and time-resolved fluorescence anisotropies. These data indicate that the participation of the ethoxylated surfactant in the mixed micelle induces an increasing hydration in the palisade layer of the micelle, which forces the probe to migrate toward the inner micellar region, where it senses a slightly less polar environment. The time-resolved fluorescence anisotropy data were analyzed on the basis of the two-step and wobbling-in-cone model. The average reorientation time of the probe molecule was found to decrease with the presence of the ethoxylated surfactant, in good agreement with steady-state fluorescence anisotropy data, suggesting a reduction of the microviscosity in the solubilization site of the probe. The behavior of all diffusion reorientation parameters was analyzed on the basis of two factors: the micellar hydration and the headgroup flexibility of both surfactants. It was concluded that the increasing participation of the ethoxylated surfactant induces a greater hydration in the micellar palisade layer, producing the formation of a less compact microenvironment where the probe experiences a faster rotational reorientation.

Characterization of mixed non-ionic surfactants n-octyl-ß-D-thioglucoside and octaethylene-glycol monododecyl ether: micellization and microstructure.

Mixed micelles of n-octyl-ß-D-thioglucoside (OTG) and octaethylene-glycol monododecyl ether (C(12)E(8)), two non-ionic surfactants belonging to the alkyl glucosides and polyoxyethylene alkyl ether families, respectively, were investigated by using light scattering and fluorescence probe techniques. From the determination of the critical micelle concentration (cmc), by the well-established pyrene 1:3 ratio method, it was found that the mixed system behaves ideally, the micellization process being clearly controlled by the ethoxylated surfactant. The micellar hydrodynamic radius as a function of temperature, composition and concentration was obtained by dynamic light scattering measurements. It was observed that the micellar size increases with temperature, this growth being more pronounced as the relative proportion of the ethoxylated surfactant was increased. The behavior of the micellar size with the total surfactant concentration was also found to be dependent on temperature and composition. The clouding temperature, characteristic of the ethoxylated surfactants, was increased with the addition of the sugar surfactant. Lastly, possible structural changes in the micellar palisade layer were examined by steady-state fluorescence anisotropy in conjunction with time-resolved fluorescence studies with the hydrophobic probe coumarin 6 (C6). The obtained results indicate that the participation of the ethoxylated surfactant induces a slightly more polar palisade layer, whereas the probe carries out a faster rotational reorientation as a result of a less compact environment. All these observations were attributed to the different structure of the head groups of both surfactants and, as a consequence, to their different hydration.

Physicochemical Studies on the Interaction between N-Decanoyl-N-methylglucamide and Bovine Serum Albumin.

The protein-surfactant system constituted by bovine serum albumin (BSA) and N-decanoyl-N-methylglucamide (MEGA-10) has been studied by using surface tension, steady-state fluorescence, and dynamic light scattering measurements. It was found that the presence of protein delays the surfactant aggregation, which was interpreted as a sign of binding between surfactant and protein. Binding studies were carried out by two different methods. First, a treatment based on surface tension measurements was used to obtain information on the number of surfactant molecules bound per protein molecule under saturation conditions. Second, the binding curve for the BSA/MEGA-10 system was determined by examining the behavior of the intrinsic BSA fluorescence upon the surfactant addition. Both approaches indicate that the binding process is essentially cooperative in nature. The results of the aggregation numbers of MEGA-10 micelles, as well as those of resonance energy transfer from tryptophan residues to 8-anilinonaphthalene-1-sulfonate, corroborate the formation of micelle-like aggregates of surfactants, smaller than the free micelles, adsorbed on the protein surface. The dynamic light scattering results were not conclusive, in the sense that it was not possible to discriminate between protein-surfactant complexes and free micelles. However, the overall results suggest the formation of "pearl necklace" complexes in equilibrium with the free micelles of the surfactant.

Effect of NaCl on the self-aggregation of n-octyl beta-D-thioglucopyranoside in aqueous medium.

This report investigates the effect of sodium chloride (NaCl) on the micellization, surface activity, and the evolution in the shape and size of n-octyl beta-D-thioglucopyranoside (OTG) aggregates. By using surface tension measurements, information was obtained on both changes in the critical micelle concentration and adsorption behavior in the air-liquid interface with the electrolyte concentration. These data were used to obtain the thermodynamic properties of micellization along with the corresponding adsorption parameters in the air-liquid interface. From extended static and dynamic light scattering measurements, the micelle molecular weight, the mean aggregation number, and the second virial coefficient, the apparent diffusion coefficient and the mean hydrodynamic radius of micelles in a range of NaCl concentrations were obtained. The light scattering data have shown that when the surfactant concentration is lower to 4.5 g/L, only spherical micelles are formed. However, an increase in the surfactant concentration induces an increase in micellar size, suggesting a rodlike growth of the micelles. This deviation of micelle geometry from spherical to rodlike is supported both by the ratio between the hydrodynamic radius and the radius of gyration and by the angular dependence of light scattering. On the other hand, the studies performed in the presence of high NaCl concentration (0.2 and 0.5 M) provide strong support for the view that the micelles may overlap together to form an entangled network above certain crossover concentration.

Stability, interaction, size, and microenvironmental properties of mixed micelles of decanoyl-N-methylglucamide and sodium dodecyl sulfate.

The mixed micellization between the nonionic surfactant decanoyl-N-methylglucamide (MEGA-10) and the common sodium dodecyl sulfate (SDS) in aqueous solutions of 0.1 M NaCl was investigated by the fluorescence probe method. The critical micelle concentrations were determined by the pyrene 1:3 ratio method. The experimental data are discussed in light of two mixing thermodynamic models within the framework of the pseudophase separation model, including the conventional regular solution theory and a recent treatment proposed by Maeda (J. Phys. Chem. B 2004, 108, 6043). This last approach provides a more appropriate description of the mixed system, particularly in two aspects: the nature of the interactions responsible for the stability of the mixed micelle and the behavior of the excess free energy per monomer of the system. By using the static quenching method, the mean micellar aggregation numbers of mixed micelles in the whole range of compositions were obtained. It was found that the micellar aggregation number initially increases with the content of the ionic component, then remains roughly constant, and, finally, decreases slightly for high content of this component. This behavior was analyzed taking into account the effects produced by the presence of the charged headgroups of sodium dodecyl sulfate, as this component increases its participation in the mixed micelle. The micropolarity of the mixed micelles was studied by the pyrene 1:3 ratio index. It was observed that the increasing participation of the ionic component induces the formation of micelles with a more dehydrated structure. Data of micellar microviscosity were obtained by using different methods, including fluorescence intensity measurements of Auramine O and steady-state fluorescence anisotropy of rhodamine B and diphenylbutadiene. The results obtained from these experiments are in good agreement and suggest the formation of mixed micelles with a less ordered structure as the content of SDS increases.