Interactions between Cationic Ion Pair Amphiphile Vesicles and Hyaluronan-A Physicochemical Study

High-resolution ultrasound spectroscopy (HR-US), size and ζ-potential titrations, and isothermal titration calorimetry (ITC) were used to characterize the interactions between hyaluronan and catanionic ion pair amphiphile vesicles composed of hexadecyltrimethylammonium-dodecylsulphate (HTMA-DS), dioctadecyldimethylammonium chloride (DODAC), and cholesterol. In addition to these methods, visual observations were performed with the selected molecular weight of hyaluronan. A very good correlation was obtained between data from size titration, HR-US, and visual observation, which indicated in lower charge ratios the formation of hyaluronan-coated vesicles. On the contrary, at higher charge ratios, coated vesicles disintegrated to a size of around 2000 nm. The intensity of these interactions and the disaggregation were dependent on the molecular weight of hyaluronan. All interactions studied by ITC showed strong exothermic behavior, and these interactions between vesicles and hyaluronan were confirmed from the first addition, independently of the molecular weight of hyaluronan.

Time-resolved detection of SDS-induced conformational changes in α-synuclein by a micro-stopped-flow system

An intrinsically disordered protein, α-synuclein (αSyn), binds to negatively charged phospholipid membranes and adopts an α-helical structure. This conformational change is also induced by interaction with sodium dodecyl sulfate (SDS), which is an anionic surfactant used in previous studies to mimic membrane binding. However, while the structure of the αSyn and SDS complex has been studied widely by various static measurements, the process of structural change from the denatured state to the folded state remains unclear. In this study, the interaction dynamics between αSyn and SDS micelles was investigated using time-resolved measurements with a micro-stopped-flow system, which has been recently developed. In particular, the time-resolved diffusion based on the transient grating technique in combination with a micro-stopped-flow system revealed the gradual change in diffusion triggered by the presence of SDS micelles. This change is induced not only by binding to SDS micelles, but also by an intramolecular conformational change. It was interesting to find that the diffusion coefficient decreased in an intermediate state and then increased to the final state in the binding reaction. We also carried out stopped-flow-kinetic measurements of circular dichroism and intramolecular fluorescence resonance energy transfer, and the D change was assigned to the formation of a compact structure derived from the helix bending on the micelle.

Dynamics of conformation changes of α-synuclein induced by the presence of SDS micelles are revealed using time-resolved diffusion, CD, and FRET measurements combined with a micro-stopped flow system.

State of Oxygen Molecules in Aqueous Supersaturated Solutions

The state of oxygen in aqueous supersaturated solutions prepared by different methods was studied using high-resolution ultrasonic spectroscopy in combination with other techniques. This allowed for nondestructive evaluation of the properties of oxygen solute particles, composed of oxygen molecules and surrounding (coordinating) molecules of water, at equilibrium, supersaturated conditions, and different temperatures and concentrations of O₂. The results were compared with the behaviors of other types of solutes in water, including H₂O₂, which has similar molecular size and mass to O₂ but is characterized by a significantly different type of interaction with water molecules. Additionally, theoretical modeling was performed to assess the ultrasonic characteristics of dispersions of oxygen nanobubbles stabilized by a surface electrical charge. The obtained data indicate a clathrate-like organization of water in the coordination shells of single molecules of O₂. We did not find any signs of formation of clusters of oxygen molecules in supersaturated solutions. No quantifiable presence of oxygen nanobubbles in the solutions was detected. The state of O₂ molecules was not affected by supersaturation within the analyzed concentration range of oxygen. The results also demonstrated the potential of the ultrasonic technique in precision real-time nondestructive monitoring of oxygen solubilization and outgassing processes.

Ultrasonic study of hyaluronan interactions with Septonex-A pharmaceutical cationic surfactant

Interactions between hyaluronan and the cationic surfactant Septonex were studied using high resolution ultrasound spectroscopy. Four different interaction regions were identified in aqueous solution in a narrow interval of surfactant concentrations. In contrast, in 0.15 mol/L NaCl solution, essentially only two principal regions were observed. Hyaluronan-Septonex aggregates were generally less compressible than Septonex monomers or micelles and their formation in water was not appreciably affected by hyaluronan molecular weight. In the presence of NaCl, the effect of hyaluronan molecular weight was more profound, which can be ascribed to its conformational sensitivity to ionic strength combined with its role as a surfactant counterion. Hyaluronan-Septonex interactions were closer to interactions with CTAB than with TTAB. Differences in structure or Krafft temperature (solubility) between Septonex and CTAB were thus not significantly reflected in these interactions. The basic characteristics of Septonex surfactant were also determined.

Driving Forces in Pressure-Induced Protein Transitions

The molecular mechanisms underlying pressure-induced protein denaturation can be analyzed based on the pressure-dependent differences in the apparent volume occupied by amino acids inside the protein and when exposed to water in an unfolded conformation. This chapter presents a volumetric analysis of the peptide group and the 20 naturally occurring amino acid side chains in the interior of the native state, the micelle-like interior of the pressure-induced denatured state, and in the unfolded conformation modeled by low-molecular analogs of proteins. The transfer of a peptide group from the protein interior to water becomes increasingly favorable as pressure increases. This observation classifies solvation of peptide groups as a major driving force in pressure-induced protein denaturation. Polar side chains do not appear to exhibit significant pressure-dependent changes in their preference for the protein interior or solvent. The transfer of nonpolar side chains from the protein interior to water becomes more unfavorable as pressure increases. An inference can be drawn that a sizeable population of nonpolar side chains remains buried inside a solvent-inaccessible core of the pressure-induced denatured state. At elevated pressures this core, owing to the absence of structural constraints, may become packed almost as tightly as the interior of the native state. The presence and partial disappearance of large intraglobular voids is another driving force facilitating pressure-induced protein denaturation. Volumetric data presented here have implications for the kinetics of protein folding and shed light on the nature of the folding transition state ensembles.

Thermal behavior of long-chain alkanoylcholine soaps

Long-chain alkanoylcholines prepared from fatty acids adopt a diversity of thermally interconvertible phases made of a bilayered structure with alkanoyl chains crystallized or interdigitated in a more or less extent depending on temperature and alkanoyl chain length.

Effect of hydrophobic interactions on volume and thermal expansivity as derived from micelle formation

Volumetric parameters have long been used to elucidate the phenomena governing the stability of protein structures, ligand binding, or transitions in macromolecular or colloidal systems. In spite of much success, many problems remain controversial. For example, hydrophobic groups have been discussed to condense adjacent water to a volume lower than that of bulk water, causing a negative contribution to the volume change of unfolding. However, expansivity data were interpreted in terms of a structure-making effect that expands the water interacting with the solute. We have studied volume and expansivity effects of transfer of alkyl chains into micelles by pressure perturbation calorimetry and isothermal titration calorimetry. For a series of alkyl maltosides and glucosides, the methylene group contribution to expansivity was obtained as 5 uL/(mol K) in a micelle (mimicking bulk hydrocarbon) but 27 uL/(mol K) in water (20 °C). The latter value is virtually independent of temperature and similar to that obtained from hydrophobic amino acids. Methylene contributions of micellization are about -60 J/(mol K) to heat capacity and 2.7 mL/mol to volume. Our data oppose the widely accepted assumption that water-exposed hydrophobic groups yield a negative contribution to expansivity at low temperature that would imply a structure-making, water-expanding effect.

Colloidal stability of hydrophobic nanoparticles in ionic surfactant solutions: definition of the critical dispersion concentration

The dispersion stability diagrams of hydrophobic boehmite nanoparticles in aqueous n-alkyltrimethylammonium bromide solutions (alkyl chain lengths 10-16) were studied over a wide range of particle and surfactant concentrations. The surfactant molecules adsorb tail-on on the particle surface, which provides the colloidal stability through electrostatic repulsion. In the stable region of each diagram, bimodal particle size distributions (50 and 500 nm) are found at lower surfactant concentration, which give way to monomodal distributions (50 nm) at higher concentration. This deagglomeration is connected with the cmc of the surfactants and can be explained by a desorption of counterions from the self-assembled surfactant layer. The desorption is caused by changes in the counterion concentration upon micellization. At low particle concentrations, the transition from the intermediate to the stable region, that is, the disappearance of the precipitate, occurs at a constant surfactant concentration. This concentration is introduced as the "critical dispersion concentration" (cdc), this being the lowest required concentration of a surfactant that is necessary to disperse the hydrophobic particles. The logarithm of the cdc shows a linear dependence on the surfactant chain length, thus a cmc-analogous behavior. The ratio cdc/cmc decreases with increasing surfactant chain length, indicating that long-chain surfactants are more efficient in dispersing nanoparticles than are their lower homologues. The existence of a system-specific critical cdc/cmc ratio, beyond which stable dispersions cannot be obtained, is proposed, which explains the disability of short-chain surfactants to disperse colloids.

A novel real-time ultrasonic method for prion protein detection using plasminogen as a capture molecule

Background

High resolution ultrasonography (HR-US) can monitor the molecular changes and biochemical interactions between proteins in real-time. The aim of this study was to use HR-US to characterize the real-time interactions between plasminogen coated beads and PrP^Sc and to determine if this approach could be applied to the identification of animals affected by prion diseases. Plasminogen, immobilized to beads, was used as a capturing tool for PrP^Sc in brain homogenates from scrapie affected sheep and the binding reaction was monitored in real-time in an ultrasonic cell.

Results

Changes in the ultrasonic parameters suggested that three processes occurred during the incubation: binding, protein-protein network formation and precipitation and that these processes occurred in a concentration dependent manner. Conversely, when homogenates from normal sheep were similarly examined, no evidence for the occurrence of these processes was found indicating the specificity of the interaction between the plasminogen coated beads and PrP^Sc.

Conclusion

These results indicate firstly, that the plasminogen coated beads binded selectively to PrP^Sc and secondly, that a HR-US system can discriminate between scrapie affected and non-affected samples and thus has potential as a tool for the rapid diagnosis for prion diseases. This approach has the significant advantage of not requiring a proteinase K pre-digestion step, which is routinely used in current PrP^Sc detection assays.

Analysis of the phase diagram and microstructural transitions in phospholipid microemulsion systems using high-resolution ultrasonic spectroscopy

In the present work, high-resolution ultrasonic spectroscopy was applied to analyze a pseudoternary phase diagram for mixtures consisting of water/isopropyl myristate/Epikuron 200 and a cosurfactant (n-propanol). Changes in the ultrasonic velocity and attenuation in the megahertz frequency range were measured in the course of titration of the oil/surfactant/cosurfactant mixture with water at 25 degrees C. The ultrasonic titration profiles showed several phase transitions in the samples, which allowed the construction of an "ultrasonic" phase diagram. Quantitative analysis of the ultrasonic parameters enabled the characterization of various phases (swollen micelles, microemulsion, coarse emulsion, and pseudo-bicontinuous) as well as the evaluation of the state of the water and the particle size. The particle size obtained for the microemulsion region ranged from 5 to 14 nm over the measured concentrations of water/isopropyl myristate/Epikuron 200 and n-propanol, which agreed well with the previous literature data.

A comparative study of the physicochemical properties of perfluorinated and hydrogenated amphiphiles

In this work we studied and compared the physicochemical properties of perfluorinated (sodium perfluoroheptanoate, C7FONa, and perfluorooctanoate, C8FONa) and hydrogenated (sodium octanoate, C8HONa, decanoate, C10HONa, and dodecanoate, C12HONa) amphiphiles. First, we determined their Krafft points to study the solubility and appropriate temperature range of micellization of these compounds. The critical micelle concentration (cmc) and ionization degree of micellization (beta) as a function of temperature (T) were estimated from conductivity data. Plots of cmc vs T appear to follow the typical U-shaped curve with a minimum T(min). The results show that the surfactants with CF2/CH2 ratio of 1.5 between alkyl chains (C12HONa-C8FONa and C10HONa-C7FONa) have nearly the same minimum value for cmc against temperature. The comparison between the cmc of hydrogenated amphiphiles and the corresponding perfluorinated amphiphiles must be done at this point. Thermodynamic functions of micellization were obtained by applying different theoretical models and choosing the one that best fit our experimental data. Although perfluorinated and hydrogenated amphiphiles present similar thermodynamic behavior, we have found a variation of 1.3 to 1.7 in the CF2/CH2 ratio, which did not remain constant with temperature. In the second part of this study the apparent molar volumes and adiabatic compressibilities were determined from density and ultrasound velocity measurements. Apparent molar volumes at infinite dilution presented the ratio 1.5 between alkyl chains again. However, apparent molar volumes upon micellization for sodium perfluoroheptanoate indicated a different aggregation pattern.

High-Q ultrasonic determination of the critical nanoaggregate concentration of asphaltenes and the critical micelle concentration of standard surfactants

Asphaltenes are known to be interfacially active in many circumstances such as at toluene-water interfaces. Furthermore, the term micelle has been used to describe the primary aggregation of asphaltenes in good solvents such as toluene. Nevertheless, there has been significant uncertainty regarding the critical micelle concentration (CMC) of asphaltenes and even whether the micelle concept is appropriate for asphaltenes. To avoid semantic debates we introduce the terminology critical nanoaggregate concentration (CNAC) for asphaltenes. In this report, we investigate asphaltenes and standard surfactants using high-Q, ultrasonic spectroscopy in both aqueous and organic solvents. As expected, standard surfactants are shown to exhibit a sharp break in sonic velocity versus concentration at known CMCs. To prove our methods, we measured known surfactants with CMCs in the range from 0.010 g/L to 2.3 g/L in agreement with the literature. Using density determinations, we obtain micelle compressibilities consistent with previous literature reports. Asphaltenes are also shown to exhibit behavior similar to that of ultrasonic velocity versus concentration as standard surfactants; asphaltene CNACs in toluene occur at roughly 0.1 g/L, although the exact concentration depends on the specific (crude oil) asphaltene. Furthermore, using asphaltene solution densities, we show that asphaltene nanoaggregate compressibilities are similar to micellar compressibilities obtained with standard nonionic surfactants in toluene. These results strongly support the contention that asphaltenes in toluene can be treated roughly within the micelle framework, although asphaltenes may exhibit small levels of aggregation (dimers, etc.) below their CNAC. Furthermore, our extensive results on known surfactants agree with the literature while the asphaltene CNACs reported here are one to two orders of magnitude lower than most previously published results. (Previous work utilized the terminology "micelle" and "CMC" for asphaltenes.) We believe that the previously reported high concentrations for asphaltene CMCs do not correspond to primary aggregation; perhaps they refer to higher levels of aggregation or perhaps to a particular surface structure.

10-Methylacridinium ion as a fluorimetric probe measuring the activity of halide anions in aqueous solutions of cationic surfactants

The quenching of fluorescence of 10-methylacridinium ion by inorganic anions in aerated aqueous solutions was studied at room temperature. In the case of cationic surfactants, with chloride and bromide anions as counterions, characteristic breaks on the Stern-Volmer plots could be observed at concentrations corresponding to the critical micelle concentration of the surfactants. It is shown that the ratio of the slopes of the two linear fragments of the plots, in the micellar and premicellar concentration ranges, gives an estimate of the value of the ionization degree, alpha, of the micelles. This approach is applicable also in aqueous-alcohol systems.

Static and dynamic light-scattering studies on micellar solutions of alkyldimethylbenzylammonium chlorides

Static (SLS) and dynamic (DLS) light-scattering techniques were applied to the study of the aggregation of dodecyl- (C12DBACl), tetradecyl- (C14DBACl), and hexadecyldimethylbenzylammonium (C16DBACl) chlorides in water and in 0.01 and 0.05 m NaCl aqueous solutions at 25 degrees C. Results of SLS measurements yielded critical micelle concentration (cmc) values for aqueous and NaCl solutions. The aggregation numbers of the micelles for the homologous surfactants are low but increase with chain length and ionic strength of the solution. Various patterns of changes of the diffusion coefficient, D, as a function of chain length, molality, and with ionic strength were found for the studied surfactants. Transformations in the structure of micelles of C14DBACl in 0.01 m NaCl occur at a concentration of surfactant of about 0.01 m. Such transformations, presumably due to rodlike structure, are the more extensive the higher the concentration of NaCl. The concentration of C16DBACl in 0.05 m NaCl covers the range where already repulsive interactions between micelles occur, as judged by the strongly negative slope of the D versus molality plot. To provide additional information on the suggested transformations, complementary viscosity measurements for C14DBACl in 0.01 m of NaCl have been performed.

Ultrasonic shear wave rheology of weak particle gels

Weak particle gels have attracted increasing attention in the last decade. These gels have a very short region of deformation over which their viscoelastic parameters are constant. They can be broken easily in response to external forces. Therefore the rheological measurements in these systems must be performed at very small deformations, which may frequently be below the accuracy limits of conventional rheological instruments. In the present paper we discuss the application of the thickness shear mode resonator technique for the measurement of viscoelastic parameters of weak particle gels in the MHz frequency range. The technique provides information on the viscoelasticity of weak gels in the time scale 10(-7)-10(-9) s. The length scale of the measurements, determined by the depth of penetration and the wavelength of the shear wave, falls in the submicron and micron range. The displacements in the shear deformations generated in this technique are extremely small, in the order of Angstroms, and the shear strain, approximately 10(-3), corresponds to the low limits in the classical dynamic rheology measurements. Only small volumes, down to 0.1 ml, of sample are required and this is another advantage of this technique. The measurements of the storage, G', and the loss, G'', moduli can be carried out non-invasively and continuously at various frequencies in the same sample during the whole length of the process of gelation. General and specific aspects of the measurements and interpretation of experimental results are discussed in the present paper.

Hydrophilicity of Polar and Apolar Domains of Amphiphiles

The hydrophilicity of polar and apolar domains of various amphiphiles was systematically estimated for their homologues and analogues by measuring the molar adiabatic compressibility of an aqueous solution at infinite dilution. The homologues of protic alkyl H(CH(2))(n)-, perfluoroalkyl F(CF(2))(n)-, and alkylphenyl H(CH(2))(n)(C(6)H(5))- groups (n=0-10) were chosen to represent apolar hydrophobic domains. The polar hydrophilic domains tested were -SO(4)Na, -SO(3)Na, -COONH(4), -N(CH(3))(3)Br, N(C(m)H(2m+1))(4)Br (m=1-5), and -NH(CH(2))(n)SO(3) (n=3, 4) groups. Also tested were the tetraphenyl ionic compounds (C(6)H(5))(4)MX (M=B/X=Na, M=P/X=Cl, M=As/X=Cl) to study the effect of the ionic sign of the core atom across the tetraphenyl apolar shell, the polyethylene glycols H(OCH(2)CH(2))(m)OH (m=1-4) to study the role of apolar -CH(2)- units in the hydrophilic oxyethylene group, and the zwitterionic dimethylaminoalkylsulfonate (CH(3))(2)NH(CH(2))(n)SO(3) homologues to study the effect of intramolecular salt formation on the hydrophilicity of the zwitterion. The adiabatic compressibility of the solution was calculated from measurement of the sound velocity and density of solutions. The introduction of laboratory automation and the numerical control of the system improved the accuracies and efficiencies of the measurements a great deal. The range of the temperature scan was 0-40 degrees C with an effective accuracy of +/-0.001 degrees C and the concentration was automatically scanned down to far below the cmc of the surfactant. The hydrophilicity of various polar and apolar substances was estimated as the decrease of molar adiabatic compressibility of the aqueous solution with increased concentration of their homologues and analogues. The hydrophobic hydration of nonpolar substances was found to be very small at room temperature and was barely detected above 40 degrees C; however, it became large as the temperature was lowered and attained a maximum at 0 degrees C. The cationic charge of quaternary ammonium N(+)(C(n)H(2n+1))(4) was found to enhance the hydrophobic hydration of methylene groups located at a distance of 4 to 6 Å from the core nitrogen atom, while the terminal negative charge of the anionic surfactant R-SO(4)(-), R-SO(3)(-), or R-COO(-) was found to decrease the hydrophobic hydration of -CH(2)- units within the same range. The hydrophilicity of quaternary ammonium and the tetraphenyl ions should be synergistically given by both hydrophobic and ionic hydrations. The hydrophilicity of the perfluoromethylene unit -CF(2)- was found to have a value comparable to that of the protic methylene unit -CH(2)-. The hydrophobic hydration seems to offer a good measure of the hydrophilicity of apolar substances; however, it does not necessarily represent the "hydrophobicity" of the apolar segment when the "surface activity" of the amphiphile is concerned. Copyright 2000 Academic Press.