Cluster phases of decorated micellar solutions with macrocyclic ligands.

An aqueous self-assembled micellar system (sodium dodecyl sulfate, SDS, decorated with various adhesive sites, cryptand Kryptofix 222 and crown ether 18-Crown-6 molecules) has been investigated by dynamic light scattering (DLS) and small angle x-ray scattering (SAXS) to have insights into the micellar structure, the micellar interactions, and the aggregation properties of the system. DLS demonstrates the existence of populations of aggregates in the submicrometer/micrometer range, while the Guinier analysis of the SAXS curves helps in detailing objects smaller than 30 nm. The aggregates of micelles are here named cluster phases of micelles (CPMs). Considering that SDS micelles in water do not aggregate at low concentration, it is shown that macrocyclic ligands induce the SDS micelle aggregation as a function of the concentration (i.e., investigated ligand/SDS molar ratios are 5.0, 1.5, 1.0, and 0.5) and hydrophobicity of the adhesive sites. The sizes and the percentages of the micelles and the CPMs have been monitored to test the stability and reversibility of the system. DLS results clearly show that the aggregation processes of the decorated micelles are reproducible at time intervals of the order of 1 month, while the stability may not be entirely maintained after a year. As an issue of particular relevance, the higher the ligand/surfactant molar ratio, the larger are the CPMs induced. The K222 ligand results in being more effective in promoting the micellar aggregation than 18C6 as a consequence of the different hydrophobicity.

ß-Connectin studies by small-angle x-ray scattering and single-molecule force spectroscopy by atomic force microscopy.

The three-dimensional structure and the mechanical properties of a ß-connectin fragment from human cardiac muscle, belonging to the I band, from I(27) to I(34), were investigated by small-angle x-ray scattering (SAXS) and single-molecule force spectroscopy (SMFS). This molecule presents an entropic elasticity behavior, associated to globular domain unfolding, that has been widely studied in the last 10 years. In addition, atomic force microscopy based SMFS experiments suggest that this molecule has an additional elastic regime, for low forces, probably associated to tertiary structure remodeling. From a structural point of view, this behavior is a mark of the fact that the eight domains in the I(27)-I(34) fragment are not independent and they organize in solution, assuming a well-defined three-dimensional structure. This hypothesis has been confirmed by SAXS scattering, both on a diluted and a concentrated sample. Two different models were used to fit the SAXS curves: one assuming a globular shape and one corresponding to an elongated conformation, both coupled with a Coulomb repulsion potential to take into account the protein-protein interaction. Due to the predominance of the structure factor, the effective shape of the protein in solution could not be clearly disclosed. By performing SMFS by atomic force microscopy, mechanical unfolding properties were investigated. Typical sawtooth profiles were obtained and the rupture force of each unfolding domain was estimated. By fitting a wormlike chain model to each peak of the sawtooth profile, the entropic elasticity of octamer was described.

Dielectric relaxation spectroscopy of lysozyme aqueous solutions: analysis of the δ-dispersion and the contribution of the hydration water.

The dielectric properties of lysozyme aqueous solutions have been investigated over a wide frequency range, from 1 MHz to 50 GHz, where different polarization mechanisms, at a molecular level, manifest. The dielectric relaxation spectra show a multimodal structure, reflecting the complexity of the protein-water interactions, made even more intricate with the increase of the protein concentration. The deconvolution of the spectra into their different components is not unambiguous and is generally a delicate process which requires caution. We have analyzed the whole relaxation region, on the basis of the sum of simple Debye-type relaxation functions, considering three main contributions. Particular attention has been payed to the δ-dispersion, intermediate between the ß-dispersion (rotational dynamics of the protein) and the γ-dispersion (orientational polarization of the water molecules). This intermediate contribution to the dielectric spectrum is attributed to the orientational polarization of water molecules in the immediate vicinity of the protein surface (hydration water). Our measurements clearly demonstrate that, at least at high protein concentrations, the δ-dispersion has a bimodal structure associated with two kinds of hydration water, i.e., tightly bound and loosely bound hydration water. In the concentration range investigated, the existence of a three-modal δ-dispersion, as recently suggested, is not supported, on the basis of statistical tests, by the analysis of the dielectric relaxations we have performed and a bimodal dispersion is accurate enough to describe the experimental data. The amount of the hydration water has been evaluated both from the dielectric parameters associated with the δ-dispersion and from the decrement of the loss peak of the γ-dispersion. The relative weight of tightly bound and loosely bound hydration water is briefly discussed.

Small-angle neutron scattering of percolative perfluoropolyether water in oil microemulsions.

A water in oil microemulsion system composed of water, surfactant, and oil, the latter two components of perfluoropolyether (PFPE) type, has been studied by small-angle neutron scattering (SANS) with the aim of knowing the microstructure of the system and to have an insight on the connection between microstructure characterization and percolation behavior. In fact, along the dilution line W/S = 11 of the phase diagram, dielectric spectroscopy and conductivity studies revealed a dynamic percolation process taking place approaching and above the dynamic percolation threshold, leading to a system composed of droplet clusters with percolation thresholds varying with temperature from a 0.501 volume fraction of the dispersed phase at 9.3 degrees C to 0.205 at 32.5 degrees C. The SANS experimental spectra of this work have been studied by modeling the microemulsion droplets as adhesive hard spheres. For all of the samples, the surfactant area per polar head has been also measured in the Porod region of the SANS spectra. Geometric and potential parameters as well as the osmotic pressure, the second virial coefficient, and the distance between droplets have been extracted from data as a function of droplets concentration. At low concentration, that is, below percolation thresholds, the droplets behave as hard spheres, whereas at threshold and above, adhesion changes significantly the samples. In fact, for each temperature, the measured size increases versus concentration from 30 to 50 A, and the area per polar head decreases correspondingly, suggesting that a process of dynamic fusion of droplets occurs in the system above threshold, that is, couples of droplets stick and unstick continuously with interdigitation of the surfactant tails.

Dynamic light scattering and atomic force microscopy imaging on fragments of beta-connectin from human cardiac muscle.

In order to investigate the protein folding-unfolding process, dynamic light scattering (DLS) and atomic force microscopy (AFM) imaging were used to study two fragments of the muscle cardiac protein beta-connectin, also known as titin. Both fragments belong to the I band of the sarcomer, and they are composed of four domains from I(27) to I(30) (tetramer) and eight domains from I(27) to I(34) (octamer). DLS measurements provide the size of both fragments as a function of temperature from 20 up to 86 degrees C, and show a thermal denaturation due to temperature increase. AFM imaging of both fragments in the native state reveals a homogeneous and uniform distribution of comparable structures. The DLS and AFM techniques turn out to be complementary for size measurements of the fragments and fragment aggregates. An unexpected result is that the octamer folds into a smaller structure than the tetramer and the unfolded octamer is also smaller than the unfolded tetramer. This feature seems related to the significance of the hydrophobic interactions between domains of the fragment. The longer the fragment, the more easily the hydrophobic parts of the domains interact with each other. The fragment aggregation behavior, in particular conditions, is also revealed by both DLS and AFM as a process that is parallel to the folding-unfolding transition.

Small-angle neutron scattering of mixed ionic perfluoropolyether micellar solutions.

Aqueous mixed micellar solutions of perfluoropolyether carboxylic salts with ammonium counterions have been studied by small-angle neutron scattering. Two surfactants differing in the tail length were mixed in proportions n2/n3 = 60/40 w/w, where n2 and n3 are the surfactants with two and three perfluoroisopropoxy units in the tail, respectively. The tails are chlorine-terminated. The mixed micellar solutions, in the concentration range 0.1-0.2 M and thermal interval 20-40 degrees C, show structural characteristics of the interfacial shell that are very similar to ammonium n2 micellar solutions previously investigated; thus, the physics of the interfacial region is dominated by the polar head and counterion. The shape and dimensions of the micelles are influenced by the presence of the n3 surfactant, whose chain length in the micelle is 2 A longer than that of the n2 surfactant. The n3 surfactant favors the ellipsoidal shape in the concentration range 0.1-0.2 M with a 1/2 ionization degree of n2 micelles. The very low surface charge of the mixed micelles is attributed to the increase in hydrophobic interactions between the surfactant tails, due to the longer n3 surfactant molecules in micelles. The closer packing of the tails decreases the micellar curvature and the repulsions between the polar heads, by surface charge neutralization of counterions migrating from the Gouy-Chapman diffuse layer, leading to micellar growth in ellipsoids with greater axial ratios.

Small-angle neutron scattering of ionic perfluoropolyether micellar solutions: role of counterions and temperature.

This paper reports a small-angle neutron scattering (SANS) characterization of perfluoropolyether (PFPE) aqueous micellar solutions with lithium, sodium, cesium and diethanol ammonium salts obtained from a chlorine terminated carboxylic acid and with two perfluoroisopropoxy units in the tail (n(2)). The counterion and temperature effects on the micelle formation and micellar growth extend our previous work on ammonium and potassium salts n(2) micellar solutions. Lithium, sodium, cesium and diethanol ammonium salts are studied at 0.1 and 0.2 M surfactant concentration in the temperature interval 28-67 degrees C. SANS spectra have been analyzed by a two-shell model for the micellar form factor and a screened Coulombic plus steric repulsion potential for the structure factor in the frame of the mean spherical approximation of a multicomponent system reduced to a generalized one component macroions system (GOCM). At 28 degrees C, for all the salts, the micelles are ellipsoidal with an axial ratio that increases from 1.6 to 4.2 as the counterion volume increases. The micellar core short axis is 13 A and the shell thickness 4.0 A for the alkali micelles, and 14 and 5.1 A for the diethanol ammonium micelles. Therefore, the core short axis mainly depends on the surfactant tail length and the shell thickness on the carboxylate polar head. The bulky diethanol ammonium counterion solely influences the shell thickness. Micellar charge and average aggregation number depend on concentration, temperature and counterion. At 28 degrees C, the fractional ionization decreases vs the counterion volume (or molecular weight) increase at constant concentration for both C = 0.1 M and C = 0.2 M. The increase of the counterion volume leads also to more ellipsoidal shapes. At C = 0.2 M, at 67 degrees C, for sodium and cesium micelles the axial ratio changes significantly, leading to spherical micelles with a core radius of 15 A, lower average aggregation number, and larger fractional ionization.