Polymorphic Self-Organization of Lauroyl Peptide in Response to pH and Concentration

Amphipathic peptides are attractive building blocks for the preparation of self-assembling, bio-inspired, and stimuli responsive nanomaterials with pharmaceutical interest. The bioavailability of these materials can be improved with the insertion of d amino acid residues to avoid fast proteolysis in vivo. With this knowledge, a new lauroyl peptide consisting of a sequence of glycine, glycine, d-serine, and d-lysine was designed. In spite of its simple sequence, this lipopeptide self-assembles into spherical micelles at acid pH, when the peptide moiety adopts disordered conformations. Self-aggregates reshape toward fibers at basic pH, following the conformational transition of the peptide region from random coil to β-sheet. Finally, hydrogels are achieved at basic pH and higher concentrations. The transition from random coil to β-sheet conformation of the peptide headgroup obtained by increasing pH was monitored by circular dichroism and vibrational spectroscopy. A structural analysis, performed by combining dynamic light scattering, small-angle X-ray scattering, transmission electron microscopy, and molecular dynamic simulations, demonstrated that the transition allows the self-assemblies to remodel from spherical micelles to rodlike shapes, to long fibers with rectangular cross-section and a head-tail-tail-head structure. The viscoelastic behavior of the hydrogels formed at the highest pH was investigated by rheology measurements.

Block copolymers as bile salt sequestrants: intriguing structures formed in a mixture of an oppositely charged amphiphilic block copolymer and bile salt

To study the formation and characterize the structure of mixed complexes of oppositely charged block copolymers and surfactants are of great significance for practical applications, e.g., in drug carrier formulations that are based on electrostatically assisted assembly. In this context, biocompatible block copolymers and biosurfactants (like bile salts) are particularly interesting. In this work, we report on the co-assembly in dilute aqueous solution between a cationic poly(N-isopropyl acryl amide) (PNIPAM) diblock copolymer and the oppositely charged bile salt surfactant sodium deoxycholate at ambient temperature. The cryogenic transmission electron microscopy (cryo-TEM) experiments revealed the co-existence of two types of co-assembled complexes of radically different morphology and inner structure. They are formed mainly as a result of the electrostatic attraction between the positively charged copolymer blocks and bile salt anions and highlight the potential of using linear amphiphilic block copolymers as bile salt sequestrants in the treatment of bile acid malabsorption and hypercholesterolemia. The first complex of globular morphology has a coacervate core of deoxycholate anions and charged copolymer blocks surrounded by a PNIPAM corona. The second complex has an intriguing tape-like supramolecular morphology of several micrometer in length that is striped in the direction of the long axis. A model is presented in which the stretched cationic blocks of several block copolymers interact electrostatically with the bile salt molecules that are associated to form a zipper-like structure. The tape is covered on both sides by the PNIPAM chains that stabilize the overall complex in solution. In addition to cryo-TEM, the mixed system was investigated in a range of molar charge fractions at a constant copolymer concentration by static light scattering, small angle X-ray scattering, and electrophoretic mobility measurements.

Supramolecular assembly of a thermoresponsive steroidal surfactant with an oppositely charged thermoresponsive block copolymer

Supramolecular rearrangements are crucial in determining the response of stimuli sensitive soft matter systems such as those formed by mixtures of oppositely charged amphiphiles. Here mixtures of this kind were prepared by mixing the cationic block copolymer pAMPTMA₃₀-b-pNIPAAM₁₂₀ and an anionic surfactant obtained by the modification of the bile salt sodium cholate. As pure components, the two compounds presented a thermoresponsive self-assembly at around 30-35 °C; a micelle formation in the case of the copolymer and a transition from fibers to tubes in the case of the bile salt derivative. When both were present in the same solution they associated into mixed aggregates that showed complex thermoresponsive features. At room temperature, the core of the aggregate was comprised of a supramolecular twisted ribbon of the bile salt derivative. The block copolymers were anchored on the surface of this ribbon through electrostatic interactions between their charged blocks and the oppositely charged heads of the bile salt molecules. The whole structure was stabilized by a corona of the uncharged blocks that protruded into the surrounding solvent. By increasing the temperature to 30-34 °C the mixed aggregates transformed into rods with smooth edges that associated into bundles and clusters, which in turn induced clouding of the solution. Circular dichroism allowed us to follow progressive rearrangements of the supramolecular organization within the complex, occurring in the range of temperature of 20-70 °C.

Crystal structure of a lithium salt of a glucosyl derivative of lithocholic acid

The crystal structure of a Li(+) salt of a glucosyl derivative of lithocholic acid (lithium 3α-(α-d-glucopyranosyl)-5β-cholan-24-oate) has been solved. The crystal belongs to the orthorhombic system, P212121 spatial group, and includes acetone and water in the structure with a 1:1:2 stoichiometry. Monolayers, having a hydrophobic interior and hydrophilic edges, are recognized in the crystal structure. Li(+) is coordinated to three hydroxyl groups of three different glucose residues, with two of them belonging to the same monolayer. A fourth molecule, located in this monolayer, is involved in the coordination of the cation through the carboxylate ion by an electrostatic interaction, thus completing a distorted tetrahedron. All Li(+)-oxygen distances values are very close to the sum of the ionic radius of Li(+) and van der Waals radius of oxygen. Each steroid molecule is linked to other five steroid molecules through hydrogen bonds. Water and acetone are also involved in the hydrogen bond network. A hierarchical organization can be recognized in the crystal, the helical assembly along 21 screw axes being left-handed.

Tailoring supramolecular nanotubes by bile salt based surfactant mixtures

An approach for tailoring self-assembled tubular structures is described. By controlling the relative composition of a two-component surfactant mixture comprising the natural bile salt lithocholate and its bolamphiphilic derivative, it was possible to finely tune the nanotube cross-section of the mixed tubular aggregates that self-associated spontaneously in aqueous solution at pH 12. The diameter was found to vary up to 50% when the stoichiometric ratio of the two bile salts was changed. The tuning of supramolecular nanochannels with such remarkable precision is of significant interest for technological applications of these materials.

Sugar-bile acid-based bolaamphiphiles: from scrolls to monodisperse single-walled tubules

The introduction of a mannose residue on carbon 3 of lithocholic acid gives rise to an asymmetric and rigid bolaamphiphilic molecule, which self-assembles in water to form elongated tubular aggregates with an outer diameter of about 20 nm. These tubular structures display a temporal evolution, where the average tube diameter decreases with time, which can be followed by time-resolved small-angle X-ray scattering experiments. Cryogenic transmission electron microscopy images collected as a function of time show that at short times after preparation tubular scrolls are formed via the rolling of layers, after which a complex transformation of the scrolls into single-walled tubules takes place. At long time scales, a further evolution occurs where the tubules both elongate and become narrower. The observed self-assembly confirms the tendency of bile acids and their derivatives to form supramolecular aggregates with an ordered packing of the constituent molecules. It also demonstrates that scrolls can be formed as intermediate structures in the self-assembly process of monodisperse single-walled tubules.

Between peptides and bile acids: self-assembly of phenylalanine substituted cholic acids

Biocompatible molecules that undergo self-assembly are of high importance in biological and medical applications of nanoscience. Peptides and bile acids are among the most investigated due to their ability to self-organize into many different, often stimuli-sensitive, supramolecular structures. With the aim of preparing molecules mixing the aggregation properties of bile acid and amino acid-based molecules, we report on the synthesis and self-association behavior of two diastereomers obtained by substituting a hydroxyl group of cholic acid with a l-phenylalanine residue. The obtained molecules are amphoteric, and we demonstrate that they show a pH-dependent self-assembly. Both molecules aggregate in globular micelles at high pH, whereas they form tubular superstructures under acid conditions. Unusual narrow nanotubes with outer and inner cross-section diameters of about 6 and 3 nm are formed by the derivatives. The diasteroisomer with α orientation of the substituent forms in addition a wider tubule (17 nm cross-section diameter). The ability to pack in supramolecular tubules is explained in terms of a wedge-shaped bola-form structure of the derivatives. Parallel or antiparallel face-to-face dimers are hypothesized as fundamental building blocks for the formation of the narrow and wide nanotubes, respectively.

Amino acid-bile acid based molecules: extremely narrow surfactant nanotubes formed by a phenylalanine-substituted cholic acid

An amino acid-substituted bile acid forms tubular aggregates with inner and outer diameters of about 3 and 6 nm. The diameters are unusually small for surfactant self-assembled tubes. The results enhance the spectrum of applications of supramolecular tubules and open up possibilities for investigating a novel class of biological amphiphiles.

Nanoparticles with a bicontinuous cubic internal structure formed by cationic and non-ionic surfactants and an anionic polyelectrolyte

Nanoparticles with an internal structure have been prepared by dispersing under dilute conditions poly(acrylic acid) with a polymerization degree n = 6000 (PAA6000) together with a cationic surfactant hexadecyltrimethylammonium hydroxide (C16TAOH) and the non-ionic surfactant penta(ethylene glycol) monododecyl ether (C12E5) in water. The nanoparticles are formed at different mixing ratios in the corresponding two-phase regions (liquid crystalline phase/dilute isotropic phase) of the C16TAPA6000 complex salt/C12E5/water ternary phase diagram. The particles consist of polyacrylate PA6000– polyions, C16TA+ surfactant ions, and C12E5. Their internal ordering was identified by small-angle X-ray scattering (SAXS) to be either bicontinuous cubic with the Ia3d crystallographic space group or normal hexagonal depending upon the amount of C12E5. The bicontinuous cubic phase, to our knowledge never observed before in polyelectrolyte–surfactant particle systems, was inferred by SAXS experiments. The data also showed that this structure is thermoresponsive in a reversible manner. The bicontinuous cubic space group transforms from Ia3d to Im3m as the temperature decreases from 25 to 15 °C. According to dynamic light scattering and electrophoretic mobility measurements, the particles have a well-defined size (apparent hydrodynamic radii RH in the range of 88–140 nm) and carry a positive net charge. The size of the nanoparticles is stable up to 1 month. The faceted nanoparticles are visualized by cryogenic transmission electron microscopy that also reveals their coexistence with thread-like C12E5 micelles.

Evidence for Sevenfold Coordination in the First Solvation Shell of Hg(II) Aqua Ion

A quite unexpected sevenfold coordination of the hydrated Hg(II) complex in aqueous solution is revealed by an extensive study combining X-ray absorption spectroscopy (XAS) and quantum mechanics/molecular dynamics (QM/MD) calculations. As a matter of fact, the generally accepted octahedral solvation of Hg(II) ion cannot be reconciled with XAS results. Next, refined QM computations point out the remarkable stability of a heptacoordinated structure with C2 symmetry, and long-time MD simulations by new interaction potentials including many-body effects reveal that the hydrated complex has a quite flexible structure, corresponding for most of the time to heptacoordinated species. This picture is fully consistent with X-ray absorption near-edge structure experimental data which unambiguously show the preference for a sevenfold instead of a sixfold coordination.

EXAFS and molecular dynamics studies of ionic solutions

This paper focuses on recent advances in the X-ray absorption spectroscopy (XAS) analysis of ionic solutions. The asymmetry of radial distribution functions g(r) associated with the solvent molecules surrounding the ions has to be taken into account to perform a reliable structural analysis. Molecular dynamics (MD) simulations provide reliable g(r)'s which can be used as starting models in the XAS data analysis. The combined MD-XAS investigation reduces meaningfully the indetermination of the structural parameters, especially for coordination numbers and Debye-Waller factors. Double-electron excitation channels can be present in the XAS spectra of ionic solutions and they have to be accounted for in the background extraction. The ability of the XAS technique to probe three-body correlation functions in ionic solutions with the aid of MD g(r1, r2, theta) has been shown. The analysis of the low-k region of the spectra allows the detection of a weak but significant hydrogen structural signal. The XAS technique is especially well suited to determine the detailed shape of the nearest-neighbor peak in the atom-atom pair correlation functions of disordered systems. The information that they contain about the short-range atom-atom pairwise interactions can be very helpful for specifying and properly modifying model potentials used in MD simulations.

K- and L-edge XAFS determination of the local structure of aqueous Nd(III) and Eu(III)

The short-range coordination of Nd(III) and Eu(III) ions in aqueous solutions has been investigated by means of K-edge x-ray absorption spectroscopy. Experimental data are presented which demonstrate that an accurate determination of the Nd(III) and Eu(III) hydration structural parameters can be obtained from the analysis of the K-edge spectra. It is shown that the large widths of the core hole states do not appreciably reduce the potential structural information of the K-edge EXAFS data. In addition the available k-range of the K-edge spectra is much wider compared to that of the L3-edge which is affected by the presence of double-electron excitations in a more severe fashion.