Concentration Fluctuation/Microheterogeneity Duality Illustrated with Aqueous 1,4-Dioxane Mixtures.

The structural properties of aqueous 1-4 dioxane mixtures are studied by computer simulations of different water and dioxane force field models, from the perspective of illustrating the link between structural properties at the molecular level and measurable properties such as radiation scattering intensities and Kirkwood-Buff integrals (KBIs). A strategy to consistently correct the KBI obtained from simulations is proposed, which allows us to obtain the genuine KBI corresponding to a given pair of molecular species, in the entire concentration range, and without necessitating excessively large system sizes. The application of this method to the aqueous dioxane mixtures, with an all-atom CHARMM dioxane model and 2 water models, namely, SPC/E and TIP3P, allows one to understand the differences in the structure of the corresponding mixtures at the molecular level, particularly concerning the role of the water aggregates and its model dependence. This study allows us to characterize the dual role played by the concentration fluctuations and the domain segregation, particularly in what concerns the calculated X-ray spectra.

Hyaluronan oligosaccharides form double-helical duplexes in water:1,4-dioxane mixed solvent.

Hyaluronic acid (HA) is a hydrophilic natural polysaccharide consisting of alternating monosaccharide units of glucuronic acid and N-acetyl glucosamine. In aqueous solutions the electrostatic repulsion of the carboxylate groups hampers the formation of supermolecular structures that can be partially stabilized by the addition of salt. Increased permittivity of the mixed water:organic solvents causes better compensation of the negative charge of HA chains by dissolved cations which changes their interactions with other molecules. In this study we simulate interactions of two HA chains in water:1,4-dioxane and water:tert-butanol mixed solvents with varying NaCl concentrations using molecular dynamics (MD). Anti-parallel double-helix-like duplexes are formed in NaCl-containing water:1,4-dioxane mixture and remain stable even when NaCl is removed. Parallel duplexes separate after a short time. In water:tert-butanol analogous duplexes are unstable. Stability of HA duplexes is thus determined by the solvent composition and the ability of its components to separate in the solvation shell of HA molecules, as well as by the mutual orientation of the chains.

Structure and dynamics of the hyaluronan oligosaccharides and their solvation shell in water: organic mixed solvents.

Hyaluronan (HA) is a natural polysaccharide occurring ubiquitously in the connective tissues of vertebrates widely used in the cosmetic and pharmaceutic industries. In numerous applications HA oligosaccharides are being chemically modified using reactions incompatible with aqueous solutions, often carried out in water:organic mixed solvents. We carry out molecular-dynamics (MD) simulations of HA oligosaccharides in water:1,4-dioxane and water:tert-butanol mixtures of different compositions. HA molecule causes a separation of the solvent components in its surroundings, especially in tert-butanol containing solutions, constituting thus a solvation shell enriched by water. Furthermore, interactions with ions are stronger than in pure water and depend on the solvent composition. Consequently, the dynamics of the HA chain varies with the solvent composition and causes observable conformational changes of the HA oligosaccharide. Composition of mixed solvents thus enables us to modify the interaction of HA with other molecules as well as its reactivity.

Salt-dependent intermolecular interactions of hyaluronan molecules mediate the formation of temporary duplex structures.

Hyaluronic acid (HA) is a natural polysaccharide present in the connective tissues of vertebrates, often used in the cosmetics and pharmaceutical industries. HA is a strongly hydrophilic macromolecule forming highly swollen random coils in aqueous solutions. Although some authors reported the secondary and tertiary structures of HA chain, others brought convincing evidence contradicting this hypothesis. This study aims at investigation of the stability and dynamics of the temporary duplex HA structures at different NaCl concentrations by molecular-dynamics (MD) simulations. The tendency to duplex formation grows with NaCl concentration reaching its maximum at 0.6 M. This profile is a result of two counteracting NaCl-concentration dependent phenomena, the growing electrostatic-repulsion screening on one side and the disturbance of hydrogen-bonds formation on the other side. Although the weak intermolecular attraction cannot lead to long-lived secondary and tertiary structures, it may influence the properties of large HA macromolecules and concentrated HA solutions.