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.

The rate and evenness of the substitutions on hyaluronan grafted by dodecanoic acid influenced by the mixed-solvent composition.

In this work, low molecular weight (17 kDa) hyaluronan was modified by dodecanoyl substituents. The activation of dodecanoic acid was mediated by benzoyl chloride towards the preparation of a mixed anhydride, which reacts in a second step with HA in water mixed with an organic solvent. The effect of the cosolvent was studied and showed an even distribution of substituents and higher reaction rate in water: 1,4-dioxane compared to water:tert-butanol where substituents occupy adjacent positions. The chemical characterization of the prepared derivatives was elucidated by NMR, FTIR spectroscopy, thermal analyses, and gas chromatography, while the distribution of substituents was evaluated by enzymatic degradation. Molecular-dynamics simulations reveal opposite solvent separations around HA and dodecanoyl chains, that is stronger in water:tert-butanol solution. The resulting incompatibility of solvation-shells of the two entities repels the reaction intermediates from the HA chain and drives them towards the already bound substituents, explaining the observed differences in the distribution evenness. Thus, the influence of the solvent on the reaction selectivity is observed by shielding reactive sites around HA. Therefore, a control of the distribution of the substituents was obtained by defining the concentration of HA and used cosolvent.

Effect of solvent and ions on the structure and dynamics of a hyaluronan molecule.

Hyaluronic acid (hyaluronan, HA) is a negatively charged polysaccharide forming highly swollen random coils in aqueous solutions. Their size decreases along with growing salt concentration, but the mechanism of this phenomenon remains unclear. We carry out molecular-dynamics simulations of a 48-monosaccharide HA oligomer in varying salt concentration and temperature. They identify the interaction points of Na+ ions with the HA chain and reveal their influence on the HA solvation-shell structure. The salt-dependent variation of the molecular size does not consist in the distribution of the dihedral angles of the glycosidic connections but is driven by the random flips of individual dihedral angles, which cause the formation of temporary hairpin-like structures effectively shortening the chain. They are induced by the frequency of cation-chain interactions that grow with the salt concentration, but is reduced by the simultaneous decrease of ions' activities. This leads to an anomalous random-coil shrinkage at 0.6 M salt concentration.