Hydration of polysaccharide hyaluronan observed by IR spectrometry. I. Preliminary experiments and band assignments

This article is the first one in a series dedicated to the study of hyaluronan as observed by IR spectrometry. The goal is to determine its hydration mechanism and the structural changes this mechanism implies. Hyaluronan is a natural polysaccharide that is widely used in biomedical applications and cosmetics. Its macroscopic properties are significantly dependent on its degree of hydration. In this article we record the IR spectrum of a several micron thick dried film and deduce that four or five residual H(2)O molecules remain around each disaccharide repeat unit in the dried film. We then compare the spectra of sodium hyaluronan and its acid form to assign vibrational bands linked to the carboxylate group. We proceed with a qualitative analysis of the spectral changes induced by changes of temperature and hygroscopicity, two independent parameters that act by modifying the hydrogen bond network of the sample. This enables us to assign most of the vibrational bands of the hydrophilic groups and to distinguish the bands that are due to these hydrophilic groups when they are or are not hydrogen bonded. It constitutes a prerequisite for the quantitative analysis of hydration spectra that will be described in the following articles of this series.

Structure and properties of a bacterial polysaccharide named Fucogel

The chemical structure of a polysaccharide named Fucogel was characterized and the position of acetylation was identified by NMR. A conformational analysis was performed on this 3-sugar repeating unit. From this, the persistence length, characterizing the stiffness of the polysaccharide, was determined and the role of the presence of acetyl group, reducing the stiffness, was pointed out. The helical conformations were also predicted, one of these being in agreement with X-ray data obtained on a similar polysaccharide. Experimental characterization of the native and deacetylated polysaccharides was developed. SEC experiments allowed us to determine the molar mass and the persistence length on the deacetylated polysaccharide. The value is in good agreement with that predicted from the molecular modeling. Microcalorimetry, rheology, and fluorescence spectroscopy demonstrated respectively that no helical conformation exists in solution but that loose interchain interactions due to the acetyl substituents exist in dilute solutions.

Structure and properties of a bacterial polysaccharide from a Klebsiella strain (ATCC 12657)

The chemical structure and the rheological behavior of the Klebsiella polysaccharide ATCC 12657 was studied and compared with data described in the literature and obtained for similar polysaccharides. The acetylated polysaccharide presents in solution a normal viscoelastic behavior with no evidence of an ordered conformation whatever the experimental conditions are. The deacetylated form can induce the formation of physical gels, in the presence of salt excess or ethanol. Microcalorimetry, optical rotation, and rheology experiments demonstrate that a thermally reversible and highly cooperative conformational transition occurs at the same temperature than a sol-gel transition. The melting of the gel and the conformational transition temperatures are dependent on the nature of cations and ionic concentration, whereas the gel strength is only influenced by polymer concentration.

Hydration of hyaluronan polysaccharide observed by IR spectrometry. II. Definition and quantitative analysis of elementary hydration spectra and water uptake

We recorded a series of spectra of sodium hyaluronan (HA) films that were in equilibrium with their surrounding humid atmosphere. The hygrometry of this atmosphere extended from 0 to 0.97% relative humidity. We performed a quantitative analysis of the corresponding series of hydration spectra that are the difference spectra of the film at a defined hygrometry minus the spectrum of the dried film (hygrometry = 0). The principle of this analysis is to use this series of hydration spectra to define a limited number (four) of "elementary hydration spectra" over which we can decompose all hydration spectra with good accuracy. This decomposition, combined with the measurements of the numbers of H(2)O molecules at the origin in these elementary hydration spectra of the three characteristic vibrational bands of H(2)O, allowed us to calculate the hydration number under different relative humidity conditions. This number compares well with that determined by thermogravimetry. Furthermore, the decomposition defines for each hygrometry value which chemical mechanisms represented by elementary hydration spectra are active. This analysis is pursued by determining for the elementary hydration spectra the number of hydrogen bonds established by each of the four alcohol groups found in each disaccharide repeat unit before performing the same analysis for amide and carboxylate groups. These results are later utilized to discuss the structure of HA at various stages of hydration.

Hydration of hyaluronan polysaccharide observed by IR spectrometry. III. Structure and mechanism of hydration

The results of the analysis of hydration spectra of Na(+) hyaluronan (HA) performed in a companion study are translated in terms of chemical mechanisms. We find that dried HA is characterized by chains having ordered parts of at least 6 disaccharide repeat units that extend over 60 A. The order is mainly due to C3O3H...O5 and C4O4H...O5 hydrogen bonds that hinder rotations around beta(1-4) and beta(1-3) glycoside bonds. Along one chain there are two-thirds of the N-H amide groups and carboxyl groups that are directly hydrogen bonded, with no water intermediate, to form N-H...(-)O-C=O hydrogen bonds, which are collateral to C3O3H...O5 hydrogen bonds. The existence of these N-H...(-)O-C=O bonds is somewhat in opposition to literature descriptions. In this dry state a "water wire" of 4-5 H(2)O molecules, which are anchored on C=O carboxyl groups and hydrating the Na(+) CO(-) ionic group, establishes hydrogen bonds on other hydrophilic groups of the same chain or other chains and remains embedded in HA, even at 104 degrees C. Hydration occurs at low hygrometry around the remaining one-third of the N-H...(-)O-C=O pairs that are not hydrogen bonded. Each of these N-H and (-)O-C=O groups is hydrated by a nanodroplet of some 25 H(2)O molecules that finds other sites for binding and hydrates 2 disaccharide repeat units. At higher hygrometry bigger nanodroplets hydrate all hydrophilic sites.

Two types of hydrophobic aggregates in aqueous solutions of chitosan and its hydrophobic derivative

The aggregation phenomena in aqueous solutions of hydrophobically modified (HM) chitosan, containing 4 mol % of n-dodecyl side chains, were studied by viscometry and fluorescence spectroscopy with pyrene as a probe. The results are compared with those for unmodified chitosan. Surprisingly, fluorescence data reveal the appearance of intermolecular hydrophobic aggregates both in chitosan and in HM chitosan. Nevertheless, these polymers exhibit quite different rheological properties: upon the formation of aggregates the viscosity of HM chitosan sharply increases, while that of unmodified chitosan raises only slightly. The aggregation models for both chitosan and its hydrophobic derivative were proposed. It was shown that in solutions of HM chitosan two types of hydrophobic domains exist: hydrophobic domains typical for different associating polymers with hydrophobic side chains and hydrophobic domains inherent to chitosan itself.

Solid state NMR for determination of degree of acetylation of chitin and chitosan

The degree of acetylation (DA) of various samples has been evaluated by 1H liquid-state NMR and 13C and 15N CP-MAS solid-state NMR over the whole range of DA. A good agreement has been found for all the experiments. The 13C and 15N CP-MAS experiments have permitted the evaluation of the chitin DA and content in the structural polysaccharides in a fungus named Aspergillus niger. The fungus structural carbohydrates mainly consist in pure chitin associated with glucans. Comparison of the 13C CP-MAS spectra with standard (1-->3)-beta-D-glucans strongly suggests that chitin and glucans are linked via covalent bonds.

Associative phenomena in galactomannan-deacetylated xanthan systems

The interaction between mesquite seed galactomannan (MSG; D-mannose to D-galactose ratio (M/G) approximately 1.1) and deacetylated xanthan (DX) in 5 mM NaCl leading to synergistic gel formation at 25 degrees C was investigated and compared with the far more studied system made of xanthan and locust bean gum (LBG; M/G approximately 3.5). Rheology and differential scanning calorimetry were used to measure temperatures of gel formation and transition enthalpy as a function of polymer composition, while circular dichroism was used to probe the conformation of DX in the LBG-DX system. MSG and DX associate at 25 degrees C with a well defined stoichiometry of 0.6:1.0 (w/w) at low ionic strength favouring the disordered coil state of DX. When LBG was used in place of MSG in water or 5 mM NaCl, two types of mechanisms of interpolymeric association are envisaged.

Comparative rheological behavior of hyaluronan from bacterial and animal sources with cross-linked hyaluronan (hylan) in aqueous solution

Using a variety of rheological techniques, the behavior of hyaluronan (M(w) 0.8-2.2 x 10(6)), cross-linked hyaluronan (hylan) (M(w) 1.8-12.5 x 10(6)), and Healon (M(w) approximately 5 x 10(6)) (a proprietary hyaluronan) was studied over a large range of molecular weights. The object was to study the effect of the cross-links in hylan on the various rheological parameters, in comparison with linear hyaluronan. There are significant differences. The Huggins constant and the critical overlap parameter C*[eta] are considerably lower for hylan and an increase in moduli at low frequencies was observed for hylan compared with the hyaluronan samples at all molecular weights studied. The results point to a difference in structure in dilute solution for hylan due to the ability to form networks, which can be removed by pressure filtration. In contrast, we do not find an increase of the steady shear viscosity and elastic modulus at higher concentrations when a homogeneous entangled network is reached. We attribute this behavior to the semirigid character of the hyaluronan chain and to the predominance of entanglements over the cross-link points present in hylan in the semidilute domain. Due to the higher apparent molecular weights that are possible with hylan structures but not with the hyaluronans currently available, a wider range of applications can be achieved with hylans when viscoelasticity is required, particularly for the viscosupplementation of synovial fluid damaged by osteoarthritis.

Study on TEMPO-mediated selective oxidation of hyaluronan and the effects of salt on the reaction kinetics

2,2,6,6-Tetramethyl-1-piperidinyloxy radical (TEMPO)-mediated oxidation of hyaluronan was studied at pH 10.2 and temperature of 0 degrees C with NaOCl as the primary oxidant. As with other polysaccharides, a high selectivity of oxidation was observed. The degradation of the polymer was essentially caused by the oxidation process. The primary oxidant and the pH of the reaction mixture did not alter the molecular weight of hyaluronan during oxidation. The kinetics of the oxidation process was investigated at different concentrations of reactants and the inorganic salts, NaBr, NaCl, and Na2SO4. An increase in the salt concentration in the mixture causes a major decrease in the rate of the oxidation, and this decrease is independent of the nature of the salt.

Conformational behavior of hyaluronan in relation to its physical properties as probed by molecular modeling

Hyaluronan (HA) is a linear charged polysaccharide whose structure is made up of repeating disaccharide units. Apparently conflicting reports have been published about the nature of the helical structure of HA in the solid state. Recent developments in the field of molecular modeling of polysaccharides offer new opportunities to reexamine the structural basis underlying the formation and stabilization of ordered structures and their interactions with counterions. The conformational spaces available and the low energy conformations for the disaccharide, trisaccharide, and tetrasaccharide segments of HA were investigated via molecular mechanics calculations using the MM3 force field. First, the results were used to access the configurational statistics of the corresponding polysaccharide. A disordered chain having a persistence length of 75 A at 25 degrees C is predicted. Then, the exploration of the stable ordered forms of HA led to numerous helical conformations, both left- and right-handed, having comparable energies. Several of these conformations correspond to the experimentally observed ones and illustrate the versatility of the polysaccharide. The double stranded helical forms have also been explored and theoretical structures have been compared to experimentally derived ones.

A new bacterial polysaccharide (YAS34). I. Characterization of the conformations and conformational transition

This paper concerns the study of the conformational transition of a new exopolysaccharide (YAS34) using experimental techniques such as optical rotation, conductimetric and microcalorimetric measurements as a function of temperature. The behaviors of this polysaccharide in the acid or sodium salt form are compared; a deacetylated sample is also prepared to demonstrate the role of substituents. For the native structure (never heated), a conformational transition is observed but the deacetylated polysaccharide exhibits no ordered conformation. Multidetection size exclusion chromatography (SEC) analyses and conductimetric experiments allowed to determine the nature of each conformation and the molecular dimensions. From these results, it is suggested that the native conformation is a double helix which by heating over T(m) (temperature corresponding to half conformational transition) dissociates into disordered single chains. In the acid and sodium salt forms, by cooling below T(m), an ordered conformation is restored. This conformation seems to be an intramolecular double helix 'hairpin-like turn' (called renatured conformation). Nevertheless an irreversible denaturation is obtained progressively in the sodium salt form when the time of heating over T(m) increases. The conformation of the deacetylated polysaccharide corresponds to that of a single flexible chain (disordered conformation). The conformational transition for the native conformation was studied also in relation to the polyelectrolytic character of the polysaccharide: stability as a function of salt nature and salt and polymer concentrations was investigated for the polymer initially in the sodium and acid forms.