Optical properties and viscosity of hyaluronic acid in mixed solvents: Evidence of conformational transition

Protective effects of manganese(II) chloride on hyaluronan degradation by oxidative system ascorbate plus cupric chloride

The degradation of several high-molar-mass hyaluronan samples was investigated in the presence of ascorbic acid itself and further by an oxidative system composed of ascorbic acid plus transition metal ions, i.e. Fe(II) or Cu(II) ions. The latter oxidative system imitates conditions in a joint synovial fluid during early phase of acute joint inflammation and can be used as a model for monitoring oxidative degradation of hyaluronan under pathophysiological conditions. The system Cu(II) plus ascorbate (the Weissberger oxidative system) resulted in a more significant decrease of hyaluronan molar mass compared to the oxidative system Fe(II) plus ascorbate. Addition of manganese(II) chloride was found to decrease the rate of the oxidative damage of hyaluronan initiated by ascorbate itself and by the Weissberger system.

Experimental approaches to hyaluronan structure

A review of the literature describing experimental studies on hyaluronan (HA) is presented. Methods sensitive to the hydrodynamic properties of HA, analyzed in neutral aqueous solution containing NaCl at physiological concentration, can be shown to fit the expected behavior of a high molecular weight linear semi-flexible polymer. The significant nonideality of HA solutions can be predicted by a simple treatment for hydrodynamic interactions between polymer chains. Nuclear magnetic resonance and circular dichroism studies of HA are also in agreement with a model incorporating dynamically formed and broken hydrogen bonds, contributing to the semi-flexibility of the polymer chain, and segmental motions on the nanosecond time scale. HA shows the capability for self-association in the formation of a viscoelastic putty state at pH 2.5 in the presence of salt, and a gel state at pH 2.5 in mixed organic/aqueous solution containing salt. Ordered and associated structures have also been observed for HA on the surfaces, especially in the presence of surface-structured water. These phenomena can be understood in terms of counterion-mediated polyelectrolyte interactions. The possibility that hyaluronan exists in vivo in environments that induce ordered structures and assemblies is discussed.

Metal ion coordination of macromolecular bioligands: formation of zinc(II) complex of hyaluronic acid

The coordination of zinc(II) ion to hyaluronate (Hya), a natural copolymer, in aqueous solution at pH 6 was investigated by potentiometric and circular dichroism (CD) spectroscopic methods, and by monitoring the changes in macroscopic properties by high-precision measurements. The zinc(II)-selective electrode, and CD measurements proved the binding of zinc(II) by Hya. A number of Hya fragments (Mr approximately 3.3 x 10(3)-1.4 x 10(6)) were studied to estimate the contributions of the polyelectrolyte effect, the solvation and host-guest interactions to the extra stabilization of the macromolecular zinc(II) complexes as compared with the monomeric unit. The zinc(II) ion activity increase reflected a stability decrease for the fragments with Mr < 4 x 10(4). This molecular weight differs from that where cleavage of the Hya skeleton starts (approximately 5 x 10(5), according to the size-exclusion gel, and anion-exchange chromatographic behavior of the Hya fragments) and from that where the polyelectrolyte effect stops (approximately 6 x 10(3)). The excess volumes and Bingham shear yield values of the solutions revealed the transformation of the coherent random coil structure stabilized by intermolecular association in the NaHya to an intramolecular association producing the globular structure of the ZnHya molecule, with a smaller but more strongly bound solvate water sheet.

Mucopolysaccharides in aqueous solutions: effect of ionic strength on titration curves

We study the changes taking place in hyaluronic acid, chondroitin 4-sulfate (C4-S) and condroitin 6-sulfate (C6-S), at ionic strengths of 0.10, 0.15, and 0.20 in NaCl, in a neutralization process in aqueous solution. We apply the equation of Henderson Hasselbalch modified for polyelectrolytes and evaluate the changes in the electrostatic free energy starting from the pK curves as a function of the dissociation degree. For a dissociation degree next to 0.4 corresponding to the -COOH group of the hyaluronic acid, we observed a change in the conformation of the three glycosaminoglycans studied. This conformational change takes place as a consequence of the break of intramolecular links and the beginning of the ionization process. The macromolecules in solution show a structure of random coil sufficiently expanded so that the interaction among the close ionizable groups is negligible.

Preparation and characterisation of copper(II) hyaluronate

Amorphous copper complexes of the general composition Cu(C14H20O11N)2 x xH2O have been prepared with high- and low-molecular-weight hyaluronic acid (HA). Optimal conditions for preparation are obtained at pH values from 5.0 to 5.5, with a molar ratio of HA versus Cu2+ of 1:1, and at a mass concentration of 5 and 10 mg/mL for high- (Mw = 1.8 x 10(6) Da) and low-molecular-weight sodium hyaluronate (Mw = 2 x 10(5) Da), respectively. The coordination polyhedron of the copper ion has been elucidated by EXAFS and XANES spectroscopy. Copper atoms are octahedrally coordinated in both cases with four equatorial Cu-O bond lengths of 1.95 A, and two axial Cu-O bonds of 2.46 A. Visible spectra of acidic aqueous solution suggest that substitution of axial oxygens by NH groups occurs at pH 6.5 or higher. If the pH value of the copper(II) hyaluronate solution increases above 6.5, the coordination of copper(II) changes. It is very likely that the N atom coming from the acetamido group enters into the coordination sphere of the copper(II) ion.

Insight into the distribution of molecular weights and higher-order structure of hyaluronans and some beta-(1 --> 3)-glucans by size exclusion chromatography

The effects of high energy ultrasound and slightly raised temperature combined with the denaturing action of dimethylsulphoxide on the molecular weight and higher-order structure of hyaluronans and some beta-(1 --> 3)-glucans were studied by means of size exclusion chromatography (SEC) technique. Some experimental conditions connected with the (bio-)polymer sample preparation prior to its SEC analysis are overviewed in the light of informational relevance of studies where the action of a physical and/or chemical agent changes the hydrodynamic size of the m omolecule.

Molecular mechanisms of posterior vitreous detachment

To determine a molecular mechanism for posterior vitreous detachment and contraction of vitreous gel, the effects of serum components on bovine vitreous were systematically studied. Incubation of vitreous gel with calf serum caused a contraction of the gel accompanied by the release of a waterlike liquid. The low molecular weight fraction (M(r) < 10,000) of serum was ineffective in this process, whereas incubation of the vitreous with the high molecular weight fraction resulted in changes in the gel structure similar to those caused by serum alone. Individually, transglutaminase (factor XIIIa) was found to be most effective in inducing gel contraction. The effect was more pronounced when transglutaminase was used with fibronectin. Since transglutaminase is known to promote collagen-collagen or collagen-fibronectin-collagen cross-links, the contraction of the vitreous in this case can be attributed to the formation of similar cross-links.

Glycosaminoglycan conformations and changes on periodate oxidation

The progressive periodate oxidation of glycosaminoglycans (GAG), including hyaluronate (HA), chondroitins (CH) (chondroitin, chondroitin 4- and 6-sulfate), dermatan sulfate (DS), and keratan sulfate (KS), were monitored by CD and high performance liquid chromatography (HPLC) using a size-exclusion column. The rate of oxidation also was measured and calculated using first- and second-order kinetics, and the data appear to fit better with first-order kinetics. In both HA and CH, the n - pi amide band at 208 nm decreases in intensity upon oxidation, but in HA it becomes positive after 16 h of periodate treatment. In CH, the band disappears, and the pi - pi amide band below 200 nm becomes optically active. Concomitantly, a second negative band near 290 nm appears for these two oxidized GAG. Oxidation causes a slight change in the CD of DS. It ordinarily displays a very weak n - pi band at 210 nm, but instead shows an intense pi - pi amide band near 190 nm. CD of KS remains unaffected by periodate. Kinetic studies, however, show a higher oxidation rate for DS than HA and CH. With the exception of KS, all other oxidized polymers shown an apparent decrease in molecular weight (higher peak retention time) in HPLC analysis. Both CD and HPLC results have been attributed to a major conformational change of HA and CH, and a minor one for DS. The ease and extent of periodate oxidation as well as the changes in molecular properties following periodate treatment are critically dependent on the configuration of the individual GAG rather than the oxidation rate. There is a distinct difference in the conformational change between HA and CH, as manifested by their dichroic behavior, that was attributed to the equatorial disposition of C-4 hydroxyl group in HA and axial disposition CH.

Self-association of hyaluronate segments in aqueous NaCl solution

The potential for self-association by hyaluronate (HA) chains in 0.15 M NaCl was investigated, using low molecular weight HA segments as a model system. HA segments were derived from the polymer by controlled enzymatic digestion, and purified by gel filtration chromatography. Seven samples of narrow molecular weight distribution were analyzed by sensitivity-enhanced polyacrylamide gel electrophoresis, and found to have the following weight-average numbers of repeating disaccharide units: A, 90; B, 51; C, 38; D, 31; E, 23; F, 18; G, 13. The segment preparations were studied in 0.15 M NaCl by capillary viscometry, low angle laser light scattering, and circular dichroism spectroscopy. The data indicate concentration-dependent intermolecular association of short segments, and a capability for intramolecular association (hairpin formation) by larger HA segments.

Role of singlet oxygen in the degradation of hyaluronic acid

To investigate the effect of singlet oxygen on the molecular properties of hyaluronic acid, the polymer was irradiated in the presence of a dye sensitizer for singlet oxygen. Viscosity and circular dichroism techniques were used to monitor these changes. The relative viscosity of the polymer solution decreased steadily with increasing duration of irradiation, indicating an apparent decrease in molecular weight of hyaluronic acid. Circular dichroism measurements of the irradiated sample, however, did not show any appreciable change in the secondary structure, but do suggest that the generated singlet oxygen changes the tertiary structure and that this change is followed by a minor depolymerization.

Glycosaminoglycans: structure and interaction

In the last few years, there has been considerable progress in the studies on glycosaminoglycans, a group of acidic polysaccharides present in the intercellular matrix of connective tissue. X-ray diffraction studies have indicated that these polymers can exist in the condensed phase in some helical form. Chiroptical and hydrodynamic measurements have provided significant information regarding the molecular conformation in solution and other physicochemical properties of the polymers. Studies related to the interaction properties of glycosaminoglycans with polypeptides, metal ions, and other molecules are numerous. This review covers mainly the results and their interpretations of both published and as yet unpublished material of the 1970s, but certain previous data are also included. A present-day concept regarding the structure and interaction properties of these molecules on the basis of various physicochemical measurements is presented. The biosynthesis and metabolism of glycosaminoglycans, and the structure of proteoglycans and glycoproteins, are not discussed.

Optical characteristics of carboxyl group in relation to the circular dichroic properties and dissociation constants of glycosaminoglycans

Circular dichroism studies of glycosaminoglycans including chemically transformed heparins at various pH values reveal that carboxyl chromophore plays an important role in the dichroic behavior of the polymers. With decreasing pH, iduronic acid-containing glycosaminoglycans show increased negative ellipticity near 220 nm whereas the polymers containing glucuronic acid display enhanced negative dichroism near 230 nm and decreased negative dichroism around 210 nm. The pH-dependent optical properties have been utilized to determine the pKa values of uronic acid moieties. The acid strengths of the iduronic acid-containing glycosaminoglycans are inherently smaller than those of corresponding glucuronic acid-containing polymers. Glycosaminoglycans in which the amino sugars are linked with iduronic acid display a very weak n leads to pi* amide transition, or none. The rotational strength at 210 nm of these polymers is largely due to iduronic acid moieties. The CD variations above 200 nm with change in pH do not indicate any major conformational transition of the molecules but the difference between dermatan sulfate and heparin can be attributed to difference either in iduronic acid conformation or in intersaccharide linkages.