Competitive inhibition evidence for specific intermolecular interactions in hyaluronate solutions

Abstract

Intermolecular associations in hyaluronate systems have been investigated by a competitive inhibition approach, monitored by low deformation oscillatory measurements of dynamic viscosity and rigidity. Solutions of sodium hyaluronate isolated by a mild procedure from rooster combs showed, under physiological conditions of pH and ionic strength, coupling behaviour typical of a transient polymer network. On addition of an equal concentration of hyaluronate segments (~60 disaccharide units), all evidence of coupling is lost and the solution rheology approximates closely to that typical of isolated chains. Such behaviour is clearly incompatible with entanglement coupling, such as occurs between synthetic polymers in solution, but parallels the behaviour of gelling polysaccharides, where co-operative interchain association is known to occur. Similar inhibition is observed in hyaluronate viscoelastic “putties” and “gels”, where further intermolecular association is promoted by suppression of interchain electrostatic repulsion and reduction of water activity. The effects are particularly pronounced for hyaluronate of lower molecular weight, where crosslinking in putties and gels may approach the minimum requirements for a cohesive network. No inhibition is observed with very short chain segments (~4 disaccharide residues) nor with longer segments (~400 disaccharides). On the basis of this evidence we suggest that hyaluronate chains interact by formation of specific co-operative junctions analogous to those characterised for plant structural polysaccharides, but having substantially shorter lifetimes. The magnitude of rheological changes on suppression of these fleeting associations by competitive inhibition suggests that they are likely to dominate the physical properties of hyaluronate in vivo.