Structure of hyaluronan during acid-catalyzed hydrolysis, a kinetic study of both chain scission and disaggregation

Abstract

Aqueous hyaluronan (HA) solutions were hydrolyzed at pH 1.1, at various temperatures, and followed by (1) determining the increase in concentration of reducing sugar ends, and by (2) measuring the reduction in light-scattering intensity. The first method measures the chain scissions per unit time. The second method measures the change in particle size in time. From the determined chain scission rate, the evolution of the weight-averaged molecular mass of chains can be calculated. Under the assumption that the formed chain fragments immediately dissociate from the parent particle this would be equal to the evolution of the particle mass. However, the observed decay of light-scattering intensity relates to a much slower decrease of weight-averaged particle mass than predicted. Therefore, chain scission does not necessarily lead to immediate particle dissociation. Temperature-dependent measurements show that both chain scission and dissociation have similar activation energies of 97 ± 3 and 101 ± 10 kJ mol⁻¹ respectively, showing that these two processes are coupled, and that scission is the rate-limiting process in the dissociation. The kinetic data have been interpreted using a model in which cleaved oligomers only dissociate from the parent particle if they are smaller than a critical number of disaccharide units. The critical number amounts to 26 to 33 units. From our studies accurate HA hydrolysis conditions can be extracted to obtain a desired molecular-weight distribution. Additional circular dichroism measurements show that at neutral pH, 80% of the bond angles in the HA random coil are consistent with a flat strand structure, and 20% are consistent with a double helix structure; under our applied hydrolysis conditions, HA completely possesses the flat strand structure.