Degradation of hyaluronic acid during freeze drying

Abstract

Degradation of sodium hyaluronate (HA-Na) and sodium free hyaluronic acid (HA-H) during freeze drying was investigated by experimental methods and theoretical calculations. ESR measurements showed that the evolution of carbohydrate radicals on freeze drying of HA-H is more than three times that of HA-Na, and implied that HA-H is structurally less stable than HA-Na. Molecular dynamics (MD) calculations on model compounds of HA with and without water molecules implied that the flexibility of HA chain without water molecules is responsible for the stability. Further, MNDO-MO calculations suggest that HA-H is much more labile against hydrogen abstraction as compared to HA-Na. Since the degradation of HA-H was depressed by the addition of a hydroxyl radical scavenger, it is likely that the degradation of HA during freeze drying is mainly governed by the secondary reaction of hydroxyl radicals derived from a small amount of carbohydrate radicals which are produced by the initial C-C bond cleavage.