Effect of microwave irradiation on the molecular and structural properties of hyaluronan

Hyaluronan (Na+ salt of hyaluronic acid, HA) was extensively depolymerised by HCl-catalyzed hydrolysis at pH 3 for up to 500 min under temperature-controlled microwave irradiation. The effects of microwave heating on the hydrodynamic properties of the polysaccharide were determined by SEC-MALLS and viscometry. The weight-average molecular mass (Mw) of HA decreased from 1.44 × 106 to ∼5000, reaching the region of higher oligosaccharides. The scission of HA chains was found to proceed randomly during the whole degradation process. Treatment of the Mw and intrinsic viscosity data according to the Mark–Houwink equation, [η] = k × Mwα suggested three relationships with α1 = 0.46 for Mw > 500,000, α2 = 0.84 for Mw between 500,000 and 50,000, and α3 = 1.13 for Mw < 50,000. The results revealed that HA with Mw > 10,000 adopts a stiffish coil conformation in solution. As monitored by FT-IR and NMR spectroscopic techniques, the primary structure of the HA chains was maintained during the microwave-assisted hydrolysis at pH 3 at 105 °C. At reaction times larger than 240 min, uv spectroscopy suggested the depolymerisation of HA was accompanied by formation of by-products produced by side reaction.