Oxidation vs. degradation in polysaccharides: Pullulan – A case study

Several oxidation protocols using TEMPO applied on various polysaccharides to convert the primary OH groups into carboxylic ones are reported, without taking into account the potential side reactions (mainly depolymerization) likely to occur when longer reaction times are employed. In this paper, water-soluble pullulan has been used as a model polysaccharide to study the influence of the reaction time on the macromolecular chain degradation during the TEMPO-oxidation. The oxidation reaction was performed at room temperature, pH = 10.5, using water as solvent, in the presence of sodium hypochlorite, sodium bromide and TEMPO. Ten samples of oxidized pullulan were collected, at different reaction times. The high amount of the sodium hypochlorite, ensures a fast conversion of all the accessible C6-OH to carboxylate groups, even at very first collected sample, according with 13C NMR spectra. All the isolated samples were purified by diafiltration through a Millipore ultrafiltration membrane, recovered by freeze-drying and analyzed by FTIR, 13C NMR, gel permeation chromatography, and zeta potential measurements. The intrinsic viscosity of the native and oxidized pullulan samples was determined in aqueous solution by using a new model proposed by Wolf. The Mark-Houwink relationship was established for fully oxidized pullulan samples: [η]=1.472×10-5Mw0.8475 (dL/g), the exponent showing very good polymer-solvent interactions. The stability of the polysaccharide backbone was discussed by means of viscosity and molecular weight evolution during oxidation reaction. The present data show a strong effect of the reaction time on the molecular weight and viscosity of the oxidized products.