Characterization of the extracellular polysaccharide of Porphyridium sp.: molecular weight determination and rheological properties

The molar mass of the extracellular polysaccharide produced by the red microalga Porphyridium sp. was determined by gel permeation chromatography (GPC) followed by multiangle laser light scattering (MALLS). Two different disaggregation treatments were used prior to the measurement: (1) sonication and (2) incubation in an aqueous solution of tris(ethylenediamine)cadmium dihydroxide (cadoxen) followed by GPC analysis using a 50% cadoxen solution in the chromatographic mobile phase. The average molar mass was found to be 2.3×106 g/mol with the size distribution lying between 2×105 and 4×106 g/mol. The viscoelastic properties of aqueous solutions of the polysaccharide before and after sonication were evaluated by rheological methods. It was found that the viscosity of the solutions decreased dramatically with increasing sonication time. Changes in the mechanical spectra of aqueous polysaccharide solutions as a result of sonication over a range of frequencies were reflected in the inversion of the ratio of the storage modulus G′ to the loss modulus G″, which indicated a shift from a ‘weak gel’ to a liquid-like state.