Isolation of water-soluble alginate from brown algae

Summary Water-soluble alginate was obtained from an aqueous extract of Kjellmaniella crassifolia by precipitation with HCl, calcium acetate or 20% ethanol in the presence of 0.05 M MgCl₂ Of these precipitation procedures, MgCl₂-ethanol gave the purest alginate preparation as judged by electrophoresis. The thin-layer and gas-liquid chromatography of its acid hydrolysate, and the IR spectra analysis of the whole alginate, suggested that the water-soluble alginate is similar to ordinary water-insoluble and alkali-soluble alginate such as Kelco alginate.However, the alginate obtained in the present work contained a great excess of mannuronic acid residues, giving an M:G ratio of about 13. Its molecular weight distribution was rather broad as with Kelco alginate, but the molecular weight of its major component was estimated to be 500 000 amu, whereas that Kelco alginate measured on the same column under the same condition was 1 700 000 amu. This suggests that water-soluble alginate was far smaller in average molecular size than Kelco alginate.