A STUDY ON THE CONSTITUTION OF ALGINIC ACID BY PARTIAL ACID HYDROLYSIS

Alginic acid can be separated into two fractions by hydrolysis with 1 M oxalic acid. One fraction (about 30% of alginic acid from L. digitata) is brought into solution in the form of oligouronides which is further hydrolysed to uronic acids. The second fraction remains insoluble even after prolonged hydrolysis. The degree of polymerization (DP) of this fraction decreases rapidly until a DP of about 25, after which only a slow decrease takes place. This insoluble material can be fractionated by precipitation with acid into two fractions, one containing mainly mannuronic acid residues and the other containing guluronic acid residues. No intermediate fraction, containing both uronic residues in approximately equal amounts, was present. The different hydrolability of the two fractions of the alginic acid molecule is most probably due to a different degree of crystallinity. Alginic acid, thus, seems to consist of protected, semi-crystalline parts with an average DP of 25, linked together with hydrolysable, non-crystalline parts. The protected parts are chain fragments consisting almost exclusively of one type of uronic acid residues. The nature of the hydrolysable parts of the alginic acid molecule has been investigated by studying the diuronides found in the acid hydrolysate of alginic acid.