Structural studies on the extracellular polysaccharides of the red algae Porphyridium cruentum and Porphyridium aerugineum

Abstract

Investigations of the mucilages exuded by the red seaweeds Porphyridlum cruentum and P. aerugineum revealed many similarities between them. Both contained D-xylose, D-glucose, D- and L-galactose, 3-0-methylxylose, 3- and 4-0-methylgalactose, and D-glucuronic acid. In P. cruentum the approximate molar proportions are 3:1:2.5:0.13:0.13:0.8 and in P. aerugineum 1.7:1.0:1.1:0˙3:0.6:0˙5. In addition P. cruentum mucilage contained a 2-0-methylhexose (0.13) and 2-0-methylglucuronic acid (0.2) whereas P. aerugineum mucilage was devoid of these two sugars, but contained 2,4-di-0-methylgalactose (0.3). This is the first time methylated sugars have been reported as constituents of these mucilages. Both polysaccharides contained about 10 half ester sulphate and appeared to be linked to about 5% protein. Attempted fractionation into homopolysaccharides was unsuccessful. The separation and characterisation of oligosaccharides containing glucose, galactose and xylose; glucose and galactose; and glucose and xylose confirmed that single polydisperse heteropolysaccharides are present. Methylation and periodate oxidation studies revealed that the glucuronic acid is 1,3-linked and is attached solely to C-3 of B-galactose in both mucilages. The 2-0-methylglucuronic acid in P. cruentum is linked to C-4 of L-galactose. Xylose, glucose and galactose are present in both mucilages as end group 1,3- and 1,4 linked residues with the galactose and glucose also present as 1,3,4 linked branch points. Desulphation and infrared studies indicate that the half ester sulphate may be located at C-6 of 1,3-linked hexose units and at C-2 and/or C-3 of 1,4 linked units. If this is so then in P. aerugineum mucilage the units must be in the unfavourable conformation. Molecular weight determinations on Sepharose 4-B of the original mucilages, the methylated materials and the polyalcohols before and after cleavage of the acetal linkages indicated a molecular weight of6 6.4 x 10 for P. cruenturn and 5 x 10 for P. aerugineum. Methylationand periodate oxidation caused considerable degradation of the polymers. The partially hydrolysed polyalcohols have molecular weights (confirmedby high pressure liquid chromatography) of about 30,000.In spite of the considerable similarity of these exudates, that from P. aerugineum has about ten times the viscosity of that from P. cruentum. It is tentatively suggested that this is connected with the different conformation of some of the sulphated residues which would alter the shape of the macromolecule. Also included in this thesis are structural studies on a purified fucoidan (Appendix I), methylation of starch extracted from Chroomonas salina (Appendix II) and a preliminary investigation of the polysaccharides from the red algae Constantinea subulifera and c simplex (Appendix III).