Carbohydrates of the brown seaweed Lessonia nigrescens

Abstract

The thesis describes the first extensive study of the carbohydrates of the brown seaweed Lessonia nigrescens, harvested in Chile. It was hoped to discover a different metabolic pattern for the stems and for the fronds of this seaweed. In the initial studies sequential extraction with water, acid and alkali and a direct alkaline extraction were applied separately to stems and fronds. Each of the extracts was hydrolysed and the hydrolysates were analysed by paper and by gas liquid chromatography allied to mass spectrometry. Crystalline mannitol was separated from both parts of the s eaweed. "Fucans" were present in all the extracts together with, in the alkaline extracts, alginic acid. Fractionation of the aqueous extract failed to reveal any laminaran, a typical (1,3)-B-linked glucan of brown seaweeds. Each of the "fucan" contained varying proportions of fucose (major), galactose, mannose, xylose, glucuronic acid and half ester sulphate. Varying proportions of protein were present in all the extracts. Molecular weights determinations revealedpolydispersity with high proportion of the molecules equal to orlarger than M of 6.7 x 10.The only significant difference between stems and frondswas the higher proportion of glucuronic acid in the former.Investigation of the alginic acids revealed that theyconsisted of mannuronic to guluronic acid in the ratio of 0.54. Viscosity and C n.m.r. measurements were carried out. Large scale partial acid hydrolysis of water soluble "fucan" from a mixture of stems and fronds led to the separation of neutral and acidic oligosaccharides, an insoluble and a soluble polymer. The neutral oligosaccharides which contained only galactose were separated into six entities and characterised by methylation and the anomeric linkage determined for two of them by enzymic and n.m.r. studies. The eight acidic oligosaccharides proved to be very complex mixtures. However, two were characterised as 3-0-(D-glucopyranosyluronic acid)-L-fucose and 3-0-(D-glucopyranosyluronic acid)-D-glucuronic acid. The insoluble residue, 10% of the starting material was mainly inorganic and protein. The soluble polymeric material, 20% of the initial material (contained 80% carbohydrate) was shown by hydrolysis, 13 molecular weight determination and C n.m.r. to consist of approximately 44 alternating units of glucuronic acid and mannose.