Abstract
The NMR spectroscopic analysis of autohydrolysate fragments obtained from the gel-forming water-soluble cell-wall sulphated polysaccharides from the green seaweed Ulva sp. allowed the identification of two major repeating units in ulvan. These were the aldobiouronic acids → 4)-β-DGl cpA-(1 → 4)-α-L-Rha p 3-sulphate and → 4)-α-L-Ido pA-(l → 4)-α-L-Rha p 3-sulphate that were given the name ulvanobiouronic acid 3-sulphate A and B, respectively. These structures failed to bind boric acid since no ulvan-boron complex was observed by 11B or 13C NMR spectroscopy at the optimum pH of ulvan gelation (7.5). Boron complexes were only observed at pH ≥ 9 with ulvanobiouronic acid A. These results indicate that the ring hydroxyl groups of the major repeating structures which contain the rarely encountered iduronic acid in plant polysaccharides are probably not involved in the boric acid fixation step during the gelation of ulvan.
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