NMR analysis of the chemical structure of ulvan and of ulvan-boron complex formation

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)-beta-DGlcpA-(1 --> 4)-alpha-L-Rhap 3-sulphate and --> 4)-alpha-L-IdopA-(1 --> 4)-alpha-L-Rhap 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 B-11 or C-13 NMR spectroscopy at the optimum FH of ulvan gelation (7.5). Boron complexes were only observed at pH greater than or equal to 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. (C) 1998 Elsevier Science Ltd. All rights reserved.