NMR ANALYSIS OF THE CHEMICAL-STRUCTURE OF ULVAN AND OF ULVAN-BORON COMPLEX-FORMATION

The NMR spectroscopic analysis of autohydrolysate fragments obtained f rom the gel-forming water-soluble cell-wall sulphated polysaccharides from the green seaweed Ulva sp. allowed the identification of two majo r 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 ulvanobiou ronic acid 3-sulphate A and B, respectively. These structures failed t o 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). Boro n complexes were only observed at pH greater than or equal to 9 with u lvanobiouronic acid A. These results indicate that the ring hydroxyl g roups of the major repeating structures which contain the rarely encou ntered iduronic acid in plant polysaccharides are probably not involve d in the boric acid fixation step during the gelation of ulvan. (C) 19 98 Elsevier Science Ltd. All rights reserved.