Biosynthetic implications of NMR analyses of alginate homo- and heteropolymers from New Zealand brown seaweeds

Homopolymeric (M- and G-) and heteropolymeric (MG-) blocks have been prepared from alginates that have been isolated from seven Southern Hemisphere brown seaweeds and from three commercial algal alginates. The blocks have been analysed by H-1 and C-13 NMR spectroscopy, and the analyses show that well-defined M- and G-homopolymers are obtained only from the five polysaccharides that have been previously designated as either high-M or high-G alginates. However, the five intermediate algal alginates (which have F-M values between 0.6 and 0.7) appear to contain mainly MG or heteropolymeric blocks. It is concluded that the results of NMR spectroscopic analysis of whole alginates can give a misleading picture of the block structure of some alginates. The NMR analyses of the homopolymers isolated from both the high-M and high-G alginates also show that the order of the D-mannuronosyl and L-guluronosyl residues in these blocks fits a first-order Markov distribution pattern. This suggests that 5-epimerization of some of the GDP-D-mannuronosyl residues at the monomer level, followed by addition copolymerization catalyzed by a GDP-guluronic acid transferase system, as originally suggested by Lin and Hassid [1,2], may contribute to the biosynthesis of the homopolymeric block structures in these high-M and high-G alginates isolated from brown algae. (C) 1997 Elsevier Science Ltd.