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 1H and 13C 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 FM 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.The NMR analysis of homopolymeric blocks from several algal alginates are shown to have first-order Markov distributions of d-mannuronosyl and l-guluronosyl residues. This is consistent with a biosynthetic pathway that involves a C-5 epimerisation at the monomer level.