Epimerization of nonnatural uronans with mannuronan C-5-epimerases to obtain alginatelike polysaccharides

Different polysaccharides containing D-mannose residues have been C-6-oxidized by a selective TEMPO-mediated hypohalite oxidation to obtain the corresponding uronans. These have been treated with various recombinant mannuronan C-5-epimerases and the resulting products were analyzed by 1H NMR spectroscopy. Oxidized konjac mannan could be epimerized to obtain a uronan with a content of about 12% alpha-L-gulopyranuronate (G) residues. On prolonged epimerization, beta-elimination was observed. The oxidized galactomannan locust bean gum could only be scarcely epimerized, probably due to steric effects exerted by its 26% alpha-D-galacturopyranosyl side groups. Oxidized, galactose-depleted guar gum with a alpha-D-galactosyl content of 11% could be epimerized to a G content of about 15%. With oxidized cellulose as a substrate, mainly beta-elimination was observed. It thus seems that the mannuronan C-5-epimerases employed recognize glucuronate residues and abstract proton-5 but are unable to perform the second epimerization step and instead yield beta-eliminated products.