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.
Crescenzi
V
V
Department of Chemistry, University La Sapienza, P.le Aldo Moro 5, 00185 Rome, Italy.
Hartmann
M
M
de Nooy
A E
AE
Rori
V
V
Masci
G
G
Skjåk-Braek
G
G
D000464:Alginates / Q000138:chemical synthesis*; D002238:Carbohydrate Epimerases / Q000737:chemistry*; D003497:Cyclic N-Oxides; D009682:Magnetic Resonance Spectroscopy; D008351:Mannans / Q000737:chemistry; D010084:Oxidation-Reduction; D010944:Plants / Q000737:chemistry; D011134:Polysaccharides / Q000138:chemical synthesis*; D011994:Recombinant Proteins / Q000737:chemistry; D014574:Uronic Acids / Q000138:chemical synthesis*; D014783:Viscosity
D000464:Alginates; D003497:Cyclic N-Oxides; D008351:Mannans; D011134:Polysaccharides; D011994:Recombinant Proteins; D014574:Uronic Acids; D002238:Carbohydrate Epimerases; C086519:mannuronan c-5-epimerase; C003959:TEMPO