CHARACTERIZATION OF 4-DEOXY-BETA-L-THREO-HEX-4-ENOPYRANOSYLURONIC ACID ATTACHED TO XYLAN IN PINE KRAFT PULP AND PULPING LIQUOR BY H-1 AND C-13 NMR-SPECTROSCOPY

A new acidic sidegroup in xylans, from both kraft pulp and pulping liq uor, was identified by NMR spectroscopy. Unmodified oligosaccharides f rom kraft pulp xylan were obtained by enzymatic hydrolysis with xylana se (Trichoderma reesei). The acidic oligosaccharides were separated fr om the neutral forms on an anion exchange resin. The new acidic sidegr oup was identified as 4-deoxy-beta-L-threo-hex-4-enopyranosyluronic ac id (hexenuronic acid) by H-1 and C-13 NMR spectroscopy. Hexenuronic ac id is a beta-elimination product of 4-O-methylglucuronic acid and is f ormed during kraft pulping. HMBC and NOESY experiments showed that hex enuronic acid is attached beta-(1 --> 2) to xylose. The NOESY data fur ther indicated that hexenuronic acid protrudes from the main xylan cha in. The pK(a) values for hexenuronic acid (3.03) and 4-O-methylglucuro nic acid (3.14) attached (1 --> 2) to xylose were determined from pH-d ependent chemical shifts.