L-Altruronic acid formed by epimerization of D-galacturonic acid methyl esters during saponification of citrus pectin.
While searching for oligosaccharides containing rhamnose residues in the endopolygalacturonase (EPG) digest of saponified citrus pectin, we found several oligomers containing, in addition to galacturonic acid, a sugar previously unreported in pectin. The 1- and 2-D 1H NMR spectra of the oligosaccharides were consistent with the sugar being a uronic acid with its 2- and 3-hydroxyls being axial and 4-hydroxyl being equatorial. MALDI-TOF mass spectrometry indicated that the oligomers consisted solely of uronic acids. Reduction of the uronic acids in the oligosaccharides converted them to galactose and altrose. The altrose was found to be the L enantiomer by comparison of its trimethylsilyl (-)-2-butyl glycosides to those of authentic D-altrose and a racemic mixture. The sugar was not found in oligosaccharides prepared from EPG digestion of citrus pectin deesterified with pectin methylesterase rather than saponification. Thus, it appears that during saponification, a small proportion of the methylesterified galacturonic acid residues in pectins is epimerized at C-5 leading to formation of L-altruronic acid residues.
Department of Biochemistry and Molecular Biology, Oklahoma State University, Stillwater 74078-3035, USA.
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D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D002849:Chromatography, Gas; D002957:Citrus; D004952:Esters; D006603:Hexuronic Acids / Q000737:chemistry*; D007536:Isomerism; D008745:Methylation; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009844:Oligosaccharides / Q000737:chemistry*; D010368:Pectins / Q000737:chemistry*; D011096:Polygalacturonase; D012210:Rhamnose / Q000032:analysis / Q000737:chemistry; D014574:Uronic Acids / Q000737:chemistry*
D004952:Esters; D006603:Hexuronic Acids; D009844:Oligosaccharides; D014574:Uronic Acids; C007819:galacturonic acid; C102528:altruronic acid; D010368:Pectins; D011096:Polygalacturonase; D012210:Rhamnose