The preparation and susceptibility to hydrolysis of novel O-galacturonoyl derivatives of carbohydrates

Abstract

D-Galacturonic acid or (1-->4)-alpha-D-galacturonan reacted in aqueous pyridine in the presence of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide with alcohols to yield esters. The alcohols that gave high yields of D-galacturonoyl derivatives were primary and included methanol, ethanol, 1-propanol, D-glucose, D-galactose, methyl beta-D-glucopyranoside, methyl beta-D-galactopyranoside, and cellulose. D-Galacturonic acid itself readily gave an O-D-galacturonoyl-D-glacturonic acid. The proposed structure of one compound, methyl 6-O-D-galacturonoyl-beta-D-glucopyranoside, was supported by 1H and 13C NMR data and the FAB mass-spectral data. Each ester was hydrolysed at pH 11 and 25 degrees C within 1 h. O-D-Galacturonoyl-D-glucose was considerably more alkali labile than O-polygalacturonoyl-D-glucose, and O-D-galacturonoylcellulose had an intermediate stability. The esters were relatively stable to cold acid, but could be hydrolysed by M trifluoroacetic acid at 100 degrees C for 1 h. The esters tested were resistant to digestion by 'Driselase', although the glycosidic bonds of O-polygalacturonoyl-D-glucose were hydrolysed to yield O-oligogalacturonoyl-D-glucoses of low molecular weight. The possible application of these analytical methods to the detection of O-uronoyl-type cross-links in cell-wall polysaccharides is discussed.