A xylogalacturonan subunit present in the modified hairy regions of apple pectin

The high molecular weight fraction (fraction A) of the modified hairy regions (MHR) from apple cell walls was treated by rhamnogalacturonase (RGase) after saponification and after deacetylation by rhamnogalacturonan acetylesterase (RGAEase). Three fractions could be recognized by size-exclusion chromatography: rhamnogalacturonan oligomers, residual stubs of the rhamnogalacturonan backbone rich in arabinan side-chains, and a fraction rich in xylose and galacturonic acid. The “xylogalacturonans” obtained after saponification and after enzymic deacetylation had rather similar sugar compositions (xylose:galacturonic acid ratios of 0.4–0.9) and molecular weights. After saponification, the xylogalacturonan was eluted as a single peak on anion-exchange chromatography whereas three peaks were obtained when the MHR was deacetylated by RGAEase, indicating variations in the degrees of methylation. One of the methyl-esterified xylogalacturonan fractions was characterized by NMR spectroscopy: the xylose residues were β-(1 → 3)-linked to some of the galacturonic acid residues within a rather high molecular weight xylo-(1 → 4)-α-galacturonan and a degree of methylation of 39 was calculated. The methyl esters were found to be equally divided between the substituted and unsubstituted galacturonosyl residues.