Quantifying structural characteristics of partially de-esterified pectins

Pectins of varying degree and pattern of methylesterification were produced through controlled de-esterification of a parent citrus pectin using Daucus carota pectin methylesterase (PME), Aspergillus aculeatus PME or sodium hydroxide (NaOH) saponification (P-, F- and C-pectins respectively). Pectins from each batch were characterised in terms of degree of methylesterification (DM) and distribution of methylesters. The latter characteristic was estimated based on the degree of blockiness (DB), the absolute degree of blockiness (DBabs) and the proportions of non-methylesterified mono-, di- and tri-galacturonic acid (NM-MDT-GalA) liberated by endo-polygalacturonase (endo-PG). The non-methylesterified polymer, poly-D-galacturonic acid (PGA) was used as control substrate for endo-PG digestion. Following data analysis, the DM dependence of DBabs was described by polynomial functions, irrespective of pectin series. However, the degree of these polynomials decreased as the distribution of non-methylesterified galacturonic acid became blockier in various pectin series. Besides, the cubic function relating DM to DBabs of randomly de-esterified C-pectins was an approximation of an exponential function. Upon endo-polygalacturonase digestion, all partially de-esterified pectins revealed similar proportions of NM-MDT-GalA at DBabs values greater or equal to some specific “critical” values. Interestingly, comparable proportions of NM-MDT-GalA were obtained after endo-PG digestion of PGA. This suggested that below a certain DM (which varied with the pattern of methylesterification), all partially de-esterified pectins were digested in the same manner as PGA.