AN EFFICIENT PROCEDURE FOR STUDYING PECTIN STRUCTURE WHICH COMBINES LIMITED DEPOLYMERIZATION AND C-13 NMR

A protocol for partial thermally-induced depolymerization of different ly methoxylated pectin samples is described. The resulting macromolecu les have been fully characterized with various complementary technique s, such as size exclusion chromatography (SEC), potentiometry, viscome try and C-13 NMR. Optimum conditions afford samples at 50-80% yield wi th weight-average molecular weights in the 4 to 20 kDa range. The majo r fraction of these polysaccharides adopts the random-coil conformatio n and such samples are suitable for C-13 NMR structural studies at roo m temperature. The methoxyl distributions of two apple pectin samples with a degree of esterification (DE) between 54 and 74% and a citrus p ectin (DE, 72%) were shown to be random in nature, whereas that of a l ightly methoxylated apple pectin (DE 39%) was partially blockwise. The carbon relaxation parameters of the depolymerized pectins attain asym ptotic values for M-w > 4 kDa. The M-w values estimated from intrinsic viscosity data with the Mark-Houwink relationship reported for native pectins are in good agreement with those obtained by either end-group analysis (NMR) or SEC. Thus, all the physicochemical data indicate th at the secondary structure of the isolated chains of depolymerized pec tin is closely related to that of the parent polymers. Finally, pectin methylesterase activity towards the depolymerized pectins was similar to that of the untreated samples.