Compositional heterogeneity in pectic polysaccharides: NMR studies and statistical analysis

Pectins are a class of heterogeneous polysaccharides used in the food indus try as a result of their ability to form gels. They are primarily composed of a (1 --> 4)-alpha-D-galactopyranosyluronic acid backbone where the carbo xylic acid group is methyl esterified at a level which depends on the sourc e of the pectin and processing conditions used to isolate the material. Of considerable interest is the distribution of the free acid and methyl ester groups along this polymer chain. NMR spectroscopy, coupled with statistica l analysis, is a powerful technique for the study of sequence distribution of monomers. Experimental conditions (temperature, pD) are reported which a re appropriate for the analysis of pectic polysaccharides. Results are pres ented from a study of sequence distribution in native, modified, and fracti onated pectins using H-1 and C-13 solution NMR methods. The triad sequence information was examined using Bernoullian and Markovian probability models in conjunction with continuous Gaussian distribution and discrete models. Intermolecular heterogeneity in pectins results in triad frequencies which reflect the distribution of acid and ester monomers for native and base sap onified pectins. Fractionation of pectin through physicochemical methods an d subsequent spectroscopic analysis provide insight into this heterogeneity . Segregation into discrete polymer populations shows a structural diversit y best approximated by a 3-component statistical model.