Pectin from leaves of birch (Betula pendula Roth.): Results of NMR experiments and hypothesis of the RG-I structure

We report that in birch leaf pectin, rhamnogalacturonan-I (RG-I) and galacturonan (HG) were found as separate polymers rather than domains of a complex macromolecule. RG-I and HG were separated by anion-exchange and size-exclusion chromatography and studied by using NMR spectroscopy. NMR spectra showed that methyl-esterified D-galactosyluronic acid residues were located only in HG. Oligosaccharides of similar structure to the backbone, but without terminal reducing residues in the NMR spectra, were found in RG-I. We hypothesize, these oligosaccharides and RG-I backbone can be covalently bound due to its co-eluted of from DEAE-cellulose and Sepharose CL-4B. This result differs from the classical RG-I model, which assumes that all Rhap and GalpA residues are located only in the RG-I backbone. In the heteronuclear multiple bond correlation (HMBC) and rotating frame Overhauser effect spectroscopy (ROESY) spectra, the correlation peaks confirming the substitution of 2,4-rhamnose residues at O-4 by only single D-galactose residues were identified.