THE STRUCTURE OF PLANT-CELL WALLS .36. STRUCTURAL-ANALYSIS OF TAMARIND SEED XYLOGLUCAN OLIGOSACCHARIDES USING BETA-GALACTOSIDASE DIGESTION AND SPECTROSCOPIC METHODS

The borohydride-reduced forms (oligoglycosyl alditols) of two isomeric octasaccharides (Glc4Xyl3Gal) that are released from xyloglucans of v arious plant species upon treatment with a fungal endo-(1 --> 4)-beta- glucanase were isolated and structurally characterized. A mixture of o ligosaccharides that is released from tamarind seed xyloglucan by the endo-(1 --> 4)-beta-glucanase was digested with a commercially availab le beta-galactosidase (Aspergillus niger). The beta-galactosidase sele ctively hydrolyzed the galactosyl residue of one of the two isomeric o ctasaccharides present in the mixture. A homogeneous preparation of th e beta-galactosidase-resistant octasaccharide was prepared by high-res olution gel-permeation chromatography of the enzyme-digestion products . Spectroscopic characterization of the oligoglycosyl alditol prepared by reduction of this octasaccharide confirmed the previously proposed structure that had been based on analysis of the mixture of isomeric octasaccharides. The availability of large amounts of the pure, reduce d octasaccharide and of a pure, reduced pentasaccharide (Glc3Xyl2) mad e it possible to completely assign their H-1 and C-13 NMR spectra. In addition, the borohydride-reduced form of the beta-D-galactosidase-sus ceptible octasaccharide isomer was purified by high pH anion-exchange chromatography of the endo-(1 --> 4)-beta-glucanase-released octasacch arides from rape-seed xyloglucan (no beta-galactosidase treatment), an d its H-1 and C-13 NMR spectra were assigned. Additional correlations between specific structural features of xyloglucan oligoglycosyl aldit ols and the positions of specific resonances in their NMR spectra were deduced and added to the extensive list that we have compiled. The ef fects of recording the NMR spectra of the xyloglucan oligoglycosyl ald itols in the presence of borate salts, which could lead to incorrect s tructural assignments, are also described.