Fine structure of a mixed-oligomer storage xyloglucan from seeds of Hymenaea courbaril

Seed storage xyloglucans of Hymenaea courbaril possess a structure composed of xylocellopentaosyl (XXXXG) and xylocellohexaosyl (XXXXXG) backbone units in addition to the more common xylocellotetraosyl (XXXG) units. Electrospray mass spectrometry confirmed that both types of units exist in the same polymer. The storage xyloglucan gives a xylocellotetraose:xylocellopentaose:xylocellohexaose ratio of about 2:1:0.2. Incomplete digestion of xyloglucan gave dimers of these units in ratios of tetramer–tetramer (T-T), tetramer–pentamer (T-P), and pentamer–pentamer (P-P) backunits of 3.1:2.1:1.0, far from the 4:4:1 ratio expected if the assembly were random. Although discovery of the xylocellopentaosyl units requires a re-evaluation of the cellobiosyl/cellotetraosyl unit mechanism of backbone synthesis, a 4–5–5–4 unit ordered assembly that accounts for the dimer ratios observed preserves a mechanism by which the even-number cellodextrin units are added. Three distinct, singly galactosylated oligomers from the xylocellopentaosyl units were isolated and characterized, with XXXLG being the predominant oligomer of this type in the polymer. Examination of galactosylation profiles of the dimers indicates that the position of these subtending residues is also not random.