STUDIES ON THE STRUCTURE OF WHEAT-ENDOSPERM ARABINOXYLANS

Water-soluble arabinoxylans from a Canadian western red spring flour (cv. Laura) were fractionated by a graded ammonium-sulphate-precipitation technique into five fractions obtained at 55% (F55), 60% (F60), 70% (F70), 80% (F80), and 100% (F100) saturation with (NH4)(2)SO4. The yields of the fractions (based on the total material recovered) were 37.5%, 22.3%, 23.6%, 11.9%, and 4.7%, respectively. The relative amount of Xylp residues doubly substituted at C-2 and C-3 with Araf increased from F55 to F100, whereas the amount of mono- and unsubstituted Xylp residues decreased in the same order. The presence of short Araf chains was more pronounced in F100 than in other samples. The content of ferulic acid was highest in F55. Fractions F55 and F100 were degraded with purified endo-beta-xylanase from Trichoderma viride. The hydrolysate fragments were separated, quantified (Bio-Gel P-2, HP-SEC), and characterized (H-1-NMR and methylation analysis). F55 appeared to be built up from three structurally different regions. The first region, I-55 (15%), was of relatively high DP and was high in terminal arabinose doubly linked to xylose residues at C-3 and C-2. The second region, II55 (40%), contained high amounts of terminal arabinoses linked to xylose residues at C-3. The third region, III55 (45%), contained high amounts of contiguously unsubstituted and C-3 monosubstituted xylose residues. Fraction F100 was built up mainly from the highly substituted region I-100 (75%), enriched in C-2,3 di- and C-2 monosubstituted xylose residues as well as in short arabinose side chains. Region III100 (18%) contained a high proportion of unsubstituted xylose residues; however, the amount of disubstituted xylose residues in this region was still higher than that of monosubstituted residues.