Studies on the structure of wheat-endosperm arabinoxylans

Abstract

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 (NH 4) 2SO 4. 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 Xyl p residues doubly substituted at C-2 and C-3 with Ara f increased from F55 to F100, whereas the amount of mono- and unsubstituted Xyl p residues decreased in the same order. The presence of short Ara f 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-β-xylanase from Trichoderma viride. The hydrolysate fragments were separated, quantified (Bio-Gel P-2, HP-SEC), and characterized ( 1H-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, II 55 (40%), contained high amounts of terminal arabinoses linked to xylose residues at C-3. The third region, III 55 (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 III 100 (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.