Variation in the degree of D-xylose substitution in arabinoxylans extracted from a European wheat flour.

Total water-extractable arabinoxylan from flour of the European bread making wheat cultivar Camp Remy was fractionated by ethanol precipitation. Both 1H-NMR spectroscopy and methylation analysis of the isolated arabinoxylan fractions showed wide variation in the degree of xylose substitution. More highly substituted arabinoxylan fractions were precipitated at higher ethanol concentrations. At ethanol concentrations of 10-30% (v/v) (1→3),(1→4)-β-d-glucan co-precipitated with arabinoxylan. Re-precipitation of the polymer mixture resulted in a partial separation of the arabinoxylan from the mixture. Arabinoxylans with wide structural variation were also isolated by extraction of the flour with aqueous ethanol solutions of decreasing concentations. By this isolation technique, fractions with the highest degree of substitution were extracted with the most concentrated (50%) ethanolic solutions. The range of structural variation in the fractions was quite similar for both isolation methods. A decrease in the proportion of unsubstituted xylose residues occurred concomitantly with a decrease in the proportion of monosubstituted xylose units and with an increase in the proportion of disubstituted xylose units. An increase in the proportion of paired disubstituted xylose units as the arabinose to xylose ratio of the arabinoxylan fractions increased, and the presence of monosubstituted xylose residues next to disubstituted xylose residues in the highly substituted fractions, were illustrated by 1H-NMR spectroscopy. Methylation analysis indicated an increase in traces of side chains of arabinose residues and in the levels of O-2 substituted xylose residues as the arabinose to xylose ratio increased. © 1995 Academic Press Limited

The relative stability of zeolite frameworks

The question is examd. as to whether there is a theor. basis to an apparent relation that has been exptl. obsd. between the lattice topol. and the Al/Si ratio of zeolites. Calcns. are presented for 3 lattices formed with different Al/Si ratios, faujasite, mordenite, and ZSM-5. The relative stabilities of these lattices were studied as a function of Al/Si ratio in the presence or absence of adsorbed water. The Na+ are taken as the compensating ions. Two contrasting techniques, lattice energy minimization and extended Hueckel theory, were used. The energy differences of Al-free lattices are very small (.apprx.10 kH) and tend to decrease function of Al content. Wide pore zeolites tend to become stabilized with respect to medium pore zeolites at high Al conten