THE MODE OF BINDING OF β‐GLUCANS AND PENTOSANS IN BARLEY ENDOSPERM CELL WALLS

Abstract

β‐Glucans in barley endosperm cell walls exist as polymers of very high molecular weight (about 4 × 107 daltons) containing firmly linked peptide sequences. This peptidic material is an essential part of the structure of the β‐glucan complex as it exists in the cell wall. Rupture of peptide bonds by hydrazinolysis or with the proteolytic enzyme thermolysin gives β‐glucans similar in size to those from short‐grown green malts (about 106 daltons). This suggests that proteolysis is the first step in β‐glucan degradation. Large β‐glucans are not all precipitated in 30% (w/v) ammonium sulphate; only 34% of the β‐glucan in a hot aqueous extract of cell walls is precipitated. The amount is increased to 63% if the cell walls have been previously dehydrated. Prolonged incubation of cell wall β‐glucan at 40°C, mechanical stress, chromatography lasting 8–10 h at or above 65°C, or chromatography in M sodium chloride causes some disassociation of high molecular weight β‐glucan to a size of about 107 daltons. Heating a solution for 1 h at 100°C does not disassociate the β‐glucan. Pentosans isolated from cell walls are not covalently linked to the β‐glucans and can be separated from them by molecular sieve chromatography. They have a higher xylose/arabinose ratio than previously reported for barley pentosans. The pentosan molecules extracted by water are smaller (106 daltons) than those extracted by alkali (5 × 106 daltons). Little difference was observed in the chemical or physical properties of cell wall materials of barley cultivars of different malting qualities. 1977 The Institute of Brewing & Distilling