The hemicellulose of sisal fibre (Agave sisalana)

Abstract

Sisal fibre holocellulose was extracted with alkaline solutions of increasing concentration and finally with alkaline borate solution. Hemicellulose fractions (1–4) were isolated from the extracts. Fractions 2 and 3 have molecular weights of 23,700 ± 500 and 25,300 ± 500, respectively. Analytical data are recorded for each fraction.

Partial acid hydrolysis of sisal hemicellulose yields mainly xylose and 2-O-(4-O-methyl-α-D-glucopyranosyluronic acid)-D-xylopyranose. The main hemicellulose fraction (2) was methylated and hydrolysed, affording 2,3,4-tri-O-methyl-D-xylose, 2,3-di-O-methyl-D-xylose, 3-O-methyl-D-xylose, and 2-O-(2,3,4-tri-O-methyl-α-D-glucopyranosyluronic acid)-3-O-methyl-D-xylopyranose in the approximate molar proporation of 1:31·9:1·5:5. The methylated polysaccharide has a degree of polymerisation of 109 ± 3. On this basis and from other evidence it is suggested that the hemicellulose is composed of chains of ca. 97 1,4-linked β-D-xylopyranose residues, approximately every eighth residue carrying a terminal 4-O-methyl-α-D-glucopyranosyluronic acid residue linked through position-2. A small degree of branching in the xylan is indicated.