Some structural features of citrus limonia gum (lemon gum)

Lemon gum contains residues of D-galactose, L-arabinose, L-rhamnose, D-glucuronic acid, and 4-O-methyl-D-glucuronic acid. Graded, acid hydrolysis affords 3-O-β-L-arabinopyranosyl-L-arabinose, 5-O-α-L-arabinopyranosyl-L-arabinose, 4-O-(4-O-methyl-α-D-glucopyranosyluronic acid)-L-arabinose, three acidic trisaccharides [including O-(4-O-methyl-α-D-glucopyranosyluronic acid)-(1→4)-O-α-L-arabinopyranosyl-(1→5)-L-arabinosel, and higher acidic oligosaccharides. The methylated gum yields 2,3,4-tri-O-methyl-L-rhamnose, 2,3,5- and 2,3,4-tri-, and 2,3-, 2,5-, and 3,5-di-O-methyl-L-arabinose, 2,4-di-O-methyl-D-galactose, and 2,3,4-tri- and 2,3-di-O-methyl-D-glucuronic acid, together with small proportions of 2,3,4,6-tetra-, and 2,3,6-, 2,4,6-, and 2,3,4-tri-O-methyl-D-galactose. Partial, acid hydrolysis of the degraded gum, obtained after graded, acid hydrolysis, affords the β-(1→3)- and β-(1→6)-linkid D-galactose disaccharides, 4-O-(4-O-methyl-α-D-glucopyranosyluronic acid)-D-galactose, 6-O-(β-D-glucopyranosyluronic acid)-D-galactose, and 4-O-(α-D-glucopyranosyluronic acid)-D-galactose. Its O-methyl derivative yields 2,3,4-tri- and 2,3-di-O-methyl-L-arabinose, 2,3,4,6-tetra-, 2,3,6- 2,4,6-, and 2,3,4-tri-, 2,6- and 2,4-di-, and 2-O-methyl-D-galactose, and 2,3,4-tri-O-methyl-D-glucuronic acid. The degraded gum was subjected to a Smith-degradation, and lemon gum to two successive Smith-degradations, and the products were examined by linkage and methylation analysis. It is concluded that the molecules of lemon gum are based on galactan frameworks of a branch-on-branch type, to which are attached rhamnose-, arabinose-, and uronic acid-containing side-chains.