Analysis of the carbohydrates of fulvic and humic acids as their partially methylated alditol acetates

Abstract

The carbohydrates of humic and fulvic acids were investigated using methylation analysis. The humic acid contains slightly more rhamnose and fucose than does fulvic acid and raw humus, the arabinose content of fulvic acid is less when compared to that of humic acid and raw humus. In addition to the common aldoses, the unmethylated humic samples contained small amounts of 2,3-di-O-methyl-rhamnose, 2,3-di-O-methyl-fucose, 2-O-methyl-rhamnose, 4-O-methyl-rhamnose, 3-O-methyl-rhamnose, 3-O-methyl-fucose and a 6-deoxy-hexose. The “average” polysaccharides of humic and fulvic acids are highly branched and extremely complex in composition. They contain at least 34 different aldose building units in addition to O-methyl-, di-O-methyl-, and amino-derivatives, ketoses and uronic acids. On average, the polysaccharides of fulvic acid are slightly less branched than those of humic acid, as indicated by a higher content of the chain unit 2,4,6-tri-O-methyl-glucose in the hydrolysate of methylated samples. In total, the polysaccharides of the different fractions are of great similarity. The common aldoses all participate as 1–2 different terminating units, 1–3 different chain units, and the hexoses as 2–4 different branching units. In methylated samples, only one 6-deoxy-hexose branching unit is found, 3-O-methyl-rhamnose, and no pentose unit. 3-O-methyl-rhamnose may also be a branching unit in unmethylated samples. The most important terminating units are 2,3,4,6-tetra-O-methyl-gluco- plus 2,3,4,6-tetra-O-methyl-manno- (15%), 2,3,4-tri-O-methyl-xylo- (6%) and 2,3,4-tri-O-methyl-fuco-pyranose (5%) units. Terminal 2,3,5-tri-O-methyl-xylo-, 2,3,5-tri-O-methyl-arabino- and 2,3,5,6-tetra-O-methyl-galacto-furanosidic units are also present (2.7-1.2%). The most important chain units are 2,4,6-tri-O-methyl-glucose (11–15%), 2,3-(or 3,4-) di-O-methylxylose (6.6%) and 3,4,6-tri-O-methyl-galactose (3.3%). The individual polysaccharides of the humic and fulvic acids are also of high complexity and most probably are present in a large number of varieties. All polysaccharides are strongly associated with the humic matrix, i.e., the aromatic and aliphatic polymeric structure composing the major part of the humic and fulvic acids.