Location of a second O-acetyl group in xanthan gum by the reductive-cleavage method

Abstract

Xanthan gum is the extracellular polysaccharide produced by the bacterium Xanthomonas campestris. Its structure has been shown’,’ to consist of a (1 -+ 4) linked /3-D-glucan backbone with P-D-Manp-(I + 4)-P-D-GlcpA-(1 -+ 2)(U-DManp-(1 + side chains substituted at O-3 of alternating glucose residues. In addition, the side chains may contain 0-acetyl and pyruvic acid acetal groups, which are located at the 6-position of the internal mannose residue and at the 4,6-position of the terminal mannose group, respectively. The amount of noncarbohydrate substituents vary depending on the strain of bacteria, the fermentation conditions and the recovery procedure3-5. Samples of xanthan gum with high acetyl content have been observed6. The proportion of acetyl groups was shown to exceed the theoretical value of one residue per pentasaccharide repeat unit. Although the location of additional acetyl groups is uncertain, it has been speculated that multiple acetylation of the internal mannosyl residue is possible in some xanthan gum samples, as is acetylation of the terminal mannosyl group’. In X. campestris, three genes have been identified8,9 that encode enzymes which catalyze modifications of mannose residues. It was later shown” that one of these genes encodes an acetylase activity, termed acetylase II, directing acetylation of the terminal mannose group; however, the position of substitution was not determined. We now report the location of a second 0-acetyl group in xanthan gum using the reductive-cleavage technique”. A sample of xanthan gum was hydrolyzed using trifluoracetic acid and analyzed for the presence of organic acids by high-performance liquid chromatography (HPLC). The results of this analysis indicated the presence of acetic acid (6.8%) and pyruvic acid (2.7%). Assuming a sodium form of xanthan gum containing an average of 1.0 acetyl groups and 0.5 pyruvic acid acetal substituents per repeat unit, the theoretical content for acetic acid and pyruvic acid is 6.3 and 4.4%,