Refined structural characterization and variability of gum arabic using partial hydrolysis

Abstract

Gum arabic (GA) consists of a mixture of complex, highly branched glycoproteins. The structural architecture of these glycoproteins remains only partly understood and has not yet been comprehensively examined across functionally different GA batches. Therefore, the aim of this study was to investigate structural differences in the carbohydrate domains and carbohydrate-protein linkages in eight qualitatively different GA samples and their respective glycoprotein fractions in detail by using different partial hydrolysis procedures. To analyze the polysaccharide architecture, all samples as well as their glycoprotein fractions arabinogalactan, arabinogalactan-protein (AGP), and glycoprotein were digested with β-1,3-galactan-cleaving enzymes to release the arabinogalactan side chains, which were subsequently analyzed by HPAEC-PAD/ESI-MSn. 18 side chain derived oligosaccharides were identified in all samples, 12 of which were newly described. However, variations in their abundance were observed. In particular, Rha1→4GlcA1→6Gal1 and Ara1→3/Ara1→4Gal1→6Gal1 were enriched in high molecular mass samples and protein-rich fractions. However, assignment of the GA quality based on these side chain oligosaccharides was not possible. The analysis of protein glycosylation after alkaline hydrolysis showed that AGP fractions contained the highest amount of Hyp and the highest content of glycosylated Hyp. However, these parameters were not related to the molecular weight or the quality of the samples. Furthermore, Hyp-arabinosides were not detected but it was evident that GA polysaccharides are degraded under alkaline conditions. Although the degree of glycosylation plays a significant role for the properties of the protein-rich fractions of GA, the structure of the polysaccharides is more important for the functionality of different GA batches.