Rhamnoarabinosyl and rhamnoarabinoarabinosyl side chains as structural features of coffee arabinogalactans

Abstract

The hot water soluble green coffee arabinogalactans, representing nearly 7% of total coffee bean arabinogalactans, were characterized by 1H and 13C NMR and, after partial acid hydrolysis, by ESI-MS/MS. Data obtained showed that these are highly branched type II arabinogalactans covalently linked to proteins (AGP), with a protein moiety containing 10% of 4-hydroxyproline residues. They possess a β-(1 → 3)-Gal p/β-(1 → 3,6)-Gal p ratio of 0.80, with a sugars composition of Rha:Ara:Gal of 0.25:1.0:1.5, and containing 2 mol% of glucuronic acid residues. Beyond the occurrence of single α- l-Ara f residues and [α- l-Ara f-(1 → 5)-α- l-Ara f-(1→] disaccharide residues as side chains, these AGPs contain unusual side chains at O-3 position of the β-(1 → 6)-linked galactopyranosyl residues composed by [α- l-Rha p-(1 → 5)-α- l-Ara f-(1→] and [α- l-Rha p-(1 → 5)-α- l-Ara f-(1 → 5)-α- l-Ara f-(1→] oligosaccharides. Rhamnoarabinosyl and rhamnoarabinoarabinosyl side chains are reported for the first time as structural features of plant arabinogalactan-proteins.