Rhamnoarabinosyl and rhamnoarabinoarabinosyl side chains as structural features of coffee arabinogalactans

Coffee beans contain 7% of a water soluble highly branched type II arabinogalactan-protein (AGP) having rhamnoarabinosyl and rhamnoarabinoarabinosyl side chains. These side chains are reported for the first time as structural features of plant AGPs. 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.