Neutral Pectin Side Chains of Amaranth (Amaranthus hypochondriacus) Contain Long, Partially Branched Arabinans and Short Galactans, Both with Terminal Arabinopyranoses

Amaranth is a pseudocereal of high nutritional value, including a high dietary fiber content. Amaranth dietary fiber was suggested to contain large amounts of neutral rhamnogalacturonan I side chains. In this study, endo-arabinanase and endo-galactanase were used to liberate arabinan and galactan oligosaccharides from amaranth fiber. The liberated oligosaccharides were identified by high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and HPLC-MS(n) using standard compounds, which were isolated from amaranth, sugar beet, potato, and red clover sprouts and characterized by one- and two-dimensional NMR spectroscopy. It was demonstrated that insoluble amaranth arabinans have linear and branched areas, with the O-3 position being the dominant branching point. Minor amounts of branches at position O-2 and double substitution were also found. Amaranth arabinans were also demonstrated to contain terminal α-(1→5)-linked l-arabinopyranose units. In addition, it was evidenced that galactans from amaranth seeds are composed of β-(1→4)-linked d-galactopyranose units, which can also be terminated with l-arabinopyranose units. In direct comparison to structural elucidation of amaranth fiber by using methylation analysis, the advantage of the enzymatic approach over methylation analysis was demonstrated.