Chemical Structure of a Novel Xylogalactan Isolated from Commercially Cultured Seagrape, <i>Caulerpa </i><i>lentillifera</i>

A novel xylogalactan was isolated from green seaweed named Seagrape, Caulerpa lentillifera, which is commercially cultured in Okinawa, Japan. D-galactose (molar ratio, 2.7) and D-xylose (1.0) were identified by HPAEC analysis. The optical rotation was estimated to be +0.005˚ at 25˚C, but decreased to −0.001˚ at 50˚C, indicating α- and β-linkages co-involved. D-galactose and D-xylose were also identified from infrared spectrum that was the first to report. The spectrum indicated that the xylogalactan was involved in both α- (small peak) and β-(large) glycosides. Well resolved 13C-NMR spectrum was obtained and assigned to be 1,4-linked α-D-galactose (large) and 1,3-linked β-D-xylose (small). All ring 13C-NMR (C-2-C-6) were assigned and some C-6 of 1,4-linked α-D-galactose residues were estimated to be a branching sugar. From 1H-NMR spectrum, 1,4-linked α-D-galactose, terminal and 1,3-linked β-D-xylose were assigned. Methylation analysis was used to identify 2,3,4-tri-O-methyl-D-Xylp (terminal; 0.6 mol), 2,4-di-O-methyl-D-Xylp (1→3-linked; 2.4), 2,3,6-tri-O-methyl-D-Galp (1→4-linked; 3.2), 2-mono-methyl-D-Xylp (1→3,4-linked; 0.3), and 2,3-di-O-methyl-D-Galp (1→4,6)-linked; 1.0). The structure of a novel xylogalactan was branching trisaccharide side-chains, β-D-Xylp-(1→3)-β-D-Xylp-(1→3)-β-D-Xylp-(1→), at C-6 of 1,4-linked α-D-Galactoses main-chain.