Chemical structure of the complex pyruvylated and sulfated agaran from the red seaweed Palisada flagellifera (Ceramiales, Rhodophyta)

A homogeneous agaran fraction from Palisada flagellifera (Laurencia complex, Rhodomelaceae, Ceramiales) was obtained by aqueous room-temperature extraction, followed by ion-exchange chromatography. This galactan presents a highly complex structure with at least 18 different types of derivatives. The A units were found mostly pyruvylated, 2-sulfated (∼34%), and 6-methylated (∼34%), with the latter partially 2- and 2,4-sulfated. Minor amounts of β-d-galactopyranosyl units 2-, 6- and 2,6-sulfated, 6-glycosylated, and non-substituted are also present. The B-units are l-sugars composed predominantly of their cyclized derivatives, 3,6-anhydrogalactose and 3,6-anhydro-2-O-methylgalactose (∼56%). The former are linked to β-d-galactosyl (6-methyl) (6-glycosylated) units, as well as to 4,6-O-(1-carboxyethylidene)-β-d-galactose 2-sulfate in the proportion of 3:1.8, respectively. A significant amount (∼18%) of the α-l-galactopyranosyl units are linked to pyruvylated β-d-galactose 2-sulfate residues. An important part of the B-units (20%) is represented by α-l-galactose 6-sulfate substituted on C-3 by xylosyl, galactosyl and/or 2,3-di-O-methylgalactose units or sulfate groups that preclude their cyclization to 3,6-anhydrogalactosyl derivative. The precursor units are present in relatively low percentages. Kinetic studies suggest that in P. flagellifera agaran the cyclizable units are linked to 6-O-methyl-β-d-galactosyl and/or β-d-galactosyl units (6-glycosylated). The structural complexity of this polysaccharide is increased by the presence of 2- and 3,6-sulfated α-l-galactoses, with the latter additionally 2-O-methylated. Therefore, the major subfraction obtained from the cold extract contains structurally complex sulfated, methylated, and pyruvylated agaran.

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► A family of sulfated, methylated, and pyruvylated agarans was isolated.
► The fine structure of a homogeneous agaran with MW 143 kDa was determined.
► Methylation analysis showed at least 18 galactopyranosyl derivatives.
► One third of the diads contain pyruvylated and 2-sulfated galactosyl units.
► Diversity of single stub side chains: xylosyl, galactosyl, and 2,3-methyl galactosyl.