Structure and anti-metapneumovirus activity of sulfated galactans from the red seaweed Cryptonemia seminervis

•dl-Hybrid galactans from Cryptonemia seminervis were partially depolymerized.•The depolymerized galactans present molecular weight of 52–64kDa.•Sulfate groups are located principally on C-2 and C-6 of β- and α-galactose.•Depolymerized and native galactans present anti-human metapneumovirus activity. The anti-HMPV (human metapneumovirus) activity was determined for sulfated dl-hybrid galactans obtained from the red seaweed Cryptonemia seminervis and their depolymerized products obtained by reductive partial hydrolysis. Structural studies carried out in three homogeneous depolymerized fractions DS-1, DS-2e and DS-3 (Mw of 51.6–63.8kDa) showed that these galactans present different chemical characteristics, as monosaccharide composition, content of sulfate groups (14.1–29.9%) and agaran:carrageenan molar ratio diads, 2.7:1 for DS-1 and DS-2e and 1:1 for DS-3. The sulfate groups are located principally on C-2 of β-d-galactopyranose and 4,6-O-(1′-carboxyethylidene)-β-d-galactopyranose residues and on C-6 of α-galactose residues. Sulfated dl-galactans and their depolymerized products exhibited antiviral activity at a very early stage of the viral infection cycle. All fractions, except DS-2e inhibited HMPV replication by binding to the viral particle. Besides depolymerized galactans DS-2e and DS-3 inhibited the recognition of cell receptor by HMPV and penetration to the host cell, respectively.