A new arabinomannan from the cell wall of the chlorococcal algae Chlorella vulgaris.

A new arabinomannan has been isolated from the cell wall of the green algae Chlorella vulgaris by extraction with 0.1 M NaOH, dialysis and SEC fractionation. M(w) was about 8000 u. Terminal, 2- and 5-O-linked arabinofuranosyl residues, and 2,6-O-linked mannopyranosyl residues were detected as the main constituents beside some minor mannose components by methylation analysis. Electrospray ionisation mass spectrometry (ESI-MS) with collision induced dissociation (CID) up to MS(3) experiments of oligosaccharides obtained by partial methanolysis or partial acid hydrolysis indicated the presence of Man→Man- and Ara→Ara- as well as Ara→Man- and also Man→Ara-sequences. MS(2) experiments gave evidence of a (1→6)-linked mannan, probably also including some (1→2)-linked mannosyl and 5-linked arabinofuranosyl residues. Mannooligosaccharides up to DP5 with mainly (1→6)-, but at higher DP also (1→2)-linkages were obtained by acid hydrolysis, and arabino oligomers up to DP4 could be detected after mild methanolysis. In accordance with results from methylation analysis and ESI-MS/CID t-β-Araf, 2-α-Araf, 5-α-Araf and 2,6-α-Manp were identified from homo- and heteronuclear 1D- and 2D NMR experiments in a molar ratio of ∼2:2:1:2. A highly branched structure is suggested with a 6- and 2-O-linked mannan main chain, comprising 5-Araf residues. Araf-β-[(1→2)-Araf-α-(1→](n) side chains with an average chain length of 2 are linked to the mannan main chain. The configuration of Ara is D. The new polysaccharide shows structural similarities with the (lipo)arabinomannans of Mycobacterium tuberculosis.