A branched β-d-(1→3,1→6)-glucan from the marine diatom Chaetoceros debilis (Bacillariophyceae) characterized by NMR

The chrysolaminaran from the marine diatom Chaetoceros debilis was isolated and characterized by NMR spectroscopy. Cells were harvested in the stationary phase of growth after the medium had been depleted of nitrate when the chrysolaminaran content was expected to be at its highest. The chrysolaminaran was isolated with an yield of 17.5 mg/L, which corresponds to15.8 pg/cell. 1H NMR indicated that the structure was similar to that of a β-(1→3) main chain with β-(1→6)-linked side chains. The degree of polymerization was found to be 30, corresponding to a molecular weight of ∼4900. Thirty-three percent of the residues were found to be β-(1→6)-linked branches. The characteristics of the β-(1→6) branching were examined by NOESY NMR, which suggested pustulan-like branches, being β-(1→6) linked chains connected to the main chain with few branch points. Confirmation of the 1H NMR data was done by 13C-DEPT, TOCSY, COSY and HMQC NMR spectroscopy. The assignment of the resonances of the main β-(1→3) and β-(1→6) chains is presented. The structure proposed from our analyses is compared against other chrysolaminaran structures.