ANALYSIS OF THE STRUCTURAL HETEROGENEITY OF LAMINARIN BY ELECTROSPRAY-IONIZATION-MASS SPECTROMETRY

Electrospray-ionisation-mass spectrometry (ESIMS) was used in conjunct ion with chemical derivatisation and degradation procedures to analyse the size heterogeneity and branching structure of laminarin from the brown alga, Laminaria digitata. Laminarin is a beta-(1 --> 3)-linked D -glucan with occasional beta-(1 --> 6)-linked branches. Electrospray-i onisation-mass spectrometry of permethylated laminarin distinguished t wo homologous series of molecules, a minor G-series containing 22-28 g lucosyl residues, and a more abundant M-series containing 20-30 glucos yl residues linked to a mannitol residue. The relative abundance of al l these molecular species could be determined simultaneously from a si ngle mass spectrum, with a mean mass error of 0.6 atomic mass units an d a mean mass accuracy of 0.011%. Both series had a mean degree of pol ymerisation of 25 glucosyl residues, and an approximately 3:1 molar ra tio of M-series to G-series molecules was maintained across the range of molecular sizes. Treatment of laminarin with periodate, followed by reduction with borohydride, degraded terminal glucosyl residues on bo th the main chain and the branches, and allowed the detection of isome rs differing solely in their degree of branching. M-series molecules w ere thus shown to contain 0, 1, 2, 3 or 4 branches, with an average of 1.3 branches per molecule; branched G-series molecules were also dete cted. Subsequent treatment with acid (Smith degradation) showed that 7 5% of the branches were single glucosyl residues. This study thus show s how the speed, resolution and mass accuracy of electrospray-ionisati on-mass spectrometry can be used in the detailed structural analysis o f a polydisperse polysaccharide.