Structural characterization of two galactofuranomannan isolated from the lichen Thamnolia vermicularis var. subuliformis

Lichens are symbiotic organisms consisting of a fungus and an algae and/or cyanobacterium. Of 13, 500 lichen species growing worldwide, less than 100 species have been investigated for polysaccharide content. Lichen polysaccharides are mainly of three different structural types: β-glucans, α-glucans and galactomannans [1]. The aim of the study was to isolate and structurally characterize two galactofuranomannans, Ths-4 and Ths-5 from the lichen Thamnolia vermicularis (Sw.) Schaer. var. subuliformis (Ehrh.) Schaer. using ethanol fractionation, anion-exchange and size exclusion chromatography. The average molecular weight of Ths-4 and Ths-5 was estimated to be 19 and 200 kDa, respectively. Structural characterization of Ths-4 and Ths-5 and their partially hydrolysed derivatives was performed by methanolysis and methylation analysis. The intact and partially hydrolysed Ths-4, was further analysed using NMR-spectroscopy (1D, COSY, NOESY, TOCSY, HSQC and HMBC).

According to the data obtained, the heteroglycans Ths-4 and Ths-5 have similar structures, but have large difference in molecular weight. The structure is composed of 3-O-linked and 5-O-linked galactofuranosyl-chains linked to a mannan core. The mannan core consists of a main chain of α-(1→6)-linked mannopyranosyl residues, substituted at O-2 with either a single α-mannopyranosyl unit or an α-Manp-(1→2)-α-Manp-(1→2)-α-Manp group in the ratio of approximately 1:3, respectively.

Acknowledgements: Danish Instrument Center for NMR Spectroscopy of Biological Macromolecules, Icelandic Council of Science, University of Iceland Research Fund, The Icelandic Research Fund for Graduate Students, Nordic Council of Ministers, The Bergthoru and Thorsteins Scheving Thorsteinsson, Finn Tønnesen, NUFU project PRO22/2002.

Reference: 1. Olafsdottir, E.S., Ingólfsdóttir, K. (2001), Planta Med 67: 199–208.