Structural characterization and solution properties of an acidic branched (1->3)-beta-D-glucan from Aureobasidium pullulans

An acidic exopolysaccharide was isolated from a selected strain of Aureobasidinum pullulans. On the basis of spectroscopic and chromatographic techniques, the polymer was identified as a beta-D-glucan containing a main chain of (1-->3)-linked beta-D-glucopyranosyl units substituted at the O-6 position by single beta-D-glucopyranosyl side chains. The ratio of units in the main chain to units in the side chain was found to be 1.4:1. The ionic character of this exopolysaccharide is due to the presence of malate residues which are linked to the polymer through ester bonds. The degree of substitution was estimated to be very low (0.05). In aqueous solution no signals are present in the NMR spectra strongly suggesting that the polymer adopts a rigid ordered conformation as further confirmed by rheological data. A solvent-induced conformational transition was observed in DMSO in which NMR spectra with good signal-to-noise ratio were obtained. The solution behaviour of the polymer is similar to that of other branched (1-->3)-beta-D-glucans in spite of both the degree of branching and the substitution with malate groups.