Conformational analysis in aqueous solution and estimation of the persistence length of exopolysaccharides produced byLactobacillus helveticusLh59 andStreptococcus macedonicusSc136

Abstract

The conformations of two exopolysaccharides, one produced by Lactobacillus helveticus Lh59 and one by Streptococcus macedonicus Sc136, were investigated by molecular modelling to get insight into their physical properties. In particular, the influence of side chains and glycosidic linkage types on the overall shapes and persistence lengths of the polysaccharides were studied. It appeared that the side chains had only minor effects on the persistence lengths of the polysaccharides, with the exception of the monosaccharide residue directly attached to the backbone. The extensiveness of the backbone is enhanced by β-D-Hexp-(1→4) structural elements, whereas α-D-Hexp-(1→3) and α-D-Hexp-(1→4) elements create structural variability by introducing bends. The occurrence of either (1→5)- or (1→6)-linked monosaccharides, resulting in one extra bond between monosaccharide units, causes the overall flexibility of the polysaccharide chain to be enhanced and extensiveness to be reduced. The presence of such flexible linkages in the backbone leads to a shorter persistence length than when present in the side chain: a value of only 4.5 nm was found for the L. helveticus Lh59 EPS with a (1→5) linkage in the backbone, and a value of 8.5 nm for the S. macedonicus Sc136 EPS with two (1→6) linkages in the side chain.Key words: conformational analysis, exopolysaccharide, Lactobacillus helveticus, molecular modelling, persistence length, Streptococcus macedonicus.