The effect of different cations on the conformational and morphological properties of the capsular polysaccharide
produced by Neisseria meningitidis group A was investigated. Circular dichroism studies
showed that the presence of Na+, NH4+ or Ca2+ ions induced different local conformations of the polysaccharide
chain through interactions with the phosphodiester group bridging the saccharide residues in the
polymer chain. Atomic force microscopy experiments confirmed that the morphology of the polysaccharide
chains was different depending on the nature of the counterion. Ammonium ions were associated
with the presence of single polymer chains in an elongated conformation, whereas sodium ions favored
the folding of the chains into a globular conformation. The addition of calcium ions produced the aggregation
of a limited number of globular polysaccharide chains to form a ‘toroidal-like’ structure.
