Calcium ion binding of three different types of oligo/polysialic acids as studied by equilibrium dialysis and circular dichroic methods.

Ca2+ binding properties of three different types of oligo/polysialic acid chains, i.e., oligo/poly(Neu5Ac), oligo/poly(Neu5Gc), and oligo/poly(KDN), were studied by equilibrium dialysis and circular dichroism. Colominic acid, high molecular weight polysialoglycoprotein (H-PSGP), low molecular weight polysialoglycoprotein (L-PSGP), and 3-deoxy-D-glycero-D-galacto-2-nonulosonic acid (KDN) glycoprotein were found to bind calcium ions with about 8-100 times the affinity of sialic acid monomer. Analysis by equilibrium dialysis of the binding of Ca2+ to colominic acid was biphasic, and the high-affinity interaction was shown to change with the degree of polymerization. Specific binding of Ca2+ to polysialic acid (polySia) caused characteristic effects in the circular dichroism spectrum. A pronounced decrease in the circular dichroism of polySia at 205 nm was observed upon addition of calcium. H-PSGP was found to bind calcium ions with 3-fold higher affinity than L-PSGP.