Configuration and interaction of the polar head group in gangliosides.

1. The interactions of gangliosides with Ca(2+) and some polar-head-group requirements for establishment of particular interactions with phosphatidylcholine were studied in monolayers at the air/145mm-NaCl interface. 2. Ganglioside-Ca(2+) interactions, as revealed by surface-potential measurements, depended on the position occupied by sialosyl residues in the oligosaccharide chain. The interactions with Ca(2+) of the single sialosyl residue of monosialogangliosides occurred above 0.1mm-CaCl(2), whereas the interaction of the cation with additional sialosyl groups in di- or tri-sialogangliosides depended on the carbohydrate residue to which the sialosyl moiety was attached. The sialosyl residue bound in sialosyl-sialosyl linkage interacted very little with Ca(2+). The sialosyl residue attached to the terminal galactose of the neutral tetrasaccharide chain interacted with Ca(2+) above 1mum-CaCl(2). 3. Experiments with mixed monolayers containing dihexadecyl phosphate and hexadecyltrimethylammonium indicated that for the occurrence of interactions of polysialogangliosides with phosphatidylcholine characterized by reductions in molecular packing and surface potential both charged groups of the phospholipid and sialosyl residues with particular dipolar properties in the ganglioside are participating. 4. Possible configurations that can explain the behaviour in monolayers were inspected with space-filling molecular models. The position of the carboxylate group of sialosyl residues with respect to the interface and to the sialosyl molecular plane can explain the different orientation of the dipole-moment vector of this residue, which depends on the position to which it is linked in the oligosaccharide chain. Favoured interactions of polysialogangliosides with phosphatidylcholine may result from a configuration allowing a partial matching of two oppositely oriented electrical vectors contributed by the zwitterionic phosphocholine group and particular sialosyl groups.