Coordination forces between lipid bilayers produced by ferricyanide and Ca2+

Attractive forces usually invoked to take place in membrane–membrane contact in aggregation are hydrogen bonding cross-linkings and hydrophobic interactions between opposing surfaces. However, little is known in relation to the presence of coordination forces in the membrane–membrane interaction. These are understood as those that may be favoured by the formation or the participation of coordination complexes between surface specific groups. In this work, we have analyzed the formation of this type of aggregates between phosphatidylcholine vesicles mediated by a coadsorption of ferricyanide and Ca2+ ions to the interface. The results obtained by surface potential measures, optical and electronic microscopy, FTIR and 1H NMR spectroscopies indicate that ferricyanide [Fe(CN)6]3− but not of ferrocyanide [Fe(CN)6]4− can form the complex when Ca2+ has been adsorbed previously to the membrane surface. In this condition, the anion is likely to act as a bridge between two opposing membranes causing a tight aggregation in which geometry and the polarizability of the ligands to Fe3+ play a role.