Cobalt and nickel affect the fluidity of negatively-charged biomimetic membranes

Elevated levels of the essential trace metals cobalt and nickel are associated with a variety of toxic effects, which are not well-understood, and may involve interactions with the lipid membrane. Fluidity changes of biomimetic lipid membranes upon exposure to CoCl₂ and NiCl₂ were studied using the fluorescent probe Laurdan, which senses changes in environment polarity. Liposomes were prepared by extrusion in 20 mM HEPES + 100 mM NaCl at pH 7.4. Additionally, dynamic light scattering was used to monitor metal induced size changes of liposomes composed of: phosphatidic acid (PA), cardiolipin (CL), phosphatidylglycerol (PG), phosphatidylserine (PS), and phosphatidylcholine (PC), with saturated and unsaturated acyl chains. Micromolar concentrations of both metals significantly rigidify negatively-charged liposomes and generally increase the melting temperature. Saturated acyl chains showed stronger metal effects in PS and PG, while no clear acyl chain preference was observed in CL and PA systems. The strength of the effect appears to be influenced greatly by both the head group and acyl chain. The rigidifying effects of cobalt were almost always much larger than those of nickel. In addition, size changes and aggregation by both metals was detected in PS or PA liposomes at molar metal/lipid ratios as low as 1/10.