²H NMR studies of the effect of the DPPC/DPPG ratio on bilayer properties in thepresence of Ca²⁺.

The presence of Ca²⁺ ions in the aqueous medium is known to influence the physical properties of bilayer membranes containing charged phospholipids such as dipalmitoylphosphatidylglycerol (DPPG). 2H NMR has been used to study the effect of Ca²⁺ on the order, dynamics and phase behaviour of bilayers containing mixtures of the anionic DPPG and a neutral lipid with identical acyl chains, dipalmitoylphosphatidylcholine (DPPC), dispersed in aqueous solutions. The effect of bilayer surface charge in this system was investigated by varying the DPPG/DPPC ratio in the presence of excess Ca²⁺. It was found that Ca²⁺ alters the temperature and width of the liquid crystal to gel bilayer transition and the quadrupole echo relaxation times of chain deuterons in a way which depends on the proportion of negatively charged lipid in the bilayer. The observed effects are consistent with a Ca²⁺-induced reduction in area per lipid of liquid crystal bilayers containing DPPG. The results do not support a preferential association of Ca²⁺ with the DPPG headgroup or segregation into DPPG or DPPC domains. The introduction of a negatively charged lipid component into DPPC membranes may alter the hydrogen bonding or modify the water layer, or both, resulting in changes in the adiabatic bilayer motions.