Does calcium turn a Zwitterionic lipid cationic?

The structure of a complex formed from DNA, calcium, and DPPC has been studied by small-angle X-ray scattering (SAXS). Previous DSC and temperature scanning ultrasound have shown that in excess lipid two DPPC phases exist, one corresponding to DNA bound lipid and the other uncomplexed lipid, not directly involved in complex formation. (1.2) We have investigated the effect of calcium on both phases within the complex structure. Contrary to the observations in the ternary DPPC/calcium/water system, the binding of calcium to the DPPC membrane in this quaternary system containing DNA appears to be significantly enhanced. We have estimated the stoichiometry of calcium binding within the complex, and we concluded that the zwitterionic lipid is directly involved in binding to DNA.