1,2-Dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) divalent cation complexes: an X-ray scattering and freeze-fracture electron microscopy study

Abstract

1,2-Dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) dispersed in 100 mM NaCl at pH 7 forms negatively charged multilamellar vesicles. After addition of divalent cation salts (Mg²⁺, Ca²⁺ and Sr²⁺) in an equimolar cation/lipid ratio, a precipitate of lipid divalent cation (2:1) complexes is formed. These complexes were examined by X-ray diffraction and freeze-fracture electron microscopy to elucidate their phase structure as a function of temperature and incubation time at low temperature. The structural and morphological analysis of the DMPG divalent cation complexes stored for 7 d at 4°C (initial state) revealed the formation of a lamellar phase with a very short lamellar repeat distance of 4.4–4.6 nm. The hydrocarbon chains of the lipid are packed in a subgel state. For all three complexes smooth fracture faces with crystalline appearance were observed in the electron micrographs. Increasing the temperature induces a subgel to liquid-crystalline phase transition at around 58–62°C for the Mg²⁺ and Sr²⁺ complexes and at 85°C for the Ca²⁺ complex. The electron micrographs of the Mg²⁺ and Sr²⁺ complexes showed that in the liquid-crystalline phase regular multilamellar vesicles or large multilamellar cylinders are formed, whereas the Ca²⁺ complex forms lamellar structures of crystalline appearance. At lower temperature (below 35°C), new phases appear: Mg²⁺ and Sr²⁺ complexes form a gel-like phase with an increased lamellar repeat distance of ∽1 nm and a phase transition temperature of about 35°C, whereas the Ca²⁺ complex transforms into a subgel phase which is very similar to the initial state. Our results show that the interaction of Mg²⁺ and Sr²⁺ with DMPG is very similar but different from that observed with Ca²⁺. The results also indicate interactions of Ca²⁺ with DMPG in the liquid-crystalline phase, whereas this is not observed for the other two ions.