New lamellar phase with pores in the chain-melting regime of an anionic phospholipid dispersion

The anionic phospholipid DMPG (dimyristoyl phosphatidylglycerol) may exhibit in water, instead of a unique melting transition of the hydrocarbon chains, a "melting regime" for pH values above 5, where the phosphate groups are deprotonated, and for low ionic strength, where charge screening is weak. The chain-melting process of DMPG starts at T_m^on (onset of the melting regime at ∼ 20°C), but the complete fluid phase exists only above T_m^off (offset of the melting regime at ∼ 30°C). In a recent paper we developed a SAXS model for a bilayer with pores to explain SAXS results obtained for concentrations up to 70 mM DMPG (F. Spinozzi, L. Paccamiccio, P. Mariani, and L. Q. Amaral, Langmuir, in print, 2010). A new lamellar phase with pores, starting 3°C above T_m^on and existing up to 4°C above T_m^off, was also identified at the higher investigated DMPG concentrations (up to 300 mM DMPG). In this paper we focus in more detail the SAXS curves obtained in the concentration interval 70-300 mM DMPG. The slope of the scattering profile in the very small q range, as well as the anomalous increase in the intensity of the bilayer band centered around 0.12 Å−1 after T_m^off, have been in particular analyzed. By using a model of water-penetrated bilayers, the volume fractions of DMPG and water molecules inside the bilayer was derived as a function of temperature.