Pore Model in the Melting Regime of a Lyotropic Biomembrane with an Anionic Phospholipid.

Abstract

Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidyl glycerol (DMPG) exhibit an unusual "melting regime", at the phase transition between the ordered (gel) and the disordered (fluid liquid crystal) state of hydrocarbon chains, depending on the ionic strength and DMPG concentration, previously attributed to the pore formation. Dispersions with 150 mM DMPG present a lamellar phase above 23 °C, within the melting regime. In this study, we present a detailed pore model for the analysis of small-angle X-ray scattering (SAXS) results and their variation with temperature, focused on the surface fractions of pores in the bilayers. Large and small toroidal pores are necessary to explain the SAXS results. Pores have DMPG in the fluid conformation, whereas the flat region of the bilayer has DMPG molecules in fluid and in gel conformations. A particular strategy was developed to estimate the charges due to the localization of mobile ions in the system, which is based on the calculation of electron densities by duly considering all molecular and ionic species that characterize the system, and the temperature dependency of their volumes. The best fit to the model of SAXS curves defines that the gel phase transforms initially, at 19.4 °C, in uncoupled bilayers with large pores (radius 93.2 ± 0.5 Å, with water channel diameter 137 ± 1 Å), which transform into small pores along the lamellar phase. The minimum intensity of the SAXS bilayer peak at 30 °C corresponds to a maximum number of small pores, and above 35 °C, the system enters into the normal lamellar fluid phase, without pores. The charge is estimated and shows that the regions with pores contains less Na+ ions per polar head; hence, when they are forming, there is a release of Na+ ions toward the bulk.