Spin Labels Detect the Coexistence of Two Lipid Domains Along the Anomalous Gel-Fluid Transition of Anionic Dmpg Bilayers

Abstract

Vesicles of the acidic lipid dimyristoyl phosphatidylglycerol (DMPG), at physiological conditions, show a highly cooperative gel-fluid transition, around 23°C. However, at low ionic strength, DMPG bilayers display a peculiar thermo-structural behavior, presenting a broad gel-fluid transition, between 18°C and 35°C. Several interesting properties of this region led to different models for DMPG dispersion at low ionic strength. Here, we use computer simulations (1) of the electron spin resonance signal of spin labels incorporated into DMPG vesicles to evaluate the structure of the vesicles along the gel-fluid transition. At temperatures below and above the phase transition, in the gel and fluid phases of the bilayer, only one lipid population can be detected. But along the phase transition region, two lipid populations coexist: one very mobile, even more fluid than lipids in the bilayer fluid phase, and another one more rigid and organized, typical of lipids in the gel phase. The more mobile population appears at ca. 18°C and disappears by the end of the phase transition. In accord with previous works (2-4), we propose that this highly mobile lipid population is associated with the formation of pores at the bilayer, and these mobile lipids are located at the edges of the pores. Hence, spin labels can monitor the increase and decrease of bilayer perforations along the membrane phase transition.Financially supported by USP, FAPESP, CNPq.(1)Schneider, Freed, Biological Magnetic Resonance, vol. 8, 1, 1989; (2) Riske, Fernandez, Nascimento, Bales, Lamy-Freund, Chem. Phys. Lipids 124, 69, 2003; (3) Riske, Amaral, Dobereiner, Lamy, Biophys. J. 86, 3722, 2004; (4) Enoki, Henriques, Lamy, Chem. Phys. Lipids 165, 826, 2012.