Effect of Electrostatic Repulsion on DMPG Bilayers

Abstract

It is well known that ionic strength plays a fundamental role in the structure of DMPG (dimyristoyl phosphatidylglycerol) anionic vesicles in water medium. In buffer, at pH values above 4 and at high ionic strength (above ∼ 100 mM), the morphology of DMPG vesicles are rather similar to that of DMPC (dimyristoyl phosphatidylcholine) vesicles. However, at low ionic strength (∼ 4 mM), DMPG dispersions display several anomalous characteristics, which were interpreted as the opening of bilayer pores along the wide bilayer gel-fluid transition region (from ∼ 18oC to 30oC)1. Here, we revisit DMPG in pure water 2, to emphasize electrostatic interactions between the polar head-groups, which will not be shielded by ions in solution. For comparison, we used several techniques that have been recently applied to DMPG in buffer: light scattering, both static (SLS) and dynamic (DLS); differential scanning calorimetry (DSC); electron spin resonance (ESR) of spin labels incorporated into the aggregates; and viscosity, turbidity and electrical conductivity measurements. DSC and spin labels indicate that, in water, the bilayer gel-fluid transition is even wider, starting around 10oC but still ending ∼ 30oC. However, high electric conductivity, high viscosity and low turbidity found only in the gel-fluid transition region for DMPG in buffer, are found at higher temperatures in water, when lipid bilayers are already in the fluid state. Moreover, different from DMPG in buffer, in water, vesicles were found to fuse along the transition region. Data suggest that the strong PG- - PG- electrostatic repulsion in water leads not only to pore formation in DMPG bilayers, but also to the opening of the vesicles and vesicle fusion.[1]Enoki,T. A.; Henriques, V. B.; M. T. Lamy, Chem. Phys. Lipids 2012, 165, 826-837. [2] Epand, R. M.; Hui, W., Febs Lett. 1986, 209, 257-260.