The interactions between phosphatidylglycerol and phosphatidylethanolamines in model bacterial membranes: The effect of the acyl chain length and saturation

In this work the Langmuir monolayer technique was applied to study the interactions between phosphatidylglycerol (PG) and phosphatidylethanolamines (PEs). Since the technique of monomolecular layers is useful method of modeling of biomembranes and the investigated lipids are the major components of Gram-negative bacteria bilayers the results obtained were discussed in the context of the properties of bacterial membranes. To investigate the influence of the phosphatidylethanolamine acyl chain structure on its miscibility and interactions with dipalmitoylphosphatidylglycerol (DPPG) the chosen phosphatidylethanolamines differed both in the length (dipalmitoylphosphatidylethanolamine (DPPE), distearoylphosphatidylethanolamine (DSPE)), and saturation (dioleoylphosphatidylethanolamine (DOPE)) of the hydrocarbon chains. The obtained results prove non ideal behaviour of all of the mixed systems studied. It was found that in monolayers between DPPG and phosphatidylethanolamines with saturated acyl chains (DPPE, DSPE) strong attractions resulting from the formation of hydrogen bonds exist. These attractions are the strongest for equimolar mixtures indicating that PG and PE molecules form the “complexes” of 1:1 proportion. In DOPE/DPPG mixed system the distance between phospholipids is larger, due to the presence of the cis double bonds in the acyl chains of DOPE molecule, which makes the formation of the hydrogen bonds more difficult. The interactions between molecules, the molecular packing and the stability of the investigated mixed systems were thoroughly discussed from the point of view of the effect of polar head and PEs acyl chain structure. Basing on the obtained results it was suggested that PG molecules play in bacterial membranes similar role as cholesterol in mammalian bilayers.