Effect of Na(+) concentration on the subgel phases of negatively charged phosphatidylglycerol.

The effect of Na(+) concentration on the subgel phase of dimyristoylphosphatidylglycerol (DMPG) was investigated by differential scanning calorimetry (DSC) and negative stain electron microscopy, and the results were compared with dipalmitoylphosphatidylglycerol (DPPG). The conversion mode of DMPG vesicle to the subgel phase by annealing at 5 degrees C was grouped into two types depending on whether Na(+) concentration is above or below 200-250 mM. For [Na(+)]>200-250 mM, the subgel phase of a crystalline superstructure of bilayers wrapped in a cylinder was attained during a 24-h period of annealing and transformed directly to the liquid crystal phase on heating. For [Na(+)]<200-250 mM, two subgel phases which transform to the gel phase on heating were observed after annealing up to 24 h. Both subgel phases showed belt-like structures composed of loosely and closely stacked lamellae, respectively, and their fractions were found to depend on Na(+) concentration. With a further annealing up to 30 days, only the closely stacked subgel phase converted subsequently into the cylindrical superstructure of a more ordered phase. Similar two subgel phases were detected for DPPG at [Na(+)]< or =100 mM. The difference in the relative enthalpy between the gel and subgel phases was investigated from the van der Waals interaction energy between the hydrocarbon chains.