The influence of charge on phosphatidic acid bilayer membranes

A complete titration of phosphatidic acid bilayer membranes was possible for the first time by the introduction of a new anaologue, $\text{1,2-}\text{dihexadecyl}\text{-sn-}\text{glycerol-3-phosphoric}$ acid, which has the advantage of a high chemical stability at extreme pH values. The synthesis of this phosphatidic acid is described and the phase transition behaviour in aqueous dispersions is compared with that of three ester phosphatidic acids; $\text{1,2-}\text{dimyristoyl}\text{-sn-}\text{glycerol-3-phosphoric}$ acid, 1,3-dimyristoylglycerol-2-phosphoric acid and $\text{1,2-}\text{dipalmitoyl}\text{-sn-}\text{glycerol-3-phosphoric}$ acid.The phase transition temperatures ($\text{T}{}_{\mathrm{<mtext>t</mtext>}}$) of aqueous phosphatidic acid dispersions at different degrees of dissociation were measured using fluorescence spectroscopy and 90° light scattering. The $\text{T}{}_{\mathrm{<mtext>t</mtext>}}$ values are comparable to the melting points of the solid phosphatidic acids in the fully protonated states, but large differences exist for the charged states.The $\text{T}{}_{\mathrm{<mtext>t</mtext>}}$ vs. pH diagrams of the four phosphatidic acids are quite similar and of a characteristic shape. Increasing ionisation results in a maximum value for the transition temperatures at pH 3.5 ($\text{p}\text{K}{}_1$). The regions between the first and the second $\text{p}\text{K}$ of the phosphatidic acids are characterised by only small variations in the transition temperatures (extended plateau) in spite of the large changes occurring in the surface charge of the membranes. The slope of the plateau is very shallow with increasing ionisation. A further decrease in the H⁺ concentration results in an abrupt change of the transition temperature. The slope of the $\text{T}{}_{\mathrm{<mtext>t</mtext>}}$ vs. pH diagram beyond $\text{p}\text{K}{}_2$ becomes very steep. This is the