Study of the phase behaviour of fully hydrated saturated diacyl phosphatidylserine fatty acid mixtures with P-31-NMR and calorimetry

We investigated the phase behaviour of saturated phosphatidylserine/fatty acid mixtures in excess water and at acidic pH with P-31-NMR and differential scanning calorimetry (DSC). The pure PS in excess water yielded a bilayer structure over a large temperature range. The chain-melting transition temperature for the pure PS system increased with decreasing pH. In the acidic environment, the PS displays a co-existence of the bilayer and an isotropic phase, probably an inverse bicontinuous cubic phase. Similar to PC/FA mixtures, the PS/FA 1:2 (mol/mol) mixtures showed an increased chain-melting phase transition temperature compared to the pure PS samples. In contrast to PC/FA 1:2 (mol/mol) mixtures which give rise to a phase transition of gel lamellar-to-inverted hexagonal upon heating, the PS/FA 1:2 (mol/mol) mixtures showed that a bilayer structure was maintained in excess water over a large temperature range. The anionic nature of PS plays a crucial role in stabilizing the bilayer structure of the PS/FA 1:2 mixtures. At acidic pH's, the PS/FA 1:2 mixtures showed an isotropic phase together with the bilayer phase. Thus, decreasing the pH tends to destabilize the PS bilayer. The presence of FA also increased the tendency to destabilize the PS bilayer and drove the phase towards the inverted topology. (C) 1997 Elsevier Science Ireland Ltd.