The polymorphic phase behaviour of mixed phosphatidylserine-phosphatidylethanolamine model systems as detected by 31P-NMR.

1. The influence of divalent cations and pH on the polymorphic phase behaviour of aqueous dispersions of phosphatidylethanolamine-phosphatidylserine systems have been investigated employing 31P-NMR techniques. 2. Phosphatidylserines, derived from both egg and soya phosphatidylcholines, stabilize a bilayer organization at 30 degrees C in mixtures with soya phosphatidylethanolamine (which assumes the hexagonal (HII) phase on hydration) when the phosphatidylserine constitutes 15 mol% or more of the phospholipid. 3. The addition of Ca2+ to equimolar soya phosphatidylserine/soya phosphatidylethanolamine mixtures triggers complete HII phase formation as detected by 31P-NMR at Ca2+:phosphatidylserine ratios, R, of 1.0 or larger. In contrast, Mg2+ is ineffective even at Mg2+:phosphatidylserine ratios of 10.0. In mixtures containing 15 mol% phosphatidylserine, Ca2+ triggers HII phase formation at R = 0.25. The Ca2+-induced polymorphic phase transitions appear to occur as a result of a structural segregation of phosphatidylserine by Ca2+ into crystalline domains, leaving the phosphatidylethanolamine free to adopt the HII phase it prefers in isolation. 4. The polymorphism of soya phosphatidylserine/soya phosphatidylethanolamine systems is markedly sensitive to the pH of the aqueous medium. At 30 degrees C equimolar mixtures exhibit a bilayer-HII transition as the pH is decreased below 4.0, whereas mixtures containing 15 mol% phosphatidylserine exhibit detectable HII phase structure at pH values below 5.5. 5. 31P-NMR studies suggest that the binding of Ca2+ to phosphatidylserine to produce crystalline structures is sensitive to the unsaturation of the acyl chains, with more unsaturated species requiring higher Ca2+:phosphatidylserine ratios for formation of crystalline Ca2+-phospholipid complexes. Studies of the binding of Ca2+ with soya phosphatidylserine indicate half maximal binding at 0.3 mM in the absence of salt, which is increased to approx. 0.8 mM in the presence of 100 mM NaCl. 6. These results suggest that the effectiveness of phosphatidylserine as a bilayer-stabilizing agent can be modulated by local changes in such biologically relevant parameters as pH, ionic strength and/or cation concentrations, and are discussed in relation to membrane fusion processes.