The polymorphic phase behavior of mixed phosphatidylserine-phosphatidylethanolamine model systems as detected by 31P-NMR. Effects of divalent cations and pH

Abstract

1. 1. The influence of divalent cations and pH on the polymorphic phase behavior of aqueous dispersions of phosphatidylethanolamine-phosphatidylserine systems have been investigated employing 31P-NMR techniques. 2. 2. Phosphatidylserines, derived from both egg and soya phosphatidylcholines, stabilize a bilayer organization at 30°C in mixtures with soya phosphatidylethanolamine (which assumes the hexagonal (H II) phase on hydration) when the phosphatidylserine constitutes 15 mol% or more of the phospholipid. 3. 3. The addition of Ca 2+ to equimolar soya phosphatidylserine/soya phosphatidylethanolamine mixtures triggers complete H II phase formation as detected by 31P-NMR at Ca 2+ : phosphatidylserine ratios, R, of 1.0 or larger. In contrast, Mg 2+ is ineffective even at Mg 2+ : phosphatidylserine ratios of 10.0. In mixtures containing 15 mol% phosphatidylserine, Ca 2+ triggers H II phase formation at R = 0.25. The Ca 2+-induced polymorphic phase transitions appear to occur as a result of a structural segregation of phosphatidylserine by Ca 2+ into crystalline domains, leaving the phosphatidylethanolamine free to adopt the H II phase it prefers in isolation. 4. 4. The polymorphism of soya phosphatidylserine/soya phosphatidylethanolamine systems is markedly sensitive to the pH of the aqueous medium. At 30°C equimolar mixtures exhibit a bilayer-H II transition as the pH is decreased below