Curvature and Bending Constants for Phosphatidylserine-Containing Membranes

Abstract

Phosphatidylserine (PS), an anionic phospholipid of significant biological relevance, forms a multilamellar phase in water with net negative surface charge at pH 7.0. In this study we mixed dioleoylPS (DOPS) with reverse hexagonal (HII)-forming phosphatidylethanolamine (DOPE), and used x-ray diffraction and osmotic stress to quantify its spontaneous curvature (1/R0p) and bending modulus (Kcp). The mixtures were stable HII phases from 5 to 30 mol% PS, providing 16 wt% tetradecane (td) was also added to relieve chain-packing stress. The fully hydrated lattice dimension increased with DOPS concentration. Analysis of structural changes gave an apparent R0p for DOPS of +144Å; opposite in sign and relatively flat compared to DOPE (−30Å). Osmotic stress of the HII phases did not detect a significantly different bending modulus (Kcp) for DOPS as compared to DOPE. At pH≤4.0, DOPS (with no td) adopted the HII phase on its own, in agreement with previous results, suggesting a reversal in curvature upon protonation of the serine headgroup. In contrast, when td was present, DOPS/td formed a lamellar phase of limited swelling whose dimension increased with pH. DOPS/DOPE/td mixtures formed HII phases whose dimension increased both with pH and with DOPS content. With tetradecane, estimates put 1/R0p for DOPS at pH 2.1 at zero. Tetradecane apparently affects the degree of dissociation of DOPS at low pH.