Curvature and bending constants for phosphatidylserine-containing membranes

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 (H(II))-forming phosphatidylethanolamine (DOPE), and used x-ray diffraction and osmotic stress to quantify its spontaneous curvature (1/R(0p)) and bending modulus (K(cp)). The mixtures were stable H(II) 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 R(0p) for DOPS of +144 A; opposite in sign and relatively flat compared to DOPE (-30 A). Osmotic stress of the H(II) phases did not detect a significantly different bending modulus (K(cp)) for DOPS as compared to DOPE. At pH < or = 4.0, DOPS (with no td) adopted the H(II) 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 H(II) phases whose dimension increased both with pH and with DOPS content. With tetradecane, estimates put 1/R(0p) for DOPS at pH 2.1 at zero. Tetradecane apparently affects the degree of dissociation of DOPS at low pH.