In addition, PS interacts with other components of the mammalian membrane such as sterols, cholesterol being the most important representative. It is known to be involved in physiologically important processes, such as membrane fusion ( Papahadjopoulos et al., 1974) and activation of various phospholipases ( Buckland and Wilton, 2000), protein kinase C ( Nishizuka, 1984), and components of the blood coagulation process ( Lentz, 1999). Phosphatidylserine (PS) is one of the most prevalent naturally occurring negatively charged membrane phospholipids. Tetradecane apparently affects the degree of dissociation of DOPS at low pH. With tetradecane, estimates put 1/ R 0p for DOPS at pH 2.1 at zero. DOPS/DOPE/td mixtures formed H II phases whose dimension increased both with pH and with DOPS content. In contrast, when td was present, DOPS/td formed a lamellar phase of limited swelling whose dimension increased with pH. At pH ≤ 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. Osmotic stress of the H II phases did not detect a significantly different bending modulus ( K cp) for DOPS as compared to DOPE. Analysis of structural changes gave an apparent R 0p for DOPS of +144 Å opposite in sign and relatively flat compared to DOPE (−30 Å). The fully hydrated lattice dimension increased with DOPS concentration. 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. 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).
Phosphatidylserine (PS), an anionic phospholipid of significant biological relevance, forms a multilamellar phase in water with net negative surface charge at pH 7.0.
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