Formation of inverted hexagonal phase in SDPE as observed by solid-state (31)P NMR

Docosahexaenoic acid (DHA), the longest and most unsaturated fatty acid commonly found in biological membranes, is known to affect various membrane properties. In a variety of cell membranes, DHA is primarily incorporated in phosphatidylethanolamines, where its function remains poorly understood. In order to understand the role of DHA in influencing membrane structure, we utilize (31)P NMR spectroscopy to study the phase behavior of 1-stearoyl-2-docosahexaenoyl-sn-glycerophosphoethanolamine (SDPE) in comparison to 1-palmitoyl-2-oleoyl-sn-glycerophosphoethanolamine (POPE) from 20 to 50 degrees C. Spectra of SDPE phospholipids show the formation of inverted hexagonal phase (H(II)) from 20 to 50 degrees C; in contrast, POPE mutilamellar dispersions exist in a lamellar liquid-crystalline phase (L(alpha)) at the same temperatures. The ability of SDPE to adopt nonbilayer phases at a physiological temperature may indicate its role in imparting negative curvature stress upon the membrane and may affect local molecular organization including the formation of lipid microdomains within biological membranes.