Docosahexaenoic Acid Affects Gel Phase by Increasing Tilt Angle

Abstract

The physical properties of docosahexasenoic acid (DHA) have dramatic effects on lipid bilayer phase behavior which could be a link to beneficial health. Its highly unsaturated structure makes fatty acid highly flexible and with a low transition temperature (Tm) of −80°C. In this investigation, we explore a binary system composed of the saturated lipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-dipalmitoyl-2-docohaxeanoyl-sn-glycero-3-phosphoethanolamine (DHA-PE, 22:6) using small angle x-ray scattering (SAXS) and atomic force microscopy (AFM). Solution SAXS is a powerful tool for structural characterization and was used to identify the different phases (Lβ' and Lα) co-existing within the membrane. In a complementary study, AFM was used to investigate the morphology of coexisting domains. In a surprising result, we observed that the DHA-PE lipid used in this study induced phase separation at concentrations as low as 0.25mol% into a DHA-PE rich liquid crystalline (Lα) phase and a DHA-PE poor gel (Lβ') phase. Also unexpectedly the thickness of the gel phase is thinner than the fluid phase as shown in the d-spacing and electron density profile. We anticipated that DHA-PE would remove or decrease the angle of tilted Lβ' phase that is composed of high concentration of DPPC, but instead it does the opposite and increase the tilt angle. However, DHA-PE still maintain its thermal property and does lower the Tm of the Lβ' phase.