Abstract
In this paper we report an X-ray diffraction study on the phase behavior of binary lipid mixtures of 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphoethanolamine (DHA-PE) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at low concentrations below 5.0 mol% DHA-PE. Our results show that DHA-PE induces phase separation into a DHA rich liquid crystalline (Lα) phase and a DHA poor gel (Lβ′) phase at overall DHA-PE concentrations as low as 0.1 mol%. In addition, we find that the structure of the Lβ′ phase, from which the DHA-PE molecules are largely excluded, is modified in the phase-separated state at low DHA-PE concentrations, with a decrease in bilayer thickness of 1.34 nm for 0.1 mol% at room temperature, compared to pure DPPC bilayers. This result is contrary to that seen in similar studies on mono-unsaturated lipids where an increase in bilayer thickness is observed. The surprising effect of such low DHA-PE concentrations on membrane structure may be important in understanding the role of highly polyunsaturated lipids in biological membrane-based structures and similar artificial surfactant systems.
Highlights
Since early epidemiological studies of the Eskimo diet linked docosahexaenoic acid (DHA) content [1]to cardiac health, polyunsaturated fatty acids have attracted a great deal of attention in the physiological, nutritional and biomedical communities
X-ray diffraction is an excellent technique for characterizing lipid membranes and in this paper, we present data for lipid membranes prepared from the binary mixture DPPC/DHA-PE
Pure DHA-PE has a gel to fluid phase transition (Tm) at −27 °C in contrast to that of DPPC at 41 °C
Summary
Since early epidemiological studies of the Eskimo diet linked docosahexaenoic acid (DHA) content [1]. DHA’s interaction with other lipids in model systems was previously studied at relatively high concentrations by Stillwell and Wassall [30,31,32] In these studies, phase separation was indicated in mixtures of DHA and sphingomyelin at 10 mol%. The TBA assay has been used in many research studies to measure lipid oxidation in cells [36,37] One drawback of this technique is that it is specific to lipid as there are other methods to form malondialdehydes. Fundamental research into the physical interactions between DHA containing lipids and the other lipids in the cell membrane is still needed, in relation to phase separation effects and their relevance to lipid domain formation. In this work we focus on the lower concentration limit of DHA membrane content and the effects of this molecule on membrane phase behavior in this regime
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