Effects of oxidized lipids on the structure of the lipid bilayer

The mixing behavior of the membrane phospholipid, 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and the oxidized phospholipid (OxPL), 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine (PGPC), was investigated to understand the biophysical effects of OxPLs on the lipid bilayer. Due to the structural differences between the membrane phospholipid DPPC and the OxPL PGPC, their mixing changes the curvature of the membrane, which causes the free energy of the system to change. We characterized the resulting states of lipid organization by using fluorescence quenching assays and electron paramagnetic resonance spectroscopy methods. In the gel phase, PGPC and DPPC mix in-homogeneously with PGPC-rich domains dispersed within the DPPC bilayer. As temperature increases and DPPC enters the liquid phase, the two-component lipid system exhibits a phase separation in which PGPC segregates in hemi-fused micelles on the lipid bilayer and eventually completely separates from the vesicle. A critical composition of mixing was at XPGPC = 0.2, below and at which DPPC and PGPC eventually mix homogeneously in a single vesicle and above which the two lipids co-exist as separate DPPC vesicles and PGPC micelles.