Mixing of Oxidized and Bilayer Phospholipids

Composition and phase dependence of the mixing of 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC), with the oxidized phospholipid, 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine (PGPC) were investigated by characterizing the aggregation states of DPPC/PGPC and DOPC/PGPC using a fluorescence quenching assay, dynamic light scattering, and time-resolved fluorescence quenching in the temperature range 5-60 °C. PGPC forms 3.5 nm radii micelles of aggregation number 33. In the gel phase, DPPC and PGPC fuse to form mixed vesicles for PGPC molar fraction, XPGPC ≤ 0.3 and coexisting vesicles and micelles at higher XPGPC. Data suggest that liquid phase DPPC at 50 °C forms mixed vesicles with segregated or hemi fused DPPC and PGPC for XPGPC ≤ 0.3. At 60 °C, DPPC and PGPC do not mix, but form coexisting vesicles and micelles. DOPC and PGPC do not mix in any proportion in the liquid phase. Two dissimilar aggregates of the sizes of vesicles and PGPC micelles were observed for all XPGPC for T ≥ 22 °C. DOPC-PGPC and DPPC-PGPC mixing is non-ideal for XPGPC > 0.3 in both gel and fluid phases resulting in exclusion of PGPC from the bilayer. Formation of mixed vesicles is favored in the gel phase but not in the liquid phase for XPGPC ≤ 0.3. For XPGPC ≤ 0.3, aggregation states change progressively from mixed vesicles in the gel phase to component segregated mixed vesicles in the liquid phase close to the chain melting transition temperature to separated coexisting vesicles and micelles at higher temperatures.