PHOSPHOLIPID LATERAL PHASE-SEPARATION AND THE PARTITION OF CIS-PARINARIC ACID AND TRANS-PARINARIC ACID AMONG AQUEOUS, SOLID LIPID, AND FLUID LIPID PHASES

The partition of cw-parinaric acid (9,11,13,15-cis,trans,trans,cis-octadecatetraenoic acid, cis-PnA) and trarns-parinaric acid (9,1 l,13,15-all-trans-octadecatetraenoic acid, trans-PnA) among aqueous, solid lipid, and fluid lipid phases has been measured by three spectroscopic parameters: absorption spectral shifts, fluorescence quantum yield, and fluorescence polarization. The solid lipid was dipalmitoyl-phosphatidylcholine (DPPC); the fluid lipid was palmitoyldocosahexaenoylphosphatidylcholine (PDPC). Mole fraction partition coefficients between lipid and water were determined by absorption spectroscopy to be, for m-PnA, 5.3 × 105 with solid lipid and 9 × 105 with fluid lipid and, for trans-PnA, 5 × 106 with solid lipid and 1.7 × 106 with fluid lipid. Ratios of the solid to the fluid partition coefficients (kps/f) are 0.6 ± 0.2 for cis-PnA and 3 ± 1 for trans-PnA. A phase diagram for codispersions of DPPC and PDPC has been constructed from the measurements of the temperature dependence of the fluorescence quantum yield and polarization of m-PnA and trans-PnA and their methyl ester derivatives. A simple analysis based on the phase diagram and fluorescence data allows additional calculations of Kps/fS which are determined to be 0.7 ± 0.2 for the cis probes and 4 ± 1 for the trans probes. The relative preference of trans-PnA for solid phase lipids and its enhanced quantum yield in solid phase lipids make it sensitive to a few percent solid. The trans probes provide evidence that structural order may persist in dispersions of these phospholipids 10 °C or more above their transition temperature. It is concluded that measurements of PnA fluorescence polarization vs. temperature are better suited than measurements of quantum yield vs. temperature for determining phospholipid phase separation. © 1979, American Chemical Society. All rights reserved.