Phospholipid lateral phase separation and the partition of cis-parinaric acid and trans-parinaric acid among aqueous, solid lipid, and fluid lipid phases

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Abstract
The partition of cis-parinaric acid (9,11,13,15-cis,trans,trans,cis-octadecatetraenoic acid, cis-PnA) and trans-parinaric acid (9,11,13,15-all-trans-octadecatetraenoic acid, trans-PnA) among aqueous, solid lipid, and fluid lipid phases was measured by 3 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 determined by absorption spectroscopy were for cis-PnA, 5.3 .times. 105 with solid lipid and 9 .times. 105 with fluid lipid and for trans-PnA, 5 .times. 106 with solid lipid and 1.7 .times. 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 was constructed from the measurements of the temperature dependence of the fluorescence quantum yield and polarization of cis-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/f,s which are 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 its sensitive to a few percent solid. The trans probes provide evidence that structural order may persist in dispersions of these phospholipids 10.degree. C or more above their transition temperature. Measurements of PnA fluorescence polarization vs. temperature are apparently better suited than measurements of quantum yield vs. temperature for determining phospholipid phase separation.