Conjugated polyene fatty acids as fluorescent probes: synthetic phospholipid membrane studies

The preparation of polyene fatty acid membrane probes cis- and trans-parinaric acid and parinaroylphosphatidylcholines and their use in studies of several one- and two- component lipid systems are described. The fluorescence quantum yield of trans-parinaric acid in dipalmitoylphosphatidylcholine at 20 degrees C is approximately 0.3; the quantum yield in aqueous solution is negligibly small. Thermal-phase transitions in single-component phospholipid dispersions are monitored with absorption and fluorescence excitation peak position, fluorescence intensity, lifetime, and polarization. The transition temperatures observed are consistent with previous determinations. Shifts in the absorption peak position are related to the bilayer expansion as it undergoes the gel to liquid-crystalline transition, while fluorescence depolarization provides semiquantitative information concerning molecular motion of the probe in the bilayer. A long fluorescence lifetime component is observed for parinaric acid in the solid phase (up to 50 ns), and a short lifetime component is observed (ca. 5 ns) in the fluid phase of dipalmitoylphosphatidylcholine; both lifetime components are observed in the transition region. In most phospholipids, cis-parinaric acid detects the melting transition at about 1 degree C lower than trans-parinaric acid. Partitioning experiments involving mixed populations of phospholipid vesicles show that trans-parinaric acid preferentially associates with solid-phase lipids, while cis-parinaric acid shows a more equal distribution between solid and fluid lipids. The binding of cis-parinaric acid to dipalmitoylphosphatidylcholine at 25 degrees C is described as partitioning of parinaric acid between lipid vesicles and the aqueous phase with a partition coefficient of 5 X 10(5). Several rates are observed in the binding process which are interpreted as rapid outer monolayer uptake and a much slower process of interlamellar exchange. The phase diagram of the binary lipid mixture dipalmitoylphosphatidylcholine-dipalmitoylphosphatidylethanolamine has also been examined and found to be essentially identical to the one constructed using a nitroxide probe.