Study of Heterogeneous Emission of Parinaric Acid Isomers Using Multifrequency Phase Fluorometry

The fluorescence lifetimes of parinaric acid (PnA) isomers have been measured in pure solvents and in synthetic phospholipid membranes over a wide temperature range. In pure solvents, dodecane, ethanol, and cyclohexanol, the emission is well described by three exponential components of approximately 40, 12, and 2 ns. The preexponential factor of the long-lifetime component is quite small, and this component is neglected in the present study. The preexponential factors of the shorter lifetime components are strongly temperature dependent. Both cis- and trans-PnA show an inversion of the fractional contribution of these two components in a naroow temperature range. The inversion temperature is lower for trans-PnA than for cis-PnA, both in ethanol and in dodecane. The different temperature behavior of the decay of the two PnA isomers can influence their behavior in phospholipids. In L-α-dimyristoylphosphatidylcholine (DMPC) and L-α-dipalmitoylphosphatidylcholine (DPPC) membranes, the emission is more complex, and more than three components are necessary to describe the fluorescence decay accurately. On the basis of their different behavior in pure solvents, we anticipate that cis-PnA will detect the phospholipid transition in DMPC and DPPC at lower temperature. In fact, this effect has been observed, but it has been attributed to different partitioning of the two isomers in the phospholipid phases rather than to their different photophysics as we suggest here. © 1984, American Chemical Society. All rights reserved