Fluorescence lifetime and time-resolved polarization anisotropy studies of acyl chain order and dynamics in lipid bilayers.

The time-resolved fluorescence intensity and anisotropy decays of cis- and trans-parinaric acids and phosphatidylcholines labeled with trans-parinaric acid have been characterized in bilayers formed by several phosphatidylcholines and by dipalmitoylphosphatidylcholine-cholesterol mixtures, at several temperatures. Both a conventional free-running nitrogen flashlamp and the novel synchrotron source at the Stanford Linear Accelerator Center (SLAC) were used as excitation sources for a modified single photon counting fluorescence lifetime apparatus. The measured emission decay kinetics of both isomers of parinaric acid were biexponential in all but one of the lipid systems examined. The fluorescence anisotropy of parinaric acid was large and constant in gel phase lipids, but showed a very rapid (approximately 2 GHz) decay of large amplitude in fluid lipids. In all lipid systems studied, the fluorescence anisotropy decayed to a nonzero asymptote, in striking contrast to the behavior observed in viscous solvent solutions. The asymptotic anisotropy was used to calculate an "order parameter" of the emission transition dipole. The value of the order parameter is quite close to that obtained by deuterium NMR. Cholesterol increased the order parameter measured in fluid dipalmitoylphosphatidylcholine but did not substantially affect the rate of angular relaxation. Experiments conducted with trans-parinaroylphosphatidylcholines yielded results virtually identical with those obtained with trans-parinaric acid.