Fluorescence dynamics of diphenyl-1,3,5-hexatriene-labeled phospholipids in bilayer membranes.

A comparative study of the dynamical fluorescence properties of three phosphatidylcholines having a diphenyl-1,3,5-hexatriene (DPH) group attached at different depths from the head group incorporated into membrane vesicles has been carried out. The probes were covalently attached to the sn-2 position of the glycerol part of the phosphatidylcholine via either carboxyl, ethyl or propanoyl links. The vesicles were composed of either dimyristoylphosphatidylcholine or dipalmitoylphosphatidylcholine. The experimental time-resolved polarized fluorescence data of the probes were analysed by two different methods: maximum entropy and global analysis. Distributed fluorescence lifetimes and correlation times of the DPH derivatives were obtained with the maximum entropy method. All DPH derivatives exhibited a bimodal distribution of fluorescence lifetimes with a dependence of the lifetime peak positions on the lipid phase, confirming previous data in the literature. The anisotropic rotational dynamics of the DPH moieties in the membranes could be described by several distributed correlation times. In the fluid phase of the membrane the residual anisotropy of free DPH became very small in contrast with those of the other probes, indicating that restriction of probe rotation is mainly imposed by the molecular geometry of the lipid probes. A two-dimensional analysis using the maximum entropy method demonstrated that both rotational correlation times were associated with the same set of fluorescence lifetimes. Global analysis of the data sets according to the general rotational diffusion model yielded weighted orientational distributions. Unexpectedly, a component of the DPH moiety oriented parallel to the membrane surface was obtained in the orientational distributions of the DPH lipids (as was reported earlier for DPH and TMA-DPH), which seems at variance with the geometric constraints imposed by the headgroups.