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 at tached at different depths from the head group incorporated into membr ane 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 compo sed of either dimyristoylphosphatidylcholine or dipalmitoylphosphatidy lcholine. The experimental time-resolved polarized fluorescence data o f the probes were analysed by two different methods: maximum entropy a nd global analysis. Distributed fluorescence lifetimes and correlation times of the DPH derivatives were obtained with the maximum entropy m ethod. All DPH derivatives exhibited a bimodal distribution of fluores cence lifetimes with a dependence of the lifetime peak positions on th e lipid phase, confirming previous data in the literature. The anisotr opic rotational dynamics of the DPH moieties in the membranes could be described by several distributed correlation times. In the fluid phas e of the membrane the residual anisotropy of free DPH became very smal l in contrast with those of the other probes, indicating that restrict ion of probe rotation is mainly imposed by the molecular geometry of t he lipid probes. A two-dimensional analysis using the maximum entropy method demonstrated that both rotational correlation times were associ ated with the same set of fluorescence lifetimes. Global analysis of t he data sets according to the general rotational diffusion model yield ed 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 geometr ic constraints imposed by the headgroups.