Effect of lipid molecular structure and gramicidin A on the core of lipid vesicle bilayers. A time-resolved fluorescence depolarization study.

We have investigated the molecular orientational order and reorientational dynamics of the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) in the core of the membrane bilayer. Vesicles of lipids of varying unsaturation and headgroup (POPC, DOPC, DLPC, DLLPC, EGGPG, DOPG, DGDG, and SQDG) were studied using the time-resolved fluorescence anisotropy of DPH. Generally, values of the second order parameter <P2> for DPH are found to be very small. However, this should not be interpreted as DPH having low orientational order as witnessed by large values of the next relevant order parameter <P4>. This implies considerable transverse populations of DPH molecules within the bilayer. In phosphatidylcholines with an acyl chain of 18 carbon atoms, the value of <P2> for DPH decreases with increasing lipid unsaturation and even attains negative values. No effect of the lipid headgroup on the order and dynamics of DPH is detected. Furthermore, we study the peptide-lipid interaction of the hydrophobic antibiotic gramicidin A (gA) in DOPC vesicles using DPH. The nonchannel conformation has an ordering effect on DPH in the bilayer core, which the channel confirmation lacks. This can be understood in terms of the geometrical shape of the gA dimer, as shown previously with the probes TMA-DPH and DPHPC [Muller, J. M., et al. (1995) Biochemistry 34, 3092]. We find that for DPH data the conventional Brownian rotational diffusion (BRD) model and the compound motion model (CMM) give equivalent fits. In this respect, DPH differs from TMA-DPH and DPHPC, for which probes only the CMM allowed a consistent interpretation of the molecular orientation.