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 reorientati onal dynamics of the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene ( DPH) in the core of the membrane bilayer. Vesicles of Lipids of varyin g unsaturation and headgroup (POPC, DOPC, DLPC, DLLPC, EGGPG, DOPG, DG DG, and SQDG) were studied using the time-resolved fluorescence anisot ropy of DPH. Generally, values of the second order parameter [P-2] for DPH are found to be very small. However, this should not be interpret ed as DPH having low orientational order as witnessed by large values of the next relevant order parameter [P-4]. This implies considerable transverse populations of DPH molecules within the bilayer. In phospha tidylcholines with an acyl chain of 18 carbon atoms, the value of [P-2 ] for DPH decreases with increasing lipid unsaturation and even attain s negative values. No effect of the lipid headgroup on the order and d ynamics of DPH is detected. Furthermore, we study the peptide-lipid in teraction of the hydrophobic antibiotic gramicidin A (gA) in DOPC vesi cles using DPH. The nonchannel conformation has an ordering effect on DPH in the bilayer core, which the channel conformation lacks. This ca n 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 conve ntional Brownian rotational diffusion (BRD) model and the compound mot ion model (CMM) give equivalent fits. In this respect, DPH differs fro m TMA-DPH and DPHPC, for which probes only the CMM allowed a consisten t interpretation of the molecular orientation.