Effect of gramicidin A on structure and dynamics of lipid vesicle bilayers. A time-resolved fluorescence...

We investigated the effects of the hydrophobic small peptide antibiotic gramicidin A (gA) on the properties of vesicle bilayers in the liquid crystalline state. Time-resolved fluorescence anisotropy experiments were performed with unilamellar vesicles of the lipids DMPC, POPC, DOPC, EGGPC, DLPC, DOPG, and SQDG containing various concentrations of gA in two different conformations using TMA-DPH and DPHPC as fluorescent probes. These analogues of DPH were taken to study the gA induced change in the structural and dynamical properties of the lipid bilayer in different portions of the hydrophobic region. The time-resolved anisotropy data were analyzed using the recently introduced compound motion model [van der Sijs, D. A., et al. (1993) Chem. Phys. Lett. 216, 559; Muller, J. M., et al. (1994) Chem. Phys. 185, 393]. In general, gA raises the order and reduces the rotational diffusion coefficient for the probes in the bilayer. In DOPC vesicles this ordering effect of gA on the bilayer is found to depend on both the conformation of the peptide and the depth in the bilayer at which the order is probed. This significant effect of gA conformation on the lipid order parameter profile suggests that the shape of the gA dimer in the bilayer, which is determined by its conformation, affects the order of the adjacent DOPC lipid acyl chains.