Effect of gramicidin addition upon the physicochemical properties of dipalmitoyl phosphatidyl choline large unilamellar vesicles

Interaction of gramicidin with DPPC/DODAB bilayer fragments

The interaction between the antimicrobial peptide gramicidin (Gr) and dipalmitoylphosphatidylcholine (DPPC)/dioctadecyldimethylammonium bromide (DODAB) 1:1 large unilamellar vesicles (LVs) or bilayer fragments (BFs) was evaluated by means of several techniques. The major methods were: 1) Gr intrinsic fluorescence and circular dichroism (CD) spectroscopy; 2) dynamic light scattering for sizing and zeta-potential analysis; 3) determination of the bilayer phase transition from extrinsic fluorescence of bilayer probes; 4) pictures of the dispersions for evaluation of coloidal stability over a range of time and NaCl concentration. For Gr in LVs, the Gr dimeric channel conformation is suggested from: 1) CD and intrinsic fluorescence spectra similar to those in trifluoroethanol (TFE); 2) KCl or glucose permeation through the LVs/Gr bilayer. For Gr in BFs, the intertwined dimeric, non-channel Gr conformation is evidenced by CD and intrinsic fluorescence spectra similar to those in ethanol. Both LVs and BFs shield Gr tryptophans against quenching by acrylamide but the Stern-Volmer quenching constant was slightly higher for Gr in BFs confirming that the peptide is more exposed to the water phase in BFs than in LVs. The DPPC/DODAB/Gr supramolecular assemblies may predict the behavior of other antimicrobial peptides in assemblies with lipids.

Study of Singlet Oxygen Equilibrium in Dioctadecyldimethylammonium Chloride Vesicles Employing 2-(n-(N,N,N-trimethylamine)-n-alkyl)- 5-alkylfuryl Halides†

Steady state photolysis and time resolved near infrared luminescence detection were employed to study the reaction kinetics of singlet oxygen with three different lipid-soluble probes incorporated in large unilamellar dioctadecyldimethylammonium chloride (DODAC) vesicles. The probes: 2-(4-(N,N,N-trimethylamine)-butyl)-5-dodecylfuryl bromide (DFTA), 2-(12-(N,N,N-trimethylamine)-dodecyl)-5-hexylfuryl bromide (HFDA) and 2-(1-(N,N,N-trimethylamine)-methyl)-5-methylfuryl iodide (MFMA) are useful in studying both singlet oxygen dynamics and its equilibrium in microcompartmentalized systems because they are actinometers in lipidic microphases. These probes contain a reactive furan ring, which will be located at different depths in the bilayer of DODAC vesicles. In the limit of the approximations, the result indicates an inhomogeneous equilibrium distribution of singlet oxygen across the bilayer. The calculated mean partitioning constant of singlet oxygen equals 2.8 and 8.3 at 20 degrees C and 40 degrees C, respectively, in the order of the previously reported constants for other microorganized systems such as sodium dodecylsulfate and cetyltrimethylammonium halide micelles and water/oil microemulsions.

Peroxyl radicals promoted changes in water permeability through gramicidin channels in DPPC and lecithin-PC vesicles

Gramicidin incorporation to DPPC or lecithin-PC large unilamellar vesicles (LUVs) leads to pore formation that, under hyper-osmotic conditions, produces a noticeable increase in the rate of trans-membrane water flow. This pore formation is more efficient in the more fluid lecithin-PC LUVs. Exposure of these vesicles to peroxyl radicals generated in the aerobic thermolysis of 2,2'-azo-bis(2-amidinopropane) (AAPH), changes the physical properties of the bilayer (as sensed employing fluorescent probes), modifies gramicidin molecules (as sensed by the decrease in Trp fluorescence) and notably reduces the transbilayer rate of water outflow. In order to evaluate if this reduced water-transport capacity is due to changes in the membrane due to lipid-peroxidation and/or direct damage to gramicidin channels, results obtained in the oxidable vesicles (lecithin-PC) were compared to those obtained in DPPC vesicles. The data obtained show that most of the water transport efficiency loss can be ascribed to a direct disruption of gramicidin channels by AAPH derived peroxyl radicals.