Metalloporphyrin intercalation in liposome membranes: ESR study

Liposomes characterized by membranes featuring diverse fluidity (liquid-crystalline and/or gel phase), prepared from egg yolk lecithin (EYL) and dipalmitoylphosphatidylcholine (DPPC), were doped with selected metalloporphyrins and the time-related structural and dynamic changes within the lipid double layer were investigated. Porphyrin complexes of Mg(II), Mn(III), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), and the metal-free base were embedded into the particular liposome systems and tested for 350 h at 24°C using the electron spin resonance (ESR) spin probe technique. 5-DOXYL, 12-DOXYL, and 16-DOXYL stearic acid methyl ester spin labels were applied to explore the interior of the lipid bilayer. Only the 16-DOXYL spin probe detected evident structural changes inside the lipid system due to porphyrin intercalation. Fluidity of the lipid system and the type of the porphyrin complex introduced significantly affected the intermolecular interactions, which in certain cases may result in self-assembly of metalloporphyrin molecules within the liposome membrane, reflected in the presence of new lines in the relevant ESR spectra. The most pronounced time-related effects were demonstrated by the EYL liposomes (liquid-crystalline phase) when doped with Mg and Co porphyrins, whereas practically no spectral changes were revealed for the metal-free base and both the Ni and Zn dopants. ESR spectra of the porphyrin-doped gel phase of DPPC liposomes did not show any extra lines; however, they indicated the formation of a more rigid lipid medium. Electronic configuration of the porphyrin’s metal center appeared crucial to the degree of molecular reorganization within the phospholipid bilayer system

Pore formation in BLM studied by means of alternate current

Porin form pores which act as molecular sieves to allow the diffusion of small, hydrophilic solutes across the outer membrane of gram-negative bacteria and mitochondria. The addition of porin to the bathing solutions of a BLM results in the formation of ion permeable pores. Pore incorporation kinetic, as detected measuring electrical membrane current, is S-shaped and a steady state is reached after many minutes depending on the porin concentration, the nature of BLM and the composition of the media. Porin incorporation kinetic was analyzed considering the phenomenon studied by means of alternate current. The membrane equivalent circuit, constituted by a parallel resistance and capacitance, was in series with an analogous circuit of the measuring device and the input voltage was applied to the whole series. The pore dependence of membrane conductance and capacitance was determined from geometrical considerations and single channel electrical characteristics. A non linear decreasing of membrane resistance, a linear decreasing of capacitance, and then a non linear increasing of reactance, are obtained increasing the number of the pores. Analytical time dependence of pore formation was introduced with a four-parameter logistic equation or some exponential functions. A set of differential equations describes the circuit. The current depends not only on the membrane resistance and capacitance but, during kinetic phase, also on the time derivative of the capacitance. Because of this time dependence of membrane electrical parameters the phase angle varies during channel formation

Interaction between model membranes and a new class of surfactants with antioxidant function.

The effect of two series of amphiphilic quaternary ammonium salts on some properties of phospholipid membranes was studied. The compounds of one series, N-benzyl-N,N-dimethyl-N-alkyl ammonium bromides, exert a destructive effect on membranes and are treated as reference compounds. The compounds of the other series, N-(3,5-di-t-butyl-4-hydroxy)benzyl-N,N-dimethyl-N-alkyl ammonium bromides, are derivatives of the former ones, exhibit antioxidant properties, and do only relatively slight damage to the membranes. The aim of the work was to explain the difference in molecular interaction with membranes between the two kinds of hydrophobic compounds. Thermodynamic methods, a new mixing technique, and monolayer and quantum calculation methods were used. It has been shown that the antioxidant molecules are less hydrophobic than those of the reference compounds and disturb the membrane organization to a lesser extent. On the basis of monolayer data, we suggest that the studied antioxidant behaves like a substitutional impurity, whereas the reference behaves like an interstitial one