Fluidity of Liposome Membranes Doped with Organic Tin Compounds: ESR Study

The kinetics of change in the fluidity of liposome membranes, obtained in the process of sonication of Egg Yolk Lecithin (EYL), with the admixture of organic tin compounds, was investigated. Five compounds were selected for the research: three differed in the length of hydrocarbon chains, (CH₃)₄Sn, (C₂H₅)₄Sn, and (C₃H₇)₃SnCl, whereas two differed in the number of aromatic rings, (C₆H₅)₂SnCl₂ and (C₆H₅)₃SnCl. The concentration of the compounds in proportion to EYL was 2 mol-%. Electron Spin (paramagnetic) Resonance (ESR) was applied using two spin probes TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and 16-DOXYL-stearic acid methyl ester (2-ethyl-2-(15-methoxy-15-oxopentadecyl)-4,4-dimethyl-3-oxazolidinyloxyl) localized at different sites within the membrane, to determine the spectroscopic parameters: partition (F) and rotation correlation time (τ), related to the membrane's fluidity. The ESR spectroscopic spectra of investigated samples were recorded from the moment of introducing the admixture to membranes for 180h. Analysing the obtained results, the following conclusions can be drawn: chain compounds slightly stiffened the membrane both on the inside (hydrophobic) layer and on the surface one, whereas ring compounds resulted in fluidization of the membrane—stronger in the case of the two-layer middle and weaker with reference to the surface layer.