Partitional and motional properties of a spin-labeled daunomycin in lipid bilayers. An ESR study

Quantifying molecular partition into model systems of biomembranes: an emphasis on optical spectroscopic methods

Optical spectroscopies have been intensively used to determine partition coefficients by a plethora of methodologies. The present review is intended to give detailed and useful information for the determination of partition coefficients and addresses several relevant aspects, namely: (i) definition and calculation of the partition coefficient between aqueous and lipidic phases; (ii) partition coefficients vs. "binding" formalisms; (iii) advantages of spectroscopic methodologies over separation techniques; (iv) formalisms for various experimental approaches based on UV-Vis absorption or fluorescence parameters (fluorescence intensity, lifetime, anisotropy and quenching); (v) experimental hints, artifacts and model limitations; and (vi) a brief survey of nonoptical techniques.

Partitioning and structural effects of the antitumor drug daunomycin on model membranes

The effects of the antitumor drug daunomycin on the phase transition and dynamic properties of phosphatidylcholine membranes were investigated using the electron paramagenetic resonance spin labeling method. Multilamellar liposomes made of saturated dimyristoylphosphatidylcholine and unsaturated egg yolk phosphatidylcholine were used. The main phase transition of saturated bilayer was significantly broadened in the presence of daunomycin. In the fluid phase of saturated membranes, daunomycin caused a decrease in the rotational motion of the spin probe 16-doxylstearic acid (16-SASL). This effect was strongly diminished by raising the temperature. In unsaturated membranes no influence of daunomycin on the rotational motion of 16-SASL was observed. It is proposed that the neutral form of daunomycin can partition into lipid bilayer where it can diffuse into deeper hydrophobic regions of the membrane and decrease the motion of alkyl chains.

Partitioning and localization of spin-labeled amantadine in lipid bilayers: an EPR study

EPR was used to study the distribution of the spin-labeled amantadine (AA-SL) between the bulk hydrophobic-hydrocarbon solvent, light paraffin oil, and water and between hydrophobic-hydrocarbon chain region of lipid membranes and water. The AA-SL molecules were soluble in both hydrophobic and polar regions of investigated systems. It was shown that the partition coefficient of AA-SL between the hydrocarbon region of the L-alpha-dimyristoylphosphatidylcholine fluid-phase membrane and water is much higher than between bulk hydrocarbon solvent and water. Furthermore, the partitioning of AA-SL into membranes of multilamellar liposomes made of L-alpha-dimyristoylphosphatidylcholine, L-alpha-dipalmitoylphosphatidylcholine, and L-alpha-distearoylphosphatidylcholine was studied as a function of temperature, indicating no abrupt change at the main phase transition of these membranes. It is also clear from our data that AA-SL can penetrate into the gel-phase membrane practically with the same partitioning as into the fluid-phase membrane. Furthermore, it was shown that at least part of the AA-SL molecule is deeply buried in the hydrocarbon chain region of the membrane.

Effects of probucol on phase transition and fluidity of phosphatidylcholine membranes: a spin label study

Spin labeling methods were applied to study the structure and dynamics of phosphatidylcholine membranes as a function of temperature and the mole fraction of probucol. Multilamellar liposomes made of dimyristoylphosphatidyclcholine, dipalmitoylphosphatidylcholine both saturated, and egg yolk phosphatidylcholine, an unsaturated membrane, were used. In fluid phase membranes probucol was found to increase the order and decrease the motional freedom of alkyl chains of lipids as shown with stearic acid spin labels. The effect of probucol on order and motional freedom is more pronounced in the membrane center (16-doxylstearic acid spin label position) than in the near polar headgroup region (5-doxylstearic acid spin label position). The presence of unsaturation in alkyl chains significantly decreased the ordering effect of probucol. The main phase transition temperature of saturated bilayers was lowered by 2 degrees C in the presence of 3 mol% of probucol and significantly broadened at higher concentrations as measured with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) partitioning. Also, pretransition was no longer observed in the presence of probucol. In gel phase membranes, the effect of probucol was complex. Close to the main phase transition the motion of alkyl chains was increased, showing a regulatory effect of probucol on membrane fluidity. It is proposed that probucol is located in the membrane center as opposed to vitamin E, which locates its phenolic -OH group at the membrane surface; therefore, it inhibits lipid peroxidation in this region which is less accessible to vitamin E.

Transport of spin-labeled tetracycline across model and biological membranes

Electron spin resonance spectra of spin labeled tetracycline (TC-SL) do not show any recognizable partitioning into a lipid bilayer or bulk hydrocarbon solvent, paraffin oil. TC-SL, however, penetrates through the model and biological membranes. It is shown that the rate of permeation depends on membrane composition and increases with temperature. In fluid phase, the rate is greater for saturated dimyristoylphosphatidylcholine than for unsaturated egg yolk phosphatidylcholine membranes. Cholesterol significantly decreases the rate, 30 mol% cholesterol decreases the rate of TC-SL permeation across egg yolk phosphatidylcholine bilayer 8 times at 37 degrees C. The rate of permeation of TC-SL across model membranes is much smaller than the rate for TEMPONE--a compound which slightly partitions into lipid bilayer, and much greater than the rate for TEMPO--choline-a positively charged compound. After addition to the suspension of Ehrlich's ascites tumor cells, the TC-SL is reduced to a non-paramagnetic form. Reduction rate is independent of oxygen concentrations, which led us to suggest that the permeation of TC-SL across the cell plasma membrane is the limiting step of reduction reaction.