Spin labels as probes for tetraphenylboron ion interaction with liposomes

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Abstract
The effects of tetraphenylboron (TFB) on the molecular organization of lipids within phosphatidylcholine (PC) liposomes were investigated using the spin-labeling method. Perturbations at the surface of the lipid were probed using stearamide and cholestane spin labels; perturbations in the hydrophobic portion were probed with spin-labeled amphiphilic fatty esters. The spin-labeling results showed that in the concentration range 1.5 mM-10 mM of the lipid-soluble anion TFB, detectable perturbations affecting both surface and interior hydrophobic portions of the lipids were observed. For all the spin labels used, the motional correlation time (.tau.c) is independent of TFB concentration for [TFB] .gtoreq. 4 mM and the Arrhenius plots (1n .tau.c vs. 1/temperature) are parallel, with a slope independent of TFB concentration, but show a decrease of motional freedom of the labels as TFB is added (i.e., concentrations below that which causes saturation). The effect of pH on surface perturbations is small but observable and indicates that the surface of the lipid is more positively charged for pH .ltoreq. 5. TFB probably adsorbs on the surface portion of the lipids and causes the largest perturbation there. Furthermore, the sites occupied by TFB and those occupied by the labels are fairly remote from each other, and saturation of the TFB sites occurs before the local environment of the spin labels is affected. Conductance measurements on black lipid membranes were done in parallel; conductance saturation probably occurs at [TFB] .gtoreq. 3 mM for PC membranes and at [TFB] .gtoreq. 1 mM in the presence of equimolar ratio of PC and cholesterol. These measurements also showed the presence of a surface charge. For the case of TFB, conductance and spin-labeling results agree quite well.

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