Transmembrane electrical currents of spin-labeled hydrophobic ions

When spin-labeled phosphonium ions are rapidly mixed with phospholipid vesicles, time-dependent changes in the electron paramagnetic resonance spectrum of the spin label are observed. These changes are interpreted in terms of transmembrane transport of the hydrophobic ion, and simple analysis of the data at different membrane potentials is shown to give the binding constant of the ion to both membrane surfaces, the permeability, and current-voltage relationship for the vesicle membrane in the presence of the hydrophobic ion. These results establish the time resolution for methods using the phosphonium ion as a probe of time-dependent potentials across vesicle membranes, as well as provide fundamental information regarding the binding and transport of hydrophobic cations across bilayers. This latter point is significant in view of the fact that hydrophobic cations have not been well characterized in planar bilayers due to their weak binding and low conductance.