Wave-guide spectroscopy on planar lipid bilayers doped with hydrophobic ions

Wave-guide spectroscopy exploits the light pipe properties of planar lipid bilayers by propagating a light wave along the plane of the bilayer. Applying this technique to the optical absorption of chromophore in the membrane, results in an enhanced sensitivity when compared to normal incidence spectroscopy. This gain factor is of the order of 100 per mm optical path along the bilayer, thus transforming the weak absorbances in lipid membranes into easily measurable quantities. Wave-guide spectroscopy has been used to measure the adsorption isotherm of hydrophobic dipicrylamine ions in a phosphatidylcholine membrane. The adsorption isotherm is linear for low aqueous concentrations, in the micromolar range however, it changes into a sublinear dependence. The addition of an inert alkali salt to the electrolyte favours the adsorption of hydrophobic ions. Current saturation is observed with the transition to the sublinear isotherm. When using the time constant for current relaxation as an indicator of changes in the magnitude of the surface potential, it does not seem to vary with the additional dipicrylamine which adsorbs in the presence of high concentrations of alkali salt in the electrolyte. A compensation of hydrophobic charge by the alkali ions from the inert electrolyte is proposed.