A laser-T-jump study of the adsorption of dipolar molecules to planar lipid membranes. I. 2,4-dichlorophenoxyacetic acid.

The adsorption of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) as well as of other dipolar molecules to the interface of artificial lipid membranes gives rise to a change of the dipole potential between the membrane interior and water. As a consequence of the adsorption of the neutral species, the conductance of planar membranes, observed in the presence of the macrocyclic ion carriers nonactin or valinomycin, may change by many orders of magnitude. Using this effect in combination with a laser-T-jump technique, the kinetics of the adsorption process were measured and were interpreted on the basis of a Langmuir-isotherm. A partition coefficient (at small concentrations) of beta HA = 4.7 x 10(-4) cm, a rate constant of desorption kHA greater than or equal to 100 s-1 and a maximum surface density ND = 7.7 x 10(13)/cm2 were found. The concentration at half saturation is KHA = 2.7 x 10(-4) M. Using these values the membrane conductance induced by the ion carrier nonactin and the shape of the current-voltage relationship as a function of the ligand concentration in water was analyzed. A maximum dipole potential of VDmax = -239 mV and a contribution of b = 3.1 x 10(-15) V cm2 per single adsorbed 2,4-D molecule was found. 74% of the dipole potential acts on the inner membrane barrier separating the two interfacial adsorption planes of nonactin. The remainder (26%) favours interfacial complex formation between nonactin and K+ from the aqueous phase. The data hold for membranes formed from dioleoyllecithin in n-decane.