Transport kinetics of hydrophobic ions in lipid bilayer membranes. Charge-pulse relaxation studies

A modified version of the charge-pulse relaxation technique with improved time resolution was applied to the study of transport kinetics of hydrophobic ions (tetraphenylborate, dipicrylamine) through lipid bilayer membranes. Besides a better time resolution the charge-pulse method has the additional advantage that the perturbation of the membrane can be kept small (voltage amplitudes between 1 and 10 mV). The results of the analysis support the model proposed earlier, according to which the overall transport takes place in three consecutive steps, adsorption of the ion from water to the interface, translocation to the opposite interface, and desorption into the aqueous phase. The translocation rate constant ki and the partition coefficient gamma of the hydrophobic ion between water and the membrane were measured for lecithins with different mono-unsaturated fatty acid residues. Increasing the chain length of the fatty acid from C16 to C24 resulted in a decrease of ki by a factor of about 9 in the case of tetraphenylborate and by a factor of about 17 in the case of dipicrylamine.