Transport of oppositely charged lipophilic probe ions in lipid bilayer membranes having various structures

Summary

A comparative study of the charge transport kinetics of oppositely charged lipophilic probe ions in lipid bilayer membranes of varying composition was carried out by using the charge pulse technique. The ions investigated were the chemical analogs tetraphenylborate, tetraphenylarsonium and tetraphenylphosphonium. Membrane structural aspects investigated were the type of solvent used in membrane formation, sterol content, and the nature of the principal lipid. The overall results indicate that the character of the transport process involving positive lipophilic probes is, in contrast to positively charged carrier complexes, very similar to that deduced in previous studies of negative lipophilic ions. The major effect on transport of lipophilic ions of both signs using differentn-alkane solvents appears to be due to changes in the thickness of the membrane hydrocarbon region. Positive ion transport is relatively sensitive to the inclusion of sterols of several types in both monoolein and lecithin membranes, as compared with negative ion transport, suggesting that a combination of sterol-induced dipolar field and fluidity changes are involved. Results involving several variations in lipid structure, with the possible exception of hydrocarbon tail saturation, when interpreted in terms of dipolar field changes deduced under the assumption of charge independent fluidity effects, are consistent with monolayer surface potential measurements.