Transport kinetics of dipicrylamine through lipid bilayer membranes. Effects of membrane structure

Abstract

Charge-pulse current-relaxation studies have been performed with lipid bilayer membranes in the presence of the hydrophobic ion dipicrylamine. From the analysis of the relaxation times and amplitudes the translocation rate constant k i of dipicrylamine as well as the partition coefficient β between membrane surface and water could be evaluated. In a first series of experiments membranes made from monoolein or dioleoylphosphatidylcholine in a number of different n- alkane solvents were studied, as well as virtually solvent-free bilayer membranes made from monolayers. The thickness d of the hydrocarbon layer of these membranes varied between 5.0 and 2.5 nm. While β was almost insensitive to variations in d, a strong decrease of k i with increasing membrane thickness was found; the observed dependence of k i on d approximately agreed with the theoretically expected influence of membrane thickness on the height of the dielectric barrier. No specific differences between Mueller-Rudin films and solvent-free (Montal-Mueller) membranes other than differences in thickness were found. In a further series of experiments the chemical structure of the lipid was systematically varied (number and position of double bonds in the hydrocarbon chain, nature of the polar head group). The translocation rate constant k i was much larger in phosphatidylethanolamine membranes than in phosphatidylcholine membranes. A strong increase of k i was found when the number of double bonds in the hydrocarbon chain was increased from one to three. These changes were discussed in terms of membrane fluidity and dielectric barrier height. Much higher values of k i were observed in lipids with ester linkage between hydrocarbon chain and glycerol backbone, as compared with the corresponding ether analogs. This finding is qualitatively consistent with determinations of dipolar potentials in monolayers of ester and ether lipids. When cholesterol is added to phosphatidylcholine membranes, the translocation rate constant k i increases up to five-fold, while the partition coefficient β remains virtually constant. The variation of k i in this case can be largely accounted for by a decrease in membrane thickness and a concomitant reduction in dielectric barrier height. In membranes made from the negatively charged lipid phosphatidylserine the partition coefficient of dipicrylamine strongly increased with ionic strength, as expected from the Gouy-Chapman theory of the surface potential.