Effect of lipid charge and solution composition on the permeability of phospholipid–gramicidin monolayers to TlI
The effect of fixed charges on membrane permeability has been addressed by an electrochemical approach using a phospholipid monolayer absorbed on an electrode. TlI transport across dioleoyl phosphatidylcholine (DOPC)–gramicidin and phosphatidylserine (PS)–gramicidin monolayers was investigated using cyclic voltammetry. The permeability of TlI in phospholipid–gramicidin monolayers adsorbed on mercury is critically dependent on the specific lipid, the electrolyte pH and the charge, concentration and charge: size ratio of the electrolyte cation. Negatively charged PS–gramicidin layers are ca. nine times more permeable to TlI than zwitterionic DOPC–gramicidin layers. The substitution of a univalent electrolyte cation by a divalent electrolyte cation and the presence of small concentrations of trivalent cations in the electrolyte reduce the permeablity of DOPC–gramicidin layers and PS–gramicidin layers but the permeability suppression is greater for PS–gramicidin layers. At solution pH values approaching the intrinsic pKa value of PS (3.6), the permeability of PS–gramicidin layers is lowered to slightly above that of DOPC–gramicidin layers. Similarly, at solution pH values of <3.5 down to and below solution pH values corresponding to the intrinsic pKa value (2.25) of DOPC, the permeability of DOPC–gramicidin layers decreases. The influence of monolayer charge on the ion transport arises because the Tl+ flux in the gramicidin channel is related to the concentration of the free ion in the region surrounding the channel mouth, which, in turn, depends on the magnitude of the surface monolayer potential. The electrolyte cation screens the surface monolayer potential to varying extents, depending on its concentration, charge and charge: size ratio, decreasing the concentration of the permeant ion in the lipid surface region. Negatively charged lipid head groups increase the surface monolayer potential thus attracting the permeant ion and increasing its concentration in the lipid surface region. Divalent and trivalent electrolyte ions are adsorbed on PS and neutralise and reverse its potential. Protonation of PS and DOPC starts at solution pH values ca. two units above the pKa values of the respective lipids, lowering and reversing the surface monolayer potential respectively. Both effects decrease the concentration of TlI at the channel mouth. Only a very small fraction of the gramicidin partitioned into the lipid layers is responsible for conducting the Tl+ ions.