EVIDENCE FOR ION CHAIN MECHANISM OF THE NONLINEAR CHARGE-TRANSPORT OFHYDROPHOBIC IONS ACROSS LIPID BILAYERS

The conductivity across a lipid bilayer by tetraphenylborate anion is increased 10-fold on the photoformation of lipophilic porphyrin cation s. The cations alone have negligible conductivity. This nonlinear phot ogenerated increase of ion conductivity is termed the photogating effe ct. Substitution of H by Cl in the para position of tetraphenylborate leads to a 100-fold enhancement of conductivity, whereas the dark cond uctivities for this and other substituted berates are the same. Moreov er, the halo-substituted borates show a large enhancement of conductiv ity in the low concentration range (10(-8) M), whereas that of tetraph enylborate is small and space charge is negligible. The enhanced ion c onductivity has great structural sensitivity to the structure of the a nion, the cation, and the lipid, whereas the partition coefficient of all the berates and the concentration of photoformed cations are only slightly affected. The photogated ion transport has a twofold larger a ctivation energy than transport in the dark. Time-resolved photocurren ts and voltages demonstrate that the translocation rate of the porphyr in cation is also enhanced 100-fold by the Cl-borate anion but only 10 -fold by the H-borate anion. For these reasons the nonlinear gating ef fect cannot be explained by electrostatics alone, but requires an ion chain or ion aggregate mechanism. Kinetic modeling of the photoinduced current with a mixed cation-anion ion chain can fit the data well. Th e photogating effect allows the direct study of ion interactions withi n the bilayer.