Ion channeling phenomena and Tl-upd induced film dynamics in model biomembranes studied with EQCN and QCI techniques

The electrochemical behavior of bilayer lipid films (l-α-dipalmitoyl phosphatidyl choline, DPPC) on Au electrodes was investigated using the electrochemical quartz crystal nanobalance (EQCN) and quartz crystal immittance (QCI) techniques. The bilayer DPPC film was modified with ion channel forming molecules of gramicidin (GR) permeable to Tl+ ions. The QCI measurements have shown that a DPPCBLM/GR film on a Au electrode is in the solid state at room temperature and behaves as a perfectly rigid film. In the potential step chronopiezogravimetric experiments, it has been found that Tl atoms are being deposited at the Au ∣ lipid film interface and form a Tl-upd interlayer. Sharp resonant frequency spikes, associated with Tl-upd formation and dissolution, were discovered. The evaluation of mechanistic models of the processes taking place during the negative potential step, indicates an intermediate phase formation, involving aggregation of lipid molecules in the electrode sub-surface region, followed by a collapse of aggregates and formation of a new bilayer lipid film on the Tl-upd surface. The formation of lipid aggregates was not observed when only 3D-Tl had been formed or dissolved. Model calculations confirm that the formation of cylindrical non-overlapping DPPCBLM aggregates can result in trapping of sufficient amounts of solvent and this process can account for the observed frequency down-shifts. The Tl-upd induced lipid film lifting and intermediate lipid aggregation were observed only in potential step experiments and, thus, represent a dynamic transition phenomenon. In contrast to this, in the potential scan piezogravimetric experiments, where the film reorganization due to the changing substrate (from Au to Tl-upd) progresses at a slower rate, no lipid aggregation was observed.