Channel-forming activity of syringomycin E in two mercury-supported biomimetic membranes

The lipodepsipeptide syringomycin E (SR-E) interacts with two mercury-supported biomimetic membranes, which consist of a self-assembled phospholipid monolayer (SAM) and of a tethered bilayer lipid membrane (tBLM) separated from the mercury surface by a hydrophilic tetraethyleneoxy (TEO) spacer that acts as an ionic reservoir. SR-E interacts more rapidly and effectively with a SAM of dioleoylphosphatidylserine (DOPS) than with one of dioleoylphosphatidylcholine (DOPC). The proximal lipid monolayer of the tBLM has no polar head region, being linked to the TEO spacer via an ether bond, while the distal monolayer consists of either a DOPC or a DOPS leaflet. The ion flow into or out of the spacer through the lipid bilayer moiety of the tBLM was monitored by potential step chronocoulometry and cyclic voltammetry. With the distal monolayer bathed by aqueous 0.1 M KCl and 0.8 μM SR-E, an ion flow in two stages was monitored with DOPC at pH 3 and 5.4 and with DOPS at pH 3, while a single stage was observed with DOPS at pH 5.4. This behavior was compared with that already described at conventional bilayer lipid membranes. The sigmoidal shape of the chronocoulometric charge transients points to an aggregation of SR-E monomers forming an ion channel via a mechanism of nucleation and growth. The ion flow is mainly determined by potassium ions, and is inhibited by calcium ions. The contribution to the transmembrane potential from the distal leaflet depends more on the nature of the lipid than that of the ion channel.