General porin OmpF forms water-filled channels in the outer membrane of E. coli bacteria. When reconstituted into planar bilayer lipid membranes, these channels can be closed (or “gated”) by high electric fields. We discover that: (i) channel gating is sensitive to the type of cations in the membrane-bathing solution according to their position in the Hofmeister series; (ii) channel gates to a “closed” state that is represented by a set of multiple sub-conformations with at least three distinctly different conformations contributing to the closed-state conductance histogram. Taken together with the nearly symmetric response to the applied voltage of changing polarity and the hysteresis phenomena reported previously by others and reproduced here, these findings suggest that the voltage-induced closure of the OmpF channel is a consequence of reversible denaturation of the protein by the high electric field. If so, the voltage-induced gating of bacterial porins can serve as an instructive model to study the physics of protein folding at the single-molecule level.
Statistical analysis of high-resolution current recordings from a single ion channel reconstituted into a planar lipid membrane allows us to study transport of antibiotics at the molecular detail. Working with the general bacterial porin, OmpF, we demonstrate that addition of zwitterionic β-lactam antibiotics to the membrane-bathing solution introduces transient interruptions in the small-ion current through the channel. Time-resolved measurements reveal that one antibiotic molecule blocks one of the monomers in the OmpF trimer for characteristic times from microseconds to hundreds of microseconds. Spectral noise analysis enables us to perform measurements over a wide range of changing parameters. In all cases studied, the residence time of an antibiotic molecule in the channel exceeds the estimated time for free diffusion by orders of magnitude. This demonstrates that, in analogy to substrate-specific channels that evolved to bind specific metabolite molecules, antibiotics have 'evolved' to be channel-specific. The charge distribution of an efficient antibiotic complements the charge distribution at the narrowest part of the bacterial porin. Interaction of these charges creates a zone of attraction inside the channel and compensates the penetrating molecule's entropy loss and desolvation energy. This facilitates antibiotic translocation through the narrowest part of the channel and accounts for higher antibiotic permeability rates.