4 Electrochemistry of Biomimetic Membranes

What Ion Flow along Ion Channels Can Tell us about Their Functional Activity

“…More generally, in an equivalent circuit consisting of resistances and capacitances, Y is a complex quantity, and the impedance Z is equal to 1/ Y , by definition. Denoting the in-phase and quadrature components of the impedance by Z′ and Z″ , respectively, a single RC mesh yields a semicircle of diameter R and center of coordinates ( R /2,0) when displayed on a Z″ vs. Z′ plot, called Nyquist plot [4]. Moreover, the angular frequency ω at the maximum of the semicircle equals the reciprocal of the time constant, RC , of the mesh.…”

“…The semicircles of the remaining RC meshes are compressed in a very narrow area close to the origin of the Z″ vs. Z′ plot, and can be visualized only by enlarging this area. To better visualize all semicircles, we have found it convenient to represent impedance spectra on a ωZ′ vs. – ωZ″ plot, briefly referred to as an “M plot”, since ωZ′ and ωZ″ are the components of the modulus function M [4]. Even in this plot, a single RC mesh yields a semicircle, with diameter C −1 and center of coordinates (1/(2 C ),0).…”

“…A major drawback of BLMs is their fragility, high sensitivity toward vibrations and mechanical shocks, and low resistance to electric fields; thus, they hardly last more than eight hours and collapse for potential differences between the solutions that bath the two sides of the BLM greater than 100–150 mV. To overcome this problem, metal-supported tethered BLMs (tBLMs) have been devised, where the BLM is anchored to the metal via a hydrophilic spacer that replaces and mimics the water phase on the metal side [3,4]. TBLMs are fabricated by tethering a “thiolipid” monolayer to the surface of a noble metal such as Au, Ag or Hg.…”

“…Direct investigation of protein functional activity via inorganic ion flow along ion channels incorporated in Au- and Ag-supported BLMs is prevented by non-negligible water electroreduction at physiological pH and by a number of defects that provide a nonselective pathway to ion flow [8,9]. To the best of our knowledge, the only tBLMs allowing the functional activity of proteins to be directly monitored by measuring the protein-induced ion flow across the lipid bilayer are mercury-supported tBLMs [3,4,10]. Thanks to its liquid state, mercury provides the lipid bilayer with a perfectly smooth surface and a lateral mobility of the lipid molecules comparable with a BLM, healing surface defects; moreover, its high hydrogen overpotential eliminates water electroreduction at physiological pH.…”

Mercury-Supported Biomimetic Membranes for the Investigation of Antimicrobial Peptides

“…Tethered bilayer lipid membranes (tBLMs) are versatile model systems that provide a defined platform for the incorporation of peptides and proteins and for the investigation of their functional activity [1,2,3]. They are obtained by tethering a “thiolipid” monolayer to the surface of a noble metal such as Au, Ag or Hg.…”