Weak nonlinearity of current-voltage characteristics of gramicidin D channels. Experiment, theory and application to the study of transmembrane transmission of information.

A new, sensitive method of measurement of current-voltage characteristics (CVC) of the type i(U) approximately U(1 + beta U2) was applied to study ionic channels formed by gramicidin D in bilayer lipid membranes (BLM). The values of the non-linearity coefficient beta of the CVC of membranes modified by gramicidin D depended on electrolyte concentration, C, and expressed the weak nonlinearity of the CVC. In symmetric electrolyte conditions, the coefficient beta(A) increased with the increasing electrolyte concentration from -17 V-2 at 0.03 mol/l KCl to 8 V-2 at 3 mol/l KCl. In asymmetric electrolyte conditions (different electrolyte concentrations at the two sides of the BLM) the coefficient beta(A) also increased with increasing C. A theory is proposed describing the weak nonlinearity of CVC of ionic channels based on electrodiffusion model. This theory assumes that the electrochemical potential mu(x) of an ion within the ion channel depends on the coordinate x along the channel axis. According to the proposed model, CVC nonlinearity is mainly determined by the ends of the ion channel. We showed that the structure of gramicidin D ionic channel is sensitive to the concentration of ions, and in fact changes at one end of the channel influence the parameters of the other channel end.