title: Use of weak acids to determine the bulk diffusion limitation of H+ ion conductance through the gramicidin channel.
abstract: The addition of 2 M formic acid at pH 3.75 increased the single channel H+ ion conductance of gramicidin channels 12-fold at 200 mV. Other weak acids (acetic, lactic, oxalic) produce a similar, but smaller increase. Formic acid (and other weak acids) also blocks the K+ conductance at pH 3.75, but not at pH 6.0 when the anion form predominates. This increased H+ conductance and K+ block can be explained by formic acid (HF) binding to the mouth of the gramicidin channel (Km = 1 M) and providing a source of H+ ions. A kinetic model is derived, based on the equilibrium binding of formic acid to the channel mouth, that quantitatively predicts the conductance for different mixtures of H+, K+, and formic acid. The binding of the neutral formic acid to the mouth of the gramicidin channel is directly supported by the observation that a neutral molecule with a similar structure, formamide (and malonamide and acrylamide), blocks the K+ conductance at pH 6.0. The H+ conductance in the presence of formic acid provides a lower bound for the intrinsic conductance of the gramicidin channel when there is no diffusion limitation at the channel mouth. The 12-fold increase in conductance produced by formic acid suggests that greater than 90% of the total resistance to H+ results from diffusion limitation in the bulk solution.
mesh_terms: D000577:Amides; D002264:Carboxylic Acids; D004553:Electric Conductivity; D005561:Formates; D006096:Gramicidin / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007473:Ion Channels / Q000502:physiology*; D008433:Mathematics; D008954:Models, Biological; D011188:Potassium; D011522:Protons
chemical_list: D000577:Amides; D002264:Carboxylic Acids; D005561:Formates; D007473:Ion Channels; D011522:Protons; C030544:formic acid; D006096:Gramicidin; D011188:Potassium