Attenuation of Proton Currents by Methanol in a Dioxolane-Linked Gramicidin A Channel in Different Lipid Bilayers

Abstract

The mobility of protons in a dioxolane-linked gramicidin A channel (D 1) is comparable to the mobility of protons in aqueous solutions (Cukierman, S., E. P. Quigley, and D. S. Crumrine. 1997. Biophys. J. 73:2489–2502). Aliphatic alcohols decrease the mobility of H + in aqueous solutions. In this study, the effects of methanol on proton conduction through D 1 channels were investigated in different lipid bilayers and at different HCl concentrations. Methanol attenuated H + currents in a voltage-independent manner. Attenuation of proton currents was also independent of H + concentrations in solution. In phospholipid bilayers, methanol decreased the single channel conductance to protons without affecting the binding affinity of protons to bilayers. In glycerylmonooleate membranes, the attenuation of single channel proton conductances qualitatively resembled the decrease of conductivities of HCl solutions by methanol. However, in both types of lipid bilayers, single channel proton conductances through D 1 channels were considerably more attenuated than the conductivities of different HCl solutions. This suggests that methanol modulates single proton currents through D 1 channels. It is proposed that, on average, one methanol molecule binds to a D 1 channel, and attenuates H + conductance. The Gibbs free energy of this process (Δ G 0) is ∼1.2 kcal/mol, which is comparable to the free energy of decrease of HCl conductivity in methanol solutions (1.6 kcal/mol). Apolar substances like urea and glucose that do not transport protons in HCl solutions and do not permeate D 1 channels decreased solution conductivity and single channel conductance by a considerably larger proportion than methanol. Cs + currents through D 1 channels were considerably less (fivefold) attenuated by methanol than proton currents. It is proposed that methanol partitions inside the pore of gramicidin channels and delays the transfer of protons between water and methanol molecules, causing a significant attenuation of the single channel proton conductance. Gramicidin channels offer an interesting experimental model to study proton hopping along a single chain of water molecules interrupted by a single methanol molecule.