Proton Transfer in Gramicidin Water Wires in Phospholipid Bilayers: Attenuation by Phosphoethanolamine

Abstract

The transfer of protons in water wires was studied in native gramicidin A (gA), and in the SS- and RR- diastereoisomers of dioxolane-linked gA channels (SS and RR channels). These peptides were incorporated into membranes comprised of distinct combinations of phospholipid headgroups and acyl chains. Quantitative relationships between single channel conductances to H + (g H) and [H +] were determined in distinct phospholipid membranes, and are in remarkable contrast with results previously obtained in monoglyceride membranes. In particular: 1), g H-[H +] relationships for the various gA channels in distinct phospholipid membranes are well fitted by single adsorption isotherms. A simple kinetic model assuming mono-occupancy of channels by protons fits said relationships. This does not occur with monoglyceride membranes. 2), Under nonsaturating [H +], g H is ∼1 order of magnitude larger in phospholipid than in monoglyceride membranes. 3), Differences between rates of H + transfer in various gA channels are still present but considerably attenuated in phospholipid relative to monoglyceride membranes. 4), Charged phospholipid headgroups affect g H via changes in [H +] at the membrane/solution interfaces. 5), Phosphoethanolamine groups caused a marked attenuation of g H relative to membranes with other phospholipid headgroups. This attenuation is voltage-dependent and tends to saturate H + currents at voltages larger than 250 mV. This effect is likely to occur by limiting the access and exit of H + in and out of the channel due to relatively strong oriented H bonds between waters and phosphoethanolamine groups at channel interfaces. The differential effects of phospholipids on proton transfer could be reasoned by considering solvation effects of side chain residues of gramicidin channels by double acyl chains and by the presence of polar headgroups facilitating the entrance/exit of protons through the channel mouths.