Modulation of Proton Transfer in the Water Wire of Dioxolane-Linked Gramicidin Channels by Lipid Membranes

Abstract

Proton conductance ( g H) in single SS stereoisomers of dioxolane-linked gramicidin A (gA) channels were measured in different phospholipid bilayers at different HCl concentrations. In particular, measurements were obtained in bilayers made of 1,2-diphytanoyl 3-phosphocholine (DiPhPC) or its ethylated derivative 1,2-diphytanoyl 3-ethyl-phosphocholine (et-DiPhPC,). The difference between these phospholipids is that in et-DiPhPC one of the phosphate oxygens is covalently linked to an ethyl group and cannot be protonated. In relatively dilute acid solutions, g H in DiPhPC is significantly higher than in et-DiPhPC. At high acid concentrations, g H is the same in both diphytanoyl bilayers. Such differences in g H can be accounted for by surface charge effects at the membrane/solution interfaces. In the linear portion of the log g H-log [H] relationship, g H values in diphytanoyl bilayers were significantly larger (∼10-fold) than in neutral glyceryl monooleate (GMO) membranes. The slopes of the linear log-log relationships between g H and [H] in diphytanoyl and GMO bilayers are essentially the same (∼0.76). This slope is significantly lower than the slope of the log-log plot of proton conductivity versus proton concentration in aqueous solutions (∼1.00). Because the chemical composition of the membrane-channel/solution interface is strikingly different in GMO and diphytanoyl bilayers, the reduced slope in g H-[HCl] relationships may be a characteristic of proton transfer in the water wire inside the SS channel. Values of g H in diphytanoyl bilayers were also significantly larger than in membranes made of the more common biological phospholipids 1-palmitoyl 2-oleoyl phosphocholine (POPC) or 1-palmitoyl 2-oleoyl phosphoethanolamine (POPE). These differences, however, cannot be accounted for by different surface charge effects or by different internal dipole potentials. On the other hand, maximum g H measured in the SS channel does not depend on the composition of the bilayer and is determined essentially by the reduced mobility of protons in concentrated acid solutions. Finally, no experimental evidence was found in support of a lateral proton movement at the phospholipid/solution interface contributing to g H in single SS channels. Protein-lipid interactions are likely to modulate g H in the SS channel.