Biophys J, September 2001, p. 1430-1438, Vol. 81, No. 3

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

Department of Physiology, Loyola University Medical Center, Maywood, Illinois 60153 USA

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.

Biophys J, September 2001, p. 1430-1438, Vol. 81, No. 3
© 2001 by the Biophysical Society   0006-3495/01/09/1430/09  $2.00

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