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Biophys J, April 2000, p. 1825-1834, Vol. 78, No. 4

Proton Mobilities in Water and in Different Stereoisomers of Covalently Linked Gramicidin A Channels

Samuel Cukierman

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

Proton conductivities in bulk solution (lambda H) and single-channel proton conductances (gH) in two different stereoisomers of the dioxolane-linked gramicidin A channel (the SS and RR dimers) were measured in a wide range of bulk proton concentrations ([H], 0.1-8000 mM). Proton mobilities (µH) in water as well as in the SS and RR dimers were calculated from the conductivity data. In the concentration range of 0.1-2000 mM, a straight line with a slope of 0.75 describes the log (gH)-log ([H]) relationship in the SS dimer. At [H] > 2000 mM, saturation is followed by a decline in gH. The gH-[H] relationship in the SS dimer is qualitatively similar to the [H] dependence of lambda H. However, the slope of the straight line in the log(lambda H)-log([H]) plot is 0.96, indicating that the rate-limiting step for proton conduction through the SS dimer is not the diffusion of protons in bulk solution. The significant difference between the slopes of those linear relationships accounts for the faster decline of µH as a function of [H] in the SS dimer in relation to bulk solution. In the high range of [H], saturation and decline of gH in the SS dimer can be accounted for by the significant decrease of µH in bulk solution. At any given [H], gH in the RR dimer is significantly smaller than in the SS. Moreover, the gH-[H] relationship in the RR stereoisomer is qualitatively different from that in the SS. Between 1 and 50 mM [H], gH can be fitted with an adsorption isotherm, suggesting the presence of a proton-binding site inside the pore (pKa approx  2), which limits proton exit from the channel. At 100 mM < [H] < 3000 mM, gH increases linearly with [H]. The distinctive shape of the gH-[H] relationship in the RR dimer suggests that the channel can be occupied simultaneously by more than one proton. At higher [H], the saturation and decline of gH in the RR dimer reflect the properties of µH in bulk solution. In the entire range of [H], protons seem to cross the SS and RR channels via a Grotthuss-like mechanism. The rate-limiting step for proton transfer in the SS dimer is probably the membrane-channel/bulk solution interface. It is also proposed that the smaller gH in the RR dimer is the consequence of a different organization and dynamics of the H-bonded network of water molecules inside the pore of the channel, resulting in a slower proton transfer and multiple pore occupancy by protons.

Biophys J, April 2000, p. 1825-1834, Vol. 78, No. 4
© 2000 by the Biophysical Society   0006-3495/00/04/1825/10  $2.00


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