In membranes containing aqueous pores (channels), the osmotic water permeability coefficient, Pf, is greater than the diffusive water permeability coefficient, Pd. In fact, the magnitude of Pf/Pd is commonly used to determine pore radius. Although, for membranes studied to date, Pf/Pd monotonically declines with decreasing pore radius, there is controversy over the value it theoretically assumes when that radius is so small that water molecules cannot overtake one another within the channel (single-file transport). In one view it should equal 1, and in another view it should equal N, the number of water molecules in the pore. Gramicidin A forms, in lipid bilayer membranes, narrow aqueous channels through which single-file transport may occur. For these channels we find that Pf/Pd ≈ 5. In contrast, for the wider nystatin and amphotericin B pores, Pf/Pd ≈ 3. These findings offer experimental support for the view that Pf/Pd = N for single-file transport, and we therefore conclude that there are approximately five water molecules in a gramicidin A channel. A similar conclusion was reached independently from streaming potential data. Using single-channel conductance data, we calculate the water permeability of an individual gramicidin A channel. In the Appendix we report that there is a wide range of channel sizes and lifetimes in cholesterol-containing membranes.
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