Rigid Rod-Shaped Polyols:  Functional Nonpeptide Models for Transmembrane Proton Channels

The present study concerns the mode of action of a rigid rod-shaped polyol 1 and the corresponding hexamer 2. Proton flux mediated by 1 is shown to be strongly favored over metal cations and anions. The modest selectivity for monovalent cations (Rb+ > Cs+ > K+ > Na+ ≈ Li+, Eisenman sequence II) is determined by the dehydration energy and is weakly influenced by the local electric (ionophoric) field. The induction of membrane defects was ruled out by the absence of dye leakage. Structural studies by circular dichroism and fluorescence spectroscopy imply that 1 aggregates in polar and nonpolar solvents, but not in lipid bilayers. Furthermore, it is shown that a very small fraction of 1 adopts a monomeric transmembrane tunnel-like structure which accounts for activity, while the remainder forms inactive self-assemblies. The above results suggest that 1 acts as a functional unimolecular proton wire which mimics the hydrogen-bonded chain mechanism found in bioenergetic systems.