Studies on Biomimetic Energy and Signal Transduction Systems

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Ion channels transport metal cations such as Na^+, K^+, and Ca^<2+> or a Cl^- anion across the membrane with large fluxes and varying selectivities. These are fundamental molecular devices for biological signal transduction systems. Their functions were tried to be mimicked by totally synthetic organic molecules. The first approach employed supramolecular assembly formation of amphiphilic molecules in bilayr lipid membranes. Oligoether carboxylate and quaternary ammonium having two long alkyl chains were combined to make an amphiphilic ion pair. These types of molecules gave single channel currents with several conductivity levels. The observation was explained by assuming the formation of supramolecular channels in the membrane.When macrocyclic compounds were employed as the unit to define the channel mouth and membrane-inserting part was introduced by ion pairing or covalent bonds, bimolecular ion channels are expected to be formed by association of two molecules in the bilayr. By th … More is way, alpha-cyclodextrin, resorcin [4] arene, and calix [6] arene were modified by long acyl chains or methyl cholate. All these gave single ion channel currents with only one conductivity level for each channel, suggesting the successful control of the channel mouth or the assembly number. All these channels were cation selective, the selectivity factors being generally 5-6 for oligoether supramolecular channels and 15-30 for molecular channels having hydrophobic inner channel wall.When electric charges are not neutralized, e.g.a combination of phosphate and ammonium ions, voltage dependence of the open and closed times were observed.Macrodipoles induced by incorporation of charges in the membrane seem to be responsible for the voltage dependence. Alternative flux control was achieved by employing azo-compound as the hydrophobic component of the supramolecular channel unit. trans-Azo affoded single channel currents, while the photo-isomerized cis could not give any stable channel currents. Less