Photocontrol of ion flux across a lipid bilayer membrane by visible light

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Lipid bilayer membranes that respond to light are of great interest for the development of the artificial visual systems for use in optoelectronic devices and optical transducers. There are many studies of the photocontrol of ion transport across lipid bilayer membranes using photoresponsive compounds such as amphiphilic azobenzene and spirobenzopyran derivatives. Most of the studies have used changes in membrane capacitance due to the disruption of lipid bilayer structures, which results from the photoisomerization of the compounds incorporated into the lipid bilayers. So far, several groups have reported photocontrollable ion transport systems, where channel or carrier molecules attaching photochromic compounds were incorporated into lipid bilayer membranes. In these systems, photochromic reaction has been conducted by irradiation of UV light (300-380nm). The prolonged UV irradiation, however, usually results in photodamage to photochromic compounds and lipid molecules.In the present study, we have chosen calix[4]arene as a basic skeleton of a photoresponsive ion carrier because its ether derivatives are known to act as Na^+ ion carriers in lipid bilayer membranes. For the synthesis of a photoresponsive carrier, dimethylaminoazobenzene was introduced as a photochromic compound to an ether derivative of p-tert-butylcalix[4]arene. Among azobenzene derivatives, dimethylaminoazobenzene has an intense absorption in the visible region, which is attributed to the π-π* transition of the trans isomer. We expected that the dimethylaminoazobenzene appended calix[4]arene derivative mediates Na^+ ion transport across a lipid bilayer membrane, and the ion transport ability can be controlled by irradiation of visible light. In this study, we have succeeded to develop a visible light-driven ion transport system using a calix[4]arene-based photoresponsive ion carrier, where Na^+ flux across a lipid bilayer can be controlled by irradiation of light (>400nm).