SINCE the announcement by Mueller et al.1 of a technique for forming thin (<100 ) lipid membranes separating two aqueous phases, there has been a growing interest in the permeability of these films to ions, neutral solutes and water. In the absence of any modifying agents, the membranes have resistances of approximately 108 Ω cm2 in 0.1 M salt solutions1. In the presence of both iodine and iodide ions, the membrane resistance falls to values of 104 Ω cm2 or lower2,3. In this state the membrane behaves as an ideal iodide ion electrode. The current carriers in the membrane appear to be polyiodides (for example, I5-), which, because of their large size and delocalized charge, have sufficient solubility in the low dielectric constant, hydrophobic interior of the membrane to enter this phase. We have now found that in this state the membrane responds to brief pulses of light in the near ultraviolet with a fast (rise time < 40 s) decrease in conductance (1 per cent) followed by a recovery of the conductance to its original value with a time course measured in milliseconds.
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