G. Eisenman, J. Hägglund, J. Sandblom, and B. Enos, “The Current-voltage Behavior of Ion Channels: Important Features of the Energy Profile of the Gramicidin Channel Deduced from the Conductance-voltage Characteristic in the Limit of Low Ion Concentration”, ujms, vol. 85, no. 3, pp. 247-257, Sep. 1980.