Current-voltage dependences for different tentative bacteriorhodopsin models were studied in order to explain the linearity of this dependence observed in our previous experiments. It is proved that the best correspondence with experiment occurs when the passive proton transport path is assumed to contain many binding sites and the thickness of the widest barrier separating these sites is not more than 1 nm. One of the following two conditions is necessary: (1) electrostatic repulsion of protons in the channel is so strong that not more than one proton can be in the channel at a time; the potential energy of a proton in this channel decreases linearly towards the active site. (2) Electrostatic repulsion between protons in the channel is moderate, and the binding energy of a proton at all channel sites is equal. Under conditions of high channel occupation, single-file effects lead to non-linearity of the current-voltage characteristics if there is no electrostatic repulsion.
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