The ionic selectivity of the bacterial porin OmpF has been investigated in several chloride salts of alkali metal cations under a wide range of concentration ratios. We report a novel way of studying the several factors contributing to ionic selectivity of large channels: measuring reversal potential beyond the gradients commonly used in channel selectivity experiments, i.e., over 50-fold salt concentration ratios. Our results reveal an additional channel feature slipped by in previous studies: the interaction of mobile ions with the protein channel transforms noticeably the ion intrinsic diffusivities. The cation/anion diffusivity ratios in the channel are approximately half of their bulk values for all alkali metal cations studied (KCl, NaCl, LiCl, and CsCl). The binding of cations to certain acidic channel residues is likely to be involved, and its contribution is essential to account for the observed traits. A simple molecular model based on statistical thermodynamics provides qualitative explanations to the experimental findings and can be useful for future, more elaborated treatments.
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