Monte Carlo study of the effect of ion and channel size on the selectivityof a model calcium channel

Our earlier simulation (J. Phys. Chem. B 2000, 104, 8903) of the selectivit y of a model calcium channel, where all ions were assumed to have the same diameter and the channel dimensions were fixed, is extended to allow for di fferent ionic size and variable channel size. We find that for equal valenc e, the channel selects cations of the smallest size. If higher valence cati ons are present, the channel selects cations with the highest valence, and as a result, Ca will replace monovalent cations. This replacement is more e fficient for larger monovalent cations than for smaller ones. This is what would be expected on the basis of the charge/space competition mechanism th at has been postulated earlier. Of course, if the size ratio is very large, size might be selected over valence. In addition, we consider the effect o f the channel diameter and find that narrow channels are less Ca++ selectiv e. This suggests that the recent theory of Nonner et al. (Biophys. J. 2000, 79, 1976) is most useful for wide, but still microscopic, channels.