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Ion selective channels control many physiological functions vital for
the
survival of the cell and the whole organism. One important class of
channels selects calcium over monovalent ion that are present at higher
concentration. Our purpose is to relate the selectivity behavior of
various
channels to a few physical parameters such as the structural amino acid
groups in the selectivity filter, the radius of the pore and the
dielectric
constant of the protein confining the pore. The L-type calcium channel is
known the have very strong selectivity, one micromolar calcium
significantly decrease the current in 100 milimolar sodium.
Our previous work has shown that to explain such a strong selectivity the
size of the ions must be taken into account in the framework of the
charge/space mechanism. This mechanism assumes that the selectivity filter
is crowded by the structural ions (the oxygen ions of the end groups of
the
four glutamic acids lining the filter) and that calcium ions are more
efficient at neutralizing the oxygen ions than the sodium ions because
they
provide twice the charge while occupying the same space. Earlier
simulations without any dielectric boundary reproduced milimolar calcium
selectivity on the basis of this mechanism.
We report results of Monte Carlo simulations for a model calcium channel
where the protein has a different dielectric constant than the
electrolyte.
We show that dielectric interface has a profound effect on the behavior of
the system. More cations are attracted into the filter because of the
negative charges induced on the wall of the pore and calcium versus sodium
selectivity is increased.
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