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Biophys J, January 1999, p. 129-148, Vol. 76, No. 1
*Istituto Nazionale per la Fisica della Materia,
This paper shows that the selectivity properties of
monovalent cation channels found in biological membranes can originate simply from geometrical properties of the inner core of the channel without any critical contribution from electrostatic interactions between the permeating ions and charged or polar groups. By using well-known techniques of statistical mechanics, such as the Langevin equations and Kramer theory of reaction rates, a theoretical equation is provided relating the permeability ratio
PB/PA between ions A and
B to simple physical properties, such as channel geometry, thermodynamics of ion hydration, and electrostatic interactions between
the ion and charged (or polar) groups. Diffusive corrections and
recrossing rates are also considered and evaluated. It is shown that
the selectivity found in usual K+, gramicidin,
Na+, cyclic nucleotide gated, and end plate channels can be
explained also in the absence of any charged or polar group.
If these groups are present, they significantly change the permeability
ratio only if the ion at the selectivity filter is in van der Waals contact with them, otherwise these groups simply affect the channel conductance, lowering the free energy barrier of the same amount for
the two ions, thus explaining why single channel conductance, as it is
experimentally observed, can be very different in channels sharing the
same selectivity sequence. The proposed theory also provides an
estimate of channel minimum radius for K+, gramicidin,
Na+, and cyclic nucleotide gated channels.
Biophys J, January 1999, p. 129-148, Vol. 76, No. 1
© 1999 by the Biophysical Society 0006-3495/99/01/129/20 $2.00
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