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Biophysical Journal 74: 37-47 (1998)
© 1998 the Biophysical Society
Biophys J, January 1998, p. 37-47, Vol. 74, No. 1
*Protein Dynamics Unit,
Brownian dynamics simulations have been carried out to
study the transport of ions in a vestibular geometry, which offers a
more realistic shape for membrane channels than cylindrical tubes.
Specifically, we consider a torus-shaped channel, for which the
analytical solution of Poisson's equation is possible. The system is
composed of the toroidal channel, with length and radius of the
constricted region of 80 Å and 4 Å, respectively, and two reservoirs
containing 50 sodium ions and 50 chloride ions. The positions of each
of these ions executing Brownian motion under the influence of a
stochastic force and a systematic electric force are determined at
discrete time steps of 50 fs for up to 2.5 ns. All of the systematic
forces acting on an ion due to the other ions, an external electric
field, fixed charges in the channel protein, and the image charges
induced at the water-protein boundary are explicitly included in the
calculations. We find that the repulsive dielectric force arising from
the induced surface charges plays a dominant role in channel dynamics.
It expels an ion from the vestibule when it is deliberately put in it.
Even in the presence of an applied electric potential of 100 mV, an ion
cannot overcome this repulsive force and permeate the channel. Only
when dipoles of a favorable orientation are placed along the sides of
the transmembrane segment can an ion traverse the channel under the
influence of a membrane potential. When the strength of the dipoles is
further increased, an ion becomes detained in a potential well, and the driving force provided by the applied field is not sufficient to drive
the ion out of the well. The trajectory of an ion navigating across the
channel mostly remains close to the central axis of the pore lumen.
Finally, we discuss the implications of these findings for the
transport of ions across the membrane.
Biophys J, January 1998, p. 37-47, Vol. 74, No. 1
© 1998 by the Biophysical Society 0006-3495/98/01/37/11 $2.00
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