Monte Carlo Simulation Study of a System with a Dielectric Boundary: Application to Calcium Channel Selectivity

Abstract

Equilibrium Monte Carlo simulations are reported for a model of a biological calcium channel in which the region that includes the channel and the selectivity filter has a different dielectric coefficient than that of the bath. These regions are separated by a sharp dielectric boundary. This simple geometry makes it possible to use the image charge method to incorporate polarization effects. We also include a description of changes in solvation energy that arise when ions move between different dielectrics; we use the Born description of hydration with empirical ionic radii that yield experimental hydration energies. In calculations of Ca²⁺vs. Na⁺ selectivity analogous to those of earlier work (Boda et al., Molec. Phys., 100, 2361 (2002)), we find that reducing the dielectric coefficient in the channel to values as low as 10 renders the channel model less calcium selective. Thus, this continuum description of polarization effects does not capture a delicate balance that seems to exist in biological channels between the energy that ions require for dehydration and the energy that ions gain by interaction with the charged groups in the pore.

Keywords:

• Polarization effect
• Monte Carlo stimulation
• Poisson's equation
• Biological channel

Acknowledgements

This work was supported by grants from the Hungarian National Research Fund (OTKA-F035222, to D.B.), DARPA (B.E., W.N.), NIH (Al 23007, to D.D.B. and T32NS07044 to DG) and NSF (CHE98-13729, to D.H.). The authors thank the organizers of the Yangtze Conference on Fluids and Interfaces where an inspiring environment was available for discussions of this work.