THE DIELECTRIC-PROPERTIES OF WATER WITHIN MODEL TRANSBILAYER PORES

Ion channels contain extended columns of water molecules within their transbilayer pores. The dynamic properties of such intrapore water hav e been shown to differ from those of water in its bulk state. In previ ous molecular dynamics simulations of two classes of model pore (paral lel bundles of Ala(20), alpha-helices and antiparallel barrels of Ala( 10) beta-strands), a substantially reduced translational and rotationa l mobility of waters was observed within the pore relative to bulk wat er. Molecular dynamics simulations in the presence of a transpore elec trostatic field (i.e., a voltage drop along the pore axis) have been u sed to estimate the resultant polarization (due to reorientation) of t he intrapore water, and hence to determine the local dielectric behavi or within the pore. It is shown that the local dielectric constant of water within a pore is reduced for models formed by parallel alpha-hel ix bundles, but not by those formed by beta-barrels. This result is di scussed in the context of electrostatics calculations of ion permeatio n through channels, and the effect of the local dielectric of water wi thin a helix bundle pore is illustrated with a simple Poisson-Boltzman n calculation.