ELECTROSTATICS OF PROTEINS - DESCRIPTION IN TERMS OF 2 DIELECTRIC-CONSTANTS SIMULTANEOUSLY

In the semi-continuum treatment of the energetics of charge formation (or transfer) inside a protein, two components of the energy are inevi tably present: the energy of interaction of the ion with the pre-exist ing intraprotein electric field, and the energy due to polarization of the medium by the newly formed charge. The pre-existing field is set up by charges (partial or full) of the protein atoms fixed in a defini te structure. The calculation of this field involves only the electron ic polarization (the optical dielectric constant is an element of(0)) of the protein because the polarization due to shifts of heavy atoms h as already been accounted for by their equilibrium coordinates. At the same time, the aqueous surroundings should be described by the static constant is an element of(sw), as the positions of water molecules ar e not fixed. The formation of a new charge, absent in the equilibrium X-ray structure, results in shifts of electrons and polar atoms, i.e., it involves all kinds of medium polarization described by the static dielectric constant of protein is an element of(s). Thus, in calculati ons of the total energy, two different dielectric constants of the pro tein are operative simultaneously. This differs from a widely used alg orithm employing one effective dielectric constant for both components of the ion's energy. (C) 1997 Wiley-Liss, Inc.