SIMPLE DIELECTRIC IMAGE CHARGE MODELS FOR ELECTROSTATIC INTERACTIONS IN METALLOPROTEINS

An investigation has been made of the electrostatic potential from an excess point charge near dielectric interfaces by the method of image charges. Two configurations were considered: one is a planar slab of l ow constant dielectric permittivity enclosed between two infinite regi ons of larger constant dielectric permittivity, and the other is a die lectric sphere of low constant dielectric permittivity embedded in an infinite region of larger constant dielectric permittivity. The system s are representative of charges near bilayer membranes and globular pr oteins, respectively. The potentials reduce to simple analytical forms in these configurations. The notion of an 'effective dielectric permi ttivity' epsilon(eff) is useful, with characteristic distance and orie ntation dependence caused by the image charge distributions. epsilon(e ff) extends from values between the two constant intrinsic dielectric permittivities, but always larger than the lower of the two, to values which exceed significantly the value in the embedding regions. The sp herical model has been compared with observed electrostatic effects in cytochrome C-551, azurin and plastocyanin. The effects refer to reduc tion potential changes caused by deprotonation, pK(a) in different oxi dation states, charged residue replacement, and the effects of charged inhibitor molecules. Conformational changes are insignificant in thes e cases. The observable quantities mostly are reproduced well by the s imple analytical potential form, pointing to the ready applicability o f such models, supplementary to detailed numerical procedures.