Incorporating the protein – dipole Langevin – dipole model into Tanford‐Kirkwood theory

Abstract

AbstractA new technique incorporating the protein–dipole Langevin–dipole (PDLD) model into the Tanford‐Kirkwood (TK) formula has been proposed which provides a rather detailed description of solvent and ionic strength effects on the electrostatic energies. Applications of this method to realistic problems have been performed and concern the solvation energies of four residues of bovine pancreatic trypsin inhibitor (BPTI) and the pK shift of His‐64 of mutant subtilisin BPN′. We focus our calculation on the back‐field effects of bulk solvent.The determination of protein‐induced dipoles is cumbersome due to the solvent screening effects. The protein‐induced dipoles are dependent on their local electric fields, which come from the protein net charges, the surrounding water molecules, and bulk solvent. The bulk solvent will usually screen the electric fields from the protein net charges and dipoles, which are defined here as the back‐field effects of bulk solvent on protein net charges and dipoles, respectively. Our calculations indicate that the back‐field effects of bulk solvent on protein dipoles can simply be ignored, introducing a relative error less than 3%; whereas such back field‐effects on protein net charges are relatively important and cannot simply be ignored, especially when considering a system of highly charged species. © 1995 John Wiley & Sons, Inc.