Reaction field effects on proton transfer in the active site of actinidin

The feasibility of the inclusion of reaction field effects in accurate ab initio self-consistent field-molecular orbital calculations was studied in the case of proton transfer in the active site of actinidin. The effects of the polarizability of the environment were included, using the direct reaction field model, which treats the environment as a set of interacting polarizable atoms. Up to 1000 of these atoms could be treated but about 300 were sufficient. The full geometry of the active site and the environment was taken into account. The stabilization of the ion pair was calculated to be 3.5 kcal. but this value may be 10 kcal depending on the geometry used. The effect of the static field from the long alpha-helix present in the enzyme was also studied. Dispersion effects are shown to be unimportant. The orientational polarizability of side chains and water molecules was not included.