We carried out ab initio fragment molecular orbital calculations for wild-type and halogenated glutathione S-transferase (GST) at the MP2/6-31G** level. To elucidate the mechanism of structural stability of the halogenated protein, we calculated the interaction energies for all residue pairs in GST by using inter-fragment interaction energy analysis based on the fragment molecular orbital method. We were able to evaluate electrostatic and van der Waals dispersion interaction energies separately and explicitly. Differences in interaction energies of each residue pair between the two structures were interpreted as the effect of halogenation. The results show that the local interaction around halogen atoms was enhanced. The enhanced interaction effects of Cl and Br were stronger than those of F and I.
