Theoretical Study of Local Environmental Effect on Electronic Structure of M-Cluster

A local environmental effect on the electronic structure of the M-cluster of nitrogenase is examined by a density functional theory (DFT) method, especially from the viewpoint of the hydrogen bond and water molecules. It is also confirmed how the local environmental effect is influenced by the outer protein environmental field. The computational results indicate that the hydrogen bonds from Ser265 significantly contribute to the stability of the LUMO, and the water molecules around the homocitric acid (HCA) strongly stabilize the HOMO when the outer protein environment is not included. On the other hand, the contribution of Ser265 is suppressed when the outer environmental field, which is approximated by the polarizable continuum model, is applied. Under the outer environmental effect, the effect of the hydrogen bonds is averaged, and the LUMO energies become only proportional to the number of hydrogen bonds. These results would provide suggestions for the ligand structure design of synthesized FeS complexes.