Ligands with radical character for high oxidation states in manganese and iron complexes

Abstract

Hybrid density functional calculations have been performed to study the ligand radical character for highly oxidized iron and manganese complexes. This investigation was undertaken because amino acid radicals are found to play key roles in the mechanisms for substrate conversion in many proteins that contain iron or manganese complexes. Large basis sets were used and dielectric cavity methods were employed to estimate the effects of the protein surrounding. Two enzymes are of particular interest in this study. For ribonucleotide reductase the possibility that the tryptophane Trp48 obtains radical character in the course of the radical transfer between Tyr122 and Cys439 was studied. Possibilities of modelling this effect by a directly coordinated unprotonated histidine or tyrosine ligand were investigated. For photosystem II, the main interest is in the possibility that a histidine ligand might affect the stability of oxygen radical states, which could be important for OO bond formation.