On the formation of Horseradish Peroxidase Compound I at high pH: New insights from ab initio molecular dynamics

Compound I formation at neutral or basic pH, starting from ferric peroxidase–peroxide complex has been described, for the first time, by ab initio molecular dynamics on doublet surface. Proton transfer involving distal residues occurs efficiently in a stepwise mechanism in which the O–O bond is broken. An energy barrier of 4.8 kcal/mol is found to obtain the final products when the FeIII[O–OH](−) complex is formed on the doublet surface. In the final part of the reaction, just before Compound I formation, a spin crossing between doublet and quartet magnetic surfaces is observed in our model system.