Mechanistic Insights into Radical-Mediated Oxidation of Tryptophan from ab Initio Quantum Chemistry Calculations and QM/MM Molecular Dynamics Simulations

摘要

An assessment of the mechanisms of (OH)-O-center dot and (OOH)-O-center dot radical-mediated oxidation of tryptophan was performed using density functional theory calculations and ab initio plane-wave Quantum Mechanics/Molecular Mechanics (QM/MM) molecular dynamics simulations. For the (OH)-O-center dot reactions, addition to the pyrrole ring at position 2 is the most favored site with a barrierless reaction in the gas phase. The subsequent degradation of this adduct through a H atom transfer to water was intermittently observed in aqueous phase molecular dynamics simulations. For the (OOH)-O-center dot reactions, addition to the pyrrole ring at position 2 is the most favored pathway, in contrast to the situation in the model system ethylene, where concerted addition to the double bond is preferred. From the (OOH)-O-center dot position 2 adduct QM/MM simulations show that formation of oxy-3-indolanaline occurs readily in an aqueous environment. The observed transformation starts from an initial rupture of the O-O bond followed by a H atom transfer with the accompanying loss of an (OH)-O-center dot radical to solution. Finally, classical molecular dynamics simulations were performed to equate observed differential oxidation rates of various tryptophan residues in monoclonal antibody fragments. It was found that simple parameters derived from simulation correlate well with the experimental data.