Mechanistic photodissociation of CO-ligated neuroglobin and subsequent rebinding processes: a theoretical study.

In the present work, density functional theory (QM) and molecular mechanics (MM) method were used to study mechanistic photodissociation of CO-ligated neuroglobin (Ngb-CO). It was found that all the electronic states investigated here are bound with respect to the Fe-CO separation, except for a couple of near-degenerate states (1E) that are repulsive. Irradiation of Ngb-CO at 533 nm leads to the system in the lowest two excited singlet states (1Q), where non-adiabatic CO dissociation proceeds with high efficiency through the intersection between 1Q and 1E. Soret band (1B) is the strongest in the absorption spectra of Ngb-CO with the peak at 415 nm. The systems in the 1B states decay to the 1E states via fast internal conversion, which is followed by the CO dissociation. The CO dissociation induces a considerable change in the structure of the Ngb protein. The initial dissociation involves a rotation of CO, which is accompanied with movement of several residues. When the Fe-C distance is larger than a critical value of 3.0 A, the CO molecules transfer more freely into the cavity of the protein. The pentacoordinated heme was found to be a transient intermediate after CO dissociation.