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J. Phys. Chem. B 2008, 112, 990-996
Mechanistic Photodissociation of CO-Ligated Neuroglobin and Subsequent Rebinding Processes: A Theoretical Study Xin Ming and Wei-Hai Fang* College of Chemistry, Beijing Normal UniVersity, Beijing 100875, People’s Republic of China ReceiVed: August 9, 2007; In Final Form: October 26, 2007
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 Å, 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.
Introduction Heme-containing globin proteins are widely distributed in bacteria, fungi, protists, plants, and animals1 and play a unique role in biology as sensors, activators, and carriers of the gaseous molecules.2 The binding of oxygen, carbon monoxide, and other ligands to heme proteins is a topic of great ongoing interest,3-8 and the reactions associated with reversible binding of the ligands have served as the basis of developing methods and models for investigating the dynamics of ligand-protein interactions. It has become clear that the understanding of the metal-ligand bond-breaking process is essential to gaining insight into the primary molecular events in heme proteins. The photosensitive Fe-ligand bond in heme proteins has allowed transient intermediates to be studied by flash photolysis methods. In combination with femtosecond transient absorption and subpicosecond time-resolved resonance Raman spectra, photoinduced ligand dissociation has been extensively used as an initial step in studies of ligand rebinding kinetics and protein relaxation processes.9-26 Beginning with the work of Gibson,27 experiments on the photolysis of the CO complex of hemoglobin (Hb) have played a central role in investigations of the mechanism of cooperative ligand binding. Transient absorption spectroscopy at room temperature showed no evidence of CO rebinding in
