Dynamics of methionine ligand rebinding in cytochrome c

Geminate recombination of the methionine ligand to the heme iron in ferrous cytochrome c protein following photodissociation displays rich kinetics. It is of particular interest to develop an understanding of fast and slow rebinding time scales, observed in experimental studies, in terms of features of the underlying complex energy landscape. The classical empirical force field in the heme pocket has been extended by incorporating ab initio potential energy surface calculations representing the ground singlet state and quintet state associated with methionine bond breaking and rebinding. An algorithm based on the Landau-Zener nonadiabatic transition theory has been employed to model the electronic surface hopping between two spin states during the process of ligand dissociation and recombination. Multiple conformational substates of the dissociated methionine ligand are found to participate in the reaction dynamics. Varying time scales for interconversion between substates lead to a mechanism elucidating the fast and slow rebinding time scales. The reaction system may be understood in terms of a two-dimensional reaction coordinate distinctly separated from the coupled bath of surrounding protein and solvent degrees of freedom. Insights into the reaction dynamics provided by this study lead to suggestions for future experiments to further probe the role of dynamic heterogeneity in the kinetics of ligand-protein binding.