Non-exponential non-Arrhenius relaxation in the course of CO rebinding to heme proteins

The dispersive transport model for relaxation of photolyzed heme proteins has been improved to take into account the coupling of the ligand-heme geminate recombination and the non-Gaussian diffusive dynamics of conformational changes in heme proteins. Contrary to the earlier deterministic version of the model, the present more rigorous formulation is based on the stochastic approach to the problem. This implies that the time evolution of protein conformations should be described in terms of the transient distribution which satisfies the Smoluchowski-type differential equation with a time-dependent diffusion coefficient. The obtained analytical solution of this equation enables us to relate main kinetic parameters of the geminate recombination and quantities characterizing the ligand-heme interaction. The derived expressions demonstrate that the reaction barrier shifts with time towards higher values following the near-stretched exponential behavior in agreement with experiment. Such a behavior is governed by the non-exponential non-Arrhenius conformational relaxation. The latter process can be identified by the characteristics “footprint” left on the experimental rebinding curve and is shown to be responsible for some kinetically different phases of the ligand-heme geminate recombination observed within distinct temperature ranges.