Molecular dynamics simulation of NO recombination to myoglobin mutants

Molecular dynamics simulations on two coupled electronic surfaces are employed to investigate the geminate recombination of nitric oxide to mutants of sperm whale myoglobin. A model for the ground and the excited states is constructed based on experimental data. The crossing between the surfaces is treated using the Landau-Zener formula. The reaction probability and the recombination curves are calculated directly by histogramming the results of an ensemble of trajectories. The experimental trend is reproduced in which the picosecond recombination rate of different mutants increases in the order Phe29 > Leu29 > Val29 > Ala29. Furthermore, in accord with the experiment on significantly longer time scales an opposite trend is obtained, in which the recombination rate for Ala29 is larger than for Phe29. These results are explained by constrained diffusion of the ligand in the heme pocket. The average and the transient volume of the heme pocket is modified by the 29 mutants.