Pulse radiolysis reduction of myoglobin. Hydrated electrons diffusion inside the protein matrix

Pulse radiolysis studies have demonstrated that the kinetics of myoglobin reduction changes with the pH of the solution. The reduction rate constant of the protein decreases with increasing pH. The net charge of the macromolecule was longtime considered to be responsible for this dependence. However, for every protein molecule bearing many reduction sites, the reduction rate of the protein would be the summation of the individual reactions of hydrated electrons on each particular reducible site. The two schemes of protein reduction were checked by the investigation of the behavior of the experimental reduction constant values (i) versus the protein charge (Bronsted approach) and (ii) versus the reducible imidazolium groups number of the protein. Although the two plots are linear and seem to assert that the two approaches are equivalent, a critical analysis sho ws that the Bronsted formulation cannot be applied to proteins. This scheme gives also erroneous different radii for the same protein. Thus, this work rationalizes the pH-dependent rate of reduction by an interaction of the hydrated electron with protonated histidine residues of the protein.