Pulse Radiolysis Reduction of Myoglobin. Hydrated Electrons Diffusion Inside the Protein Matrix

Abstract

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 (Brönsted 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 shows that the Brönsted 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.

Importance of radiolytic reactions during high-LET irradiation modalities: LET effect, role of O2 and radiosensitization by nanoparticles

“…The latter reports near diffusion-controlled reaction rates of e − aq with DNA/RNA components and evidence for the reaction of the aqueous electron reaction with dsDNA. Proof of reducible sites in proteins was also given in some experiments (LeTilly et al 1997). H• and e − aq are mostly skipped in radiobiology because molecular oxygen competes efficiently to scavenge these species and forms superoxide radical.…”

Redox-Induced Structural Dynamics of Fe-Heme Ligand in Myoglobin by X-Ray Absorption Spectroscopy

“…Irradiation of heme proteins in solution at cryogenic temperatures is a way to generate intermediate states of the Feheme complex (Blumenfeld, 1981;Prusakov et al, 1985;Parak and Prusakov, 1994). Radiolysis of the solvent produces thermalized electrons that can reach the reducible sites of the protein even at low temperature (i.e., the porphyrin group and the histidine residues; see Pin et al, 1989;Le Tilly et al, 1997). Consequently the heme iron Fe(III) is reduced, but at low temperature the protein structure remains frozen in its initial conformation.…”

Generation of High-Oxidation States of Myoglobin in the Nanosecond Time-Scale by Laser Photoionization

“…The absorbance in the 500-800 nm region and the effect of nitrous oxide are characteristic of the hydrated electron (e,,-) (22) and the formation of this species indicates the ejection of an electron from myoglobin by the laser light (photoionization). The decay of the hydrated electron in argon-saturated solutions, accompanied by further changes in the Soret region, can be attributed to the reaction of eaqwith myoglobin (23).…”

“…It is interesting that electrons ejected from the heme group at the protein core are observed as solvated electrons in the bulk solution. Evidence for the reciprocal phenomenon, i. e. the migration of electrons from the solvent through the protein matrix, has been inferred from recent pulse radiolysis studies (23).…”