Pulse Radiolysis Studies of the Reactions of Carbonate Radical Anion with Myoglobin and Hemoglobin

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 108; no. 27; pp. 5800 - 5805
• Main Authors: Boccini, Francesca, Domazou, Anastasia S, Herold, Susanna
• Format: Journal Article
• Language: English
• Published: American Chemical Society 08.07.2004
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp049063k

The reactions of carbonate radical anion [systematic name:  trioxidocarbonate(•1−)] with different forms of myoglobin and hemoglobin were studied by pulse radiolysis in N2O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature. The reactions of CO3 •- with metMb and metHb involve only amino acid residues of the globin and no oxidation of the iron is observed. The second-order rate constants measured are (4.7 ± 0.3) × 107 and (1.9 ± 0.3) × 108 M-1 s-1, for metMb and metHb, respectively. The carbonate radical anion-mediated oxidation of oxyHb proceeds in two steps:  First, CO3 •- generates radical(s) in the globin which then, over a longer time scale, oxidize the iron center to finally produce ∼40% of metHb. The rate constants obtained for the two steps are (2.1 ± 0.1) × 108 and (1.0 ± 0.2) × 102 s-1, respectively. For the reaction between CO3 •- and oxyMb, at all wavelengths studied we obtained kinetic traces that could be fitted to a single-exponential expression. Two distinct two step mechanisms were proposed to explain the kinetic data. The reaction of CO3 •- with oxyMb proceeds either according to a mechanism identical to that observed for the reaction with oxyHb but with a significantly faster rate of electron transfer from the globin radical(s) to the iron (>6 × 104 s-1) or according to a concurring mechanism in which CO3 •- oxidizes directly both ∼50% of the iron center and amino acid residue(s) of the globin.