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

Abstract

The reactions of carbonate radical anion [systematic name: trioxidocarbonate(•1-)] with different forms of myoglobin and hemoglobin were studied by pulse radiolysis in N 2 O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature. The reactions of CO 3 •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) × 10 7 and (1.9 ( 0.3) × 10 8 M -1 s -1 , for metMb and metHb, respectively. The carbonate radical anion-mediated oxidation of oxyHb proceeds in two steps: First, CO 3 •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) × 10 8 and (1.0 ( 0.2) × 10 2 s -1 , respectively. For the reaction between CO 3 •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 CO 3 •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 × 10 4 s -1 ) or according to a concurring mechanism in which CO 3 •oxidizes directly both ∼50% of the iron center and amino acid residue(s) of the globin.

Influence of bicarbonate and carbonate ions on sonochemical degradation of Rhodamine B in aqueous phase

“…The substitution of • OH with CO 3 •− could enhance degradation if the latter, although less reactive than • OH, undergoes radical-radical recombination at a lesser extent than the hydroxyl radical. The combination of • OH is known to be 275 times higher than that the reaction of CO 3 •− with itself [10,20,21]. At higher dye concentrations, the slight negative effect of bicarbonate ions on the rate of dye destruction may be due to the much slower formation of carbonate radical.…”

“…For many years, by analogy to the carbonate and bicarbonate anions, it was assumed that this radical normally exists as the protonated form (HCO 3 • ) in the neutral to basic pH range, but it is now firmly established that no protonation of CO 3 •− (reaction (10)) could be observed in the pH range of 0-10 [8], which leaves CO 3 •− as the only relevant carbonate radical species to be considered in this study and in most aquatic chemistry applications. This radical is a strong one-electron oxidant (1.78 [9] and 1.59 V [10] vs. Normal Hydrogen Electrode (NHE) at pH 7.0 and 12.5, respectively). Carbonate radical may be formed when using ultrasound for water treatment (quenching of hydroxyl radical by HCO 3 − and CO 3 2− ).…”

Reactions of heme proteins with carbonate radical anion

“…•-does not react directly with ferric iron of methemoglobin and metmyoglobin [9,10]. The steady-state radiolysis experiments presented here show that the absorbance in the Soret band (characteristic for the heme protein) of catalase and HRP decreases with the dose, i.e., with an increase of the cumulative concentration of CO 3…”

“…•-with Fe(III) forms of these proteins, only the oxidation of amino acids takes place [9]. In our earlier studies, we have shown that peroxidase-like activity of cytochrome c significantly increases after the reaction with radiolytically generated CO 3…”

“…Recently, it has been reported that CO 3 •-directly oxidizes the Fe(II) heme center of cytochrome c [8]. It has also been found that, in the case of the reaction of oxyhemoglobin and oxymyoglobin with CO 3 •-, the initially formed amino acid radicals oxidize the iron center [9,10]. In the reaction of CO 3…”