Benon H. J. Bielski

Reactivity of HO2/O−2 Radicals in Aqueous Solution

Kinetic data for the superoxide radical (HO2⇄O−2 +H+, pK=4.8) in aqueous solution have been critically assessed. Rate constants for reactions of O−2 and HO2 with more than 300 organic and inorganic ions, molecules and other transient species have been tabulated.

REEVALUATION OF THE SPECTRAL AND KINETIC PROPERTIES OF HO2 AND O2‐ FREE RADICALS

Abstract— The spectra and molar absorbances of the HO2 and O2‐ free radicals have been redetermined in aqueous formate solutions by pulse and stopped‐flow radiolysis as well as by 60Co gamma‐ray studies. The extinction coefficients at the corresponding maxima and 23°C are 225= 1400 ± 80 M‐1 cm‐1 and 225= 2350 ± 120 M‐1 cm‐1 respectively. Reevaluation of earlier published rate data in terms of the new extinction coefficients yielded the following rate constants for the spontaneous decay of HO2 and O2‐: KHo2+HO2= (8.60 ± 0.62) × 105M‐1 s‐1; KHo2+O2‐= (1.02 ± 0.49) × 108M‐1 s‐1; KHo2+O2‐ < 0.35 M‐1 s‐1. For the equilibrium HO2→ O2‐+ H+ the dissociation constant is KHo2= (2.05 ± 0.39) × 10‐5M or pKHO2= 4.69 ± 0.08. G(O2‐) has been evaluated as a function of formate concentration.

SOME PROPERTIES OF THE ASCORBATE FREE RADICAL

A study of the reactivity of the ascorbate radical with biological materials was performed using dopamine, cytochrome C, O/sub 2/, methanol, lactate, pyruvate, fumarate, and L-..cap alpha..-ketoglutarate. Interferring conditions were controlled by use of a modified Durrum fast kinetics spectrophotometer on line with a Van de Graaf generator. The reaction conditions and rate constants are presented in tabular form. (DDA)

Study of the superoxide radical chemistry by stopped-flow radiolysis and radiation induced oxygen consumption. [Electron beams]

The chemical reactivity of the superoxide radical has been studied at pH 10 and 23/sup 0/C by absorption spectroscopy in a fast kinetics spectrophotometer and by radiation induced oxygen consumption in a modified stopped-flow radiolysis apparatus on line with a Van de Graaff electron generator. The latter method differentiates between oxidation and reduction reactions and yields information on the corresponding stoichiometry. A number of examples are used to demonstrate the versatility of these techniques. Rate constants were determined for the reaction of superoxide radicals with: ferricytochrome c, k = 2.6 +- 0.2 x 10/sup 5/ M/sup -1/ s/sup -1/ at pH 9.0; ascorbate, k = 1.52 +- 0.1 x 10/sup 5/ M/sup -1/ s/sup -1/ at pH 9.9; nitroblue tetrazolium, k = 5.94 +- 0.5 x 10/sup 4/ M/sup -1/ s/sup -1/ at pH 9.8. Some 19 other compounds (buffers, carboxylic acids, chelating agents, etc.) were shown to be inert toward superoxide radicals.

Reaction of Ferrate (VI)/Ferrate (V) with Hydrogen Peroxide and Superoxide Anion - a Stopped-Flow and Premix Pulse Radiolysis Study

The reduction of ferrate(VI) to ferrate(V) by superoxide ions was studied over the pH range 2.6-13.0 using the premix pulse radiolysis technique. The pH dependence indicates that only the unstable protonated forms of ferrate, H2FeO4 (pKa3.5) and HFeO4- (pKa7.3) are reactive, k(HFeO4(-) + O2) = (1.7 +/- 0.2) x 10(7) M-1 s-1. The stable ferrate ion, FeO4(2-), showed no significant reactivity towards either hydrogen peroxide or superoxide anion. The rate constants for the spontaneous dimerization and decomposition of the protonated ferrates, e.g. k(HFeO4(-) + HFe04) approximately 250 M-1s-1, are orders of magnitude slower than their corresponding reduction reduction by superoxide indicating an outer-sphere mode of electron transfer for the latter process. In contrast the ferrate(VI) species H3FeO4+ (pKa = 1.6 +/- 0.2), H2FeO4, and HFeO4- oxidize hydrogen peroxide, e.g. k(HFeO4(-) + H2O2) = 170 M-1 s-1), at rates which correspond closely to their dimerization rates suggesting an inner-sphere controlled mechanism.

Pulse radiolysis studies of alkaline iron(III) and iron(VI) solutions. Observation of transient iron complexes with intermediate oxidation states.

The first spectroscopic evidence for complexes containing iron formally in the IV and V oxidation states in the presence of a simple ligand, i.e., OH/sup -/ and P/sub 2/O/sub 7//sup 4 -/ in alkaline solution is reported. These transient species are obtained by pulse radiolysis of alkaline ferric (Fe(III)) and ferrate (Fe(VI)) solutions by using the hydroxyl radical or its conjugate base, O/sup -/, and the aquated electron, e/sub aq//sup -/, as the respective oxidizing and reducing agents. 18 references, 1 figure.

Enzyme-catalyzed free radical reactions with nicotinamide-adenine nucleotides: I. Lactate dehydrogenase-catalyzed chain oxidation of bound NADH by superoxide radicals

The radiation-induced oxidation of NADH by Superoxide radicals proceeds in the presence of lactate dehydrogenase by a chain mechanism. The reaction is initiated by O2− radicals, and propagated by O2. The chain length is a function of [NADH]/[lactate dehydrogenase], the concentration of O2, the dose rate, and pH. The chain reaction can be inhibited by addition of ascorbic acid.

Some observations on the chemistry of KO2-DMSO solutions

Dimethyl sulfoxide (DMSO) is the most commonly used non-aqueous solvent for the superoxide ion (0;). Three main sources for 0; in DMSO are potassium superoxide (KOs) [ 11, tetramethylammonium superoxide [2] ((CH&N’O;) and electrochemical reduction of dissolved O2 in the presence of supporting electrolytes [3]. A comprehensive discussion of these solutions is given in [4]. The solubility of KOz in DMSO can be increased by a factor of 30 by addition of 18crown-6 ether, which complexes K’. Materials were stored, solutions made up and manipulations carried out in a dry-box under purified nitrogen or helium. Cytochrome c from horse heart, and nitroblue tetrazolium were from Sigma Chemical Co. The 18crown-6 ether from Alfa products was purified by precipitation from dimethoxyethane [lo]. Potassium hydride O(H) dispersed in mineral oil, was obtained from Aldrich Chemicals and was washed with hexane and dried under Nz. Spectra were obtained with a Cary 210 spectrophotometer.

THE OXIDATION OF PHENOL BY FERRATE(VI) AND FERRATE(V). A PULSE RADIOLYSIS AND STOPPED-FLOW STUDY

Potassium ferrate, K2FeO4, is found to oxidize phenol in aqueous solution (5.5 < or = pH < or = 10) by a process which is second order in both reactants; -d[FeVI]/dt=k1[FeVI][phenol], k1 = 10(7)M-1s-1. Product analysis by HPLC showed a mixture of hydroxylated products, principally paraquinone, and biphenols that indicate that oxidation of phenol occurs by both one-electron and two-electron pathways. The two-electron oxidant, producing both para- and ortho-hydroxylated phenols is considered to be ferrate(V) which is itself produced by the initial one-electron reduction of ferrate(VI). The rate of ferrate(V) reaction with phenol was determined by pre-mix stopped flow pulse-radiolysis and found to be k7 = (3.8 +/- 0.4) x 10(5)M-1s-1.

Reactivity of ferrate(V) with carboxylic acids: A pre-mix pulse radiolysis study

Abstract Rates of reduction of ferrate(VI) to ferrate(V) by a number of organic acid and ester radicals (monocarboxylic acids, dicarboxylic acids, amino acids, malonic acid esters), generated by the pulse radiolysis technique, vary from 10 7 –10 9 dm 3 mol -1 s -1 . The rate at which these radicals reduce ferrate(VI) depends upon the nature of the substituents at the α-carbon atom and decrease in the order α- C-NH 2 >α-C-OH>α-C-H.A similar dependence upon the α-C-groups(s) has been observed for the oxidation of the parent organic acid by ferrate(V), for which the rate constants vary from 10 1 -10 6 dm 3 mol -1 s -1 . An oxidation mechanism is being proposed in which ferrate(V) oxidizes the carboxylic acid by a two-electron process. The rate of the oxidation process is dependent on the protonation of ferrate(V). For example, in the oxidation of gluconic acid with H 2 Fe V O 4 - /HF e V O 4 2- ; k 10 (H 2 Fe V O 4 - + gluconate) = 1.1 × 10 6 dm 3 mol -1 s -1 and k 11 (HFe V O 4 2- + gluconate) = 2.0 × 10 5 dm 3 mol -1 s -1 . The oxidation mechanisms for malate and asparate by OH radicals and ferrate(V) are compared.