Electrochemical studies of the reactivity of superoxide ion with several alkyl halides in dimethyl sulfoxide

Merritt, Margaret V.; Sawyer, Donald T.

doi:10.1021/jo00832a011

Cited by 102 publications

(78 citation statements)

References 2 publications

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“…The superoxide ion is stable in aprotic solvents. Solutions of alkylamine superoxide in aprotic solvents have been used to study the reactivity of the superoxide ion as a nucleophilic reagent (5).…”

Section: Introductionmentioning

confidence: 99%

Solvation of Cl⁻ and O₂⁻ with H₂O, CH₃OH, and CH₃CN in the gas phase

Yamdagni¹,

Payzant²,

Kebarle³

1973

Can. J. Chem.

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Determination of the temperature dependence of the equilibrium constants K,,,,,-, for the reactions A-B. = A-B,-, + B where A -equals C1-and 0,-and B is HOH, C H 3 0 H , or CH3CN leads t o the corresponding AH0 ,,-,,,,, AGO .-,,,, and i\So,-,,,, values. The experimental technique is based on mass spectron~etric detection of ions escaping from a high pressure ion source. At 11 = 1, CI-is solvated most strongly by methanol, then CH3CN and HOH. At higher a cross over is observed with water becoming the best solvent. These results are in agreement with the positive transfer enthalpies and free energies for CI-from the liquid solvents HOH -> C H 3 0 H and HOH -> CH3CN reported in the literature. 0,-is solvated more strongly than C1-appearing thus as an ion of "size" intermediate between C1-and F-. Again C H 3 0 H gives the highest interaction for 12 = I , however for n > 1 water gives stronger interactions. 0,-est solvat6 plus fortement que C1-indiquant ainsi que la taille de cet ion est intermediaire entre C1-et F -. C H 3 0 H montre encore la plus forte interaction pour II = 1, cependant pour 11 > 1, c'est I'eau qui conduit aux interactions les plus fortes.[Traduit par le journal]Can. J. Chem., 51. 2507 (1973) Introduction A comparative study of the gas phase solvation of the halide ions by water and acetonitrile lllolecules was published earlier (1). The present work extends the study to the 0,-ion and includes methanol as a new solvent. The 0,-ion is an important ion in the ionosphere and troposphere (2) where it results from electron capture by 0,. The superoxide ion 0,-is stable also in condensed phases. Thus paramagnetism and X-ray crystal studies of the solid metal superoxides (NaO,, KO,, etc.) have shown (3) that these contain discrete free radical anions 0 , -. The 0,-ion although unstable in aqueous solution plays an important role in biological systems (4). The superoxide ion is stable in aprotic solvents. Solutions of alkylamine superoxide in aprotic solvents have been used to study the reactivity of the superoxide ion as a nucleophilic reagent (5).In the gas phase the 0,-ion forms stable hydrates 0,-(H,O),, (6) and other solvates: 0,-(CH,OH),,, 0,-(CH,CN),, etc. Thus the stabilities of these solvates can be studied and

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Section: Introductionmentioning

confidence: 99%

Solvation of Cl⁻ and O₂⁻ with H₂O, CH₃OH, and CH₃CN in the gas phase

Yamdagni¹,

Payzant²,

Kebarle³

1973

Can. J. Chem.

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“…We suggest that the initial steps in the photodegradation of pheomelanin involve the photochemically mediated transfer of an electron from the pigment to oxygen as outlined in scheme 1. Production of superoxide per se does not account for the destruction of the pheomelanin chromophore. Superoxide has been reported to exhibit nucleophilic reactivity (30), to behave as a one-electron reducing agent (31), and to behave as a oneelectron oxidizing agent (32 In conclusion, evidence has been presented that oxygen reacts with photochemically excited pheomelanin by means of an electron transfer affording the superoxide anion. Although many other aspects of the photolysis are still unknown (e.g., involvement of singlet oxygen, the mechanism of pigment destruction and the possible involvement of hydroxyl radicals), the identification of superoxide as a primary photoproduct has serious implications.…”

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confidence: 96%

Photodestruction of pheomelanin: role of oxygen.

Chedekel¹,

Smith²,

Post³

et al. 1978

Proc. Natl. Acad. Sci. U.S.A.

170

127

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Pheomelanin, the red-brown polymeric pigment in the skin and hair of red-headed humans, is composed of a protein fraction covalently bound to a colored chromophore. Photolysis of aerated aqueous pheomelanin solutions, isolated from human red hair, results in destruction of the chromophore and liberation of the peptide fraction. The rate of photolysis depends on the pH and the concentration of both pigment and oxygen and is slightly inhibited by the enzyme superoxide dismutase (superoxide:superoxide oxidoreductase EC 1.15.1.1). Pheomelanin photolyzed in the presence of nitroblue tetrazolium results in the formation of a blue diformazan, whether or not oxygen is present. Superoxide dismutase inhibits the aerobic photoreduction of nitroblue tetrazolium whereas, in the absence of oxygen, no inhibition is observed. These experiments strongly suggest the involvement of superoxide in the aerobic photolysis of pheomelanin and point out a possible mechanism for ultraviolet-induced cell damage in redheads. The black or brown eumelanins of human skin afford protection against the damaging effects of the ultraviolet component in sunlight (1)(2)(3)(4). Light of these wavelengths, 280-380 nm, is nondestructive to eumelanin and produces reversible changes that are believed to function as mechanisms in this pigment's photoprotective ability. Two such reversible reactions are (i) immediate pigment darkening (3,5,6), an apparent oxidation-reduction reaction resulting in the darkening of preformed pigment, and (ii) an increase in the number and a change in the nature of the unpaired electrons in the pigment (7-9).Fair-skinned humans exhibit a number of abnormal reactions to sunlight, including freckling (10) and a high susceptibility to skin cancer (11)(12)(13)(14). These have usually been attributed to the fact that the skin of these people has a poor tanning capacity, sunburns readily, and contains little pigment. Although pheomelanin, the red-brown or yellow pigment found in the hair of fair-skinned humans (15,16), has yet to be isolated from human skin, there is an increasing amount of indirect evidence that it occurs in melanosomes found in various parts of the body, including the skin (17-22). Pheomelanin is readily photodegraded under physiologically relevant conditions (23, 24), an observation that has led us to suggest the following four mechanisms by which ultraviolet light may deleteriously affect cells containing this pigment: (i) loss of a pigment purportedly responsible for photoprotection; (ii) formation of dermatitic or carcinogenic photoproducts; (Mii) formation of dermatitic or carcinogenic compounds by reactions of photochemically produced intermediates with normal cell constituents; and (iv) formation of photoproducts that inhibit the enzymatic systems responsible for repair of ultraviolet-induced damage. In this paper we support the viability of mechanism iii by presenting evidence that a highly reactive intermediate is formed during the course of the photolysis.The publication costs of this article we...

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“…Sawyer and co-workers [1][2][3] pioneered work on superoxide ion (O 2 •Ϫ ), particularly the direct electrochemical reduction of dissolved oxygen gas in aprotic solvents to form O 2 •Ϫ according to the following reaction…”

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confidence: 99%