One-electron reduction of nitrobenzenes by .alpha.-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction

Jagannadham, V.; Steenken, S.

doi:10.1021/ja00334a015

Cited by 118 publications

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“…The carbon- centered radical at C-5' (structure 1), produced by hydrogenatom abstraction by a radical species of NCS-Chrom, forms a labile nitroxyl radical adduct (structure 2) with misonidazole (RNO2) (reaction 1). The addition of carboncentered radicals to the oxygen of the nitro group of tetranitromethane and nitrobenzenes to give intermediate nitroxide radicals has been documented by electron spinresonance spectroscopy and pulse-radiolysis methods (13)(14)(15). While such adducts can act as intermediates in one-electron oxidations, as has been described in radiationsensitization reactions with pyrimidines and their derivatives (15), they also can undergo fragmentation (reaction 2) to form oxy radicals (structure 3) and the nitroso reduction product of misonidazole, a two-electron process (16).…”

Section: Discussionmentioning

confidence: 99%

3'-Formyl phosphate-ended DNA: high-energy intermediate in antibiotic-induced DNA sugar damage.

Chin

Kappen

Goldberg

1987

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

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Under anaerobic conditions where the nitroaromatic radiation-sensitizer misonidazole substitutes for dioxygen, DNA strand breakage (gaps with phosphate residues at each end) by the nonprotein chromophore of the antitumor antibiotic neocarzinostatin (NCS-Chrom) is associated with the generation of a reactive form of formate from the C-5' of deoxyribose of thymidylate residues. Such lesions account for a minority (10-15%) of the strand breakage found in the aerobic reaction without misonidazole. Amino-containing nucleophiles such as tris(hydroxymethyl)aminomethane (Tris) and hydroxylamine act as acceptors for the activated formate. The amount of [3H formyl hydroxamate produced from DNA labeled with [5'-3H]thymidine is comparable to the spontaneously released thymine. During the course of the reaction, misonidazole undergoes a DNA-dependent reduction and subsequent conjugation with glutathione used to activate NCSChrom. From these and earlier results, we propose a possible mechanism in which the carbon-centered radical formed at C-5' by hydrogen atom abstraction by thiol-activated NCSChrom reacts anaerobically with misonidazole to form a nitroxyl-radical-adduct intermediate, which fragments to produce an oxy radical at C-5'. j3-Fragmentation results in cleavage between C-5' and C-4' with the generation of 3'-formyl phosphate-ended DNA, a high-energy form of formate, which spontaneously hydrolyzes, releasing formate and creating a 3'-phosphate end, or transfers the formyl moiety to available nucleophiles. A similar mechanism, involving dioxygen addition, is probably responsible for the 10-15% DNA gap formation in the aerobic reaction.as to position the reactive ring system containing the two acetylenic bonds close to the C-5' of deoxyribose of mainly thymidylate residues.Upon activation by thiol [glutathione in the cell (2)], the DNA-bound drug is converted into a species, presumably free-radical form, that abstracts a hydrogen atom from C-5' to generate a carbon-centered radical, whether or not dioxygen is present (3). Addition of dioxygen to the radical at C-5' forms a peroxyl radical species that undergoes reduction by thiol to generate a DNA strand break with a nucleoside 5'-aldehyde group at the 5' end and a phosphate at the 3' end.Such lesions account for more than 80% of the strand breaks produced (4). The remaining 10-15% of the strand breaks are actually gaps with phosphates at both ends. The mechanism for their production has not been clear, although it has been thought to be related to the formation of formate from C-5' (5). In addition to single-strand breaks, DNA bases (mainly thymine) are also released (6, 7), in part generating abasic sites. When nitroaromatic radiation sensitizers, such as misonidazole, are substituted for dioxygen in the reaction, gaps with phosphates at both ends constitute the main form of DNA breakage (8), and this is associated with base (mainly thymine) release and a 4-to 5-fold increase (over the aerobic reaction) in the release of a soluble material labeled with tri...

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Section: Discussionmentioning

confidence: 99%

3'-Formyl phosphate-ended DNA: high-energy intermediate in antibiotic-induced DNA sugar damage.

Chin

Kappen

Goldberg

1987

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

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“…From one of the author's work published from elsewhere [14], now it is the turn for "N" when it replaces "C" on some free radical reactions, though it is not the effect of hetero atom on Hammett's ρ value but on the effect of hetero atom on the formation reaction (k r ) of nitroxide radical and its heterolysis (k s ) is still quite stanch and striking. The example of these reactions is the reactions of nitrobenzene and 4-nitropyridine with α-hydroxy methyl and α-hydroxy ethyl radicals.…”

Section: Resultsmentioning

confidence: 99%

Effect of Hetero Atom on the Hammett’s Reaction Constant (ρ) from the Physical Basis of Dissociation Equilibriums of (Dithio) Benzoic Acids and (Thio) Phenols and Its Application to Solvolysis Reactions and Some Free Radical Reactions

Jagannadham

Sanjeev

2012

Advances in Physical Chemistry

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The emergence of putative Hammett equation in mid 1930s was a boon to physical-organic chemists to elucidate the reaction mechanisms of several organic reactions. Based on the concept of this equation several hundreds of papers have emerged in chemical literature in the last century on the effect of structure, on reactivity, and very few on thermodynamic stability and kinetic reactivity of intermediates. In this article an attempt is made to explain the effect of hetero atom on Hammett's reaction constant (ρ) taking the dissociation equilibriums of benzoic acids, dithiobenzoic acids, phenols, and thiophenols.

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“…In addition to the above example, there were several other reactions involving various reducing radicals generated from alcohols and nucleic acid type bases like 6-methyl uracil, 6-methyl-i-cytosine and 6-methyl dihydrouracil with fourteen nitrobenzenes [14,17,20] and some solvolysis reactions of benzyl-gem-dichlorides, dibromides and diazides [21,22,23]. Some gas-phase reactions:…”

Section: Discussionmentioning

confidence: 99%

The ‘<i>Yard Stick</i>’ to Interpret the Entropy of Activation in Chemical Kinetics: A Physical-Organic Chemistry Exercise

Sanjeev¹,

Padmavathi²,

Jagannadham³

2018

WJCE

Self Cite

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No physical or physical-organic chemistry laboratory goes without a single instrument. To measure conductance we use conductometer, pH meter for measuring pH, colorimeter for absorbance, viscometer for viscosity, potentiometer for emf, polarimeter for angle of rotation, and several other instruments for different physical properties. But when it comes to the turn of thermodynamic or activation parameters, we don't have any meters. The only way to evaluate all the thermodynamic or activation parameters is the use of some empirical equations available in many physical chemistry text books. Most often it is very easy to interpret the enthalpy change and free energy change in thermodynamics and the corresponding activation parameters in chemical kinetics. When it comes to interpretation of change of entropy or change of entropy of activation, more often it frightens than enlightens a new teacher while teaching and the students while learning. The classical thermodynamic entropy change is well explained by Atkins [1] in terms of a sneeze in a busy street generates less additional disorder than the same sneeze in a quiet library (Figure 1) [2]. The two environments are analogues of high and low temperatures, respectively. In this article making use of Eyring equation a factor usually called 'universal factor' is derived and made use as a 'yard stick' to interpreting the change in entropy of activation for physical or physical-organic chemistry senior undergraduate and graduate students' class-room.Peter Atkins Figure 1. Keywords: entropy, universal factor, kineticsCite This Article: R. Sanjeev, D. A. Padmavathi, and V. Jagannadham, "The 'Yard Stick' to Interpret the

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One-electron reduction of nitrobenzenes by .alpha.-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction

Cited by 118 publications

References 0 publications

3'-Formyl phosphate-ended DNA: high-energy intermediate in antibiotic-induced DNA sugar damage.

3'-Formyl phosphate-ended DNA: high-energy intermediate in antibiotic-induced DNA sugar damage.

Effect of Hetero Atom on the Hammett’s Reaction Constant (ρ) from the Physical Basis of Dissociation Equilibriums of (Dithio) Benzoic Acids and (Thio) Phenols and Its Application to Solvolysis Reactions and Some Free Radical Reactions

The ‘<i>Yard Stick</i>’ to Interpret the Entropy of Activation in Chemical Kinetics: A Physical-Organic Chemistry Exercise

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