Spectral Probe for Electron Transfer and Addition Reactions of Azide Radicals with Substituted Quinoxalin-2-Ones in Aqueous Solutions

Abstract

The azide radical (N3●) is one of the most important one-electron oxidants used extensively in radiation chemistry studies involving molecules of biological significance. Generally, it was assumed that N3● reacts in aqueous solutions only by electron transfer. However, there were several reports indicating the possibility of N3● addition in aqueous solutions to organic compounds containing double bonds. The main purpose of this study was to find an experimental approach that allows a clear assignment of the nature of obtained products either to its one-electron oxidation or its addition products. Radiolysis of water provides a convenient source of one-electron oxidizing radicals characterized by a very broad range of reduction potentials. Two inorganic radicals (SO4●−, CO3●−) and Tl2+ ions with the reduction potentials higher, and one radical (SCN)2●− with the reduction potential slightly lower than the reduction potential of N3● were selected as dominant electron-acceptors. Transient absorption spectra formed in their reactions with a series of quinoxalin-2-one derivatives were confronted with absorption spectra formed from reactions of N3● with the same series of compounds. Cases, in which the absorption spectra formed in reactions involving N3● differ from the absorption spectra formed in the reactions involving other one-electron oxidants, strongly indicate that N3● is involved in the other reaction channel such as addition to double bonds. Moreover, it was shown that high-rate constants of reactions of N3● with quinoxalin-2-ones do not ultimately prove that they are electron transfer reactions. The optimized structures of the radical cations (7-R-3-MeQ)●+, radicals (7-R-3-MeQ)● and N3● adducts at the C2 carbon atom in pyrazine moiety and their absorption spectra are reasonably well reproduced by density functional theory quantum mechanics calculations employing the ωB97XD functional combined with the Dunning’s aug-cc-pVTZ correlation-consistent polarized basis sets augmented with diffuse functions.

Influence of Ionizing Radiation on Spontaneously Formed Aggregates in Proteins or Enzymes Solutions

“…In contrast to the • OH radical, radical appears to react primarily via one-electron transfer and it is much more selective in its reaction than is the hydroxyl radical. However, there are several reports indicating the possibility of azide radical addition to organic compounds containing double bonds in aqueous solutions [ 9 , 10 ]. One of the methods of protein cross-linking is the formation of dityrosine bridges, which are formed as a result of the recombination of long-lived tyrosyl radicals (TyrO • ) generated under oxidative conditions.…”

The Physical Chemistry and Chemical Physics (PCCP) Section of the International Journal of Molecular Sciences in Its Publications: The First 300 Thematic Articles in the First 3 Years

“…Within the framework of this direction, only 11 articles have been published [ 224 , 225 , 226 , 227 , 228 , 229 , 230 , 231 , 232 , 233 , 234 ], among which there is 1 review. In terms of its subject matter, this direction, to some extent, intersects with both Thematic Area 5 and Thematic Area 6, since ions and radicals are among the objects of Thematic Area 5, and the structure and reactivity of these compounds, obtained as a result of theoretical research (carried out, as a rule, by quantum–chemical methods), are among the objects of Thematic Area 6.…”

“…By analogy, with the systematization of articles indicated in Thematic Area 5, here, it is also advisable to subdivide all publications presented in it into two groups, in the first of which there will be articles on organic compounds, in the second, articles on inorganic compounds. Following this differentiation, the first of these groups should include 7 articles, namely [ 227 , 228 , 229 , 230 , 231 , 232 , 234 ], and the second, 4 articles [ 224 , 225 , 226 , 233 ]. The most significant among them, in our opinion, is review [ 231 ], devoted to the physical chemistry of one of the most stable known radicals, 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl (also called diphenylpycrylhydrazil or DPPH), which was opened in 1992, and will make it exactly 100 years old, in 2022.…”

“…The most significant among them, in our opinion, is review [ 231 ], devoted to the physical chemistry of one of the most stable known radicals, 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl (also called diphenylpycrylhydrazil or DPPH), which was opened in 1992, and will make it exactly 100 years old, in 2022. The rest of the articles of this thematic direction, the objects of which were organic compounds, for the most part, were also related to the study of radical particles; for example, in [ 230 ], this was the azide radical, in [ 228 , 229 ], the radicals formed as a result of the interaction of 5-nitro-2-chloropyrimidine and 2-nitrofuran, respectively, with a flux of low-energy electrons. The only exception here is paper [ 232 ], in which the object of study was the C 6 H 6 + cation formed during the ionization of benzene as a result of electron impact.…”

Substituent Effect in the Cation Radicals of Monosubstituted Benzenes

“…They are not stable, and in the subsequent anodic reaction, the second electron or a proton can be lost to form a dication or a neutral radical. Cation radicals are also formed as a response to radiolysis [40][41][42][43]: photolysis or ion or particle-beam irradiation; external alpha, beta, and gamma radiation; or radiation by the embedded radionuclides, etc. Different types of radiation produce direct proteins, carbohydrates, DNA and RNA damage, or indirectly destroy the biomolecules by generating water radical ions [43][44][45][46][47].…”