Abstract
The widespread interest in free radicals in biology extends far beyond the effects of ionizing radiation, with recent attention largely focusing on reactions of free radicals derived from peroxynitrite (i.e., hydroxyl, nitrogen dioxide, and carbonate radicals). These radicals can easily be generated individually by reactions of radiolytically-produced radicals in aqueous solutions and their reactions can be monitored either in real time or by analysis of products. This review first describes the general principles of selective radical generation by radiolysis, the yields of individual species, the advantages and limitations of either pulsed or continuous radiolysis, and the quantitation of oxidizing power of radicals by electrode potentials. Some key reactions of peroxynitrite-derived radicals with potential biological targets are then discussed, including the characterization of reactions of tyrosine with a model alkoxyl radical, reactions of tyrosyl radicals with nitric oxide, and routes to nitrotyrosine formation. This is followed by a brief outline of studies involving the reactions of peroxynitrite-derived radicals with lipoic acid/dihydrolipoic acid, hydrogen sulphide, and the metal chelator desferrioxamine. For biological diagnostic probes such as ‘spin traps’ to be used with confidence, their reactivities with radical species have to be characterized, and the application of radiolysis methods in this context is also illustrated.
Highlights
A dramatic expansion of the scope of free radicals in biology followed the discovery of a role in biology for peroxynitrite (peroxynitrite refers to the sum of peroxynitrite anion (ONOO− ) and its conjugated acid, peroxynitrous acid (ONOOH, pKa = 6.8)) [6,7,8,9,10] formed by the reaction of O2 − with nitric oxide ( NO, nitrogen monoxide)
Many specific free radicals can be generated by exploiting this knowledge and radiolysis can be applied to studying diverse reactions of biological interest involving free radicals produced in both normal and pathological conditions in the absence of radiation
We have illustrated here how this experience can be applied in the wider field of free radicals in biology
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. In this article, reflecting the wider interests noted above, we focus on outlining and illustrating the use of radiolysis methods to generate specific, individual free radicals of interest in the context of peroxynitrite biochemistry and follow their reactions with amino acids, proteins, and other biologically relevant targets. Studies on oxidation of tryptophan (TrpH) and tyrosine (TyrOH), either free or in peptides, exploited the selectivity of secondary inorganic oxidants ((SCN)2 − , Br2 − , N3 , CO3 − , SeO3 − and NO2 ) to oxidize these amino acids without secondary reactions such as H-abstraction from side-chains [24,25,26] These selective oxidants are generated by reaction of OH with the appropriate ion (e.g., N3 − (Equation (2)), NO2 − (Equation (3)), CO3 2− (Equation (4)) or HCO3 − (Equation (5))),. The outstanding advantage of radiolysis as a technique to study radical reactions is that the concentrations of radicals are well-defined, and controllable, but even the radical yields need careful attention in deriving quantitative information such as extinction coefficients of species and the rate constants for radical-radical reactions
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