Oxidative stress has been proven to be related to the onset of a large number of health disorders. This chemical stress is triggered by an excess of free radicals, which are generated in cells because of a wide variety of exogenous and endogenous processes. Therefore, finding strategies for efficiently detoxifying free radicals has become a subject of a great interest, from both an academic and practical points of view. Melatonin is a ubiquitous and versatile molecule that exhibits most of the desirable characteristics of a good antioxidant. The amount of data gathered so far regarding the protective action of melatonin against oxidative stress is overwhelming. However, rather little is known concerning the chemical mechanisms involved in this activity. This review summarizes the current progress in understanding the physicochemical insights related to the free radical-scavenging activity of melatonin. Thus far, there is a general agreement that electron transfer and hydrogen transfer are the main mechanisms involved in the reactions of melatonin with free radicals. However, the relative importance of other mechanisms is also analyzed. The chemical nature of the reacting free radical also has an influence on the relative importance of the different mechanisms of these reactions. Therefore, this point has also been discussed in detail in the current review. Based on the available data, it is concluded that melatonin efficiently protects against oxidative stress by a variety of mechanisms. Moreover, it is proposed that even though it has been referred to as the chemical expression of darkness, perhaps it could also be referred to as the chemical light of health.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-079X.2011.00916.x

Melatonin as a natural ally against oxidative stress: a physicochemical examination

Nitrogen dioxide and carbonate radical anion: two emerging radicals in biology.

Peroxynitrite is a short-lived and reactive biological oxidant formed from the diffusion-controlled reaction of the free radicals superoxide (O2•–) and nitric oxide (•NO) In this review, we first analyze the biochemical evidence for the formation of peroxynitrite in vivo and the reactions that lead to it Then, we describe the principal reactions that peroxynitrite undergoes with biological targets and provide kinetic and mechanistic details In these reactions, peroxynitrite has roles as (1) peroxide, (2) Lewis base, and (3) free radical generator Physiological levels of CO2 can change the outcome of peroxynitrite reactions The second part of the review assesses the formation of protein 3-nitrotyrosine (NO2Tyr) by peroxynitrite-dependent and -independent mechanisms, as one of the hallmarks of the actions of •NO-derived oxidants in biological systems Moreover, tyrosine nitration impacts protein structure and function, tyrosine kinase signal transduction cascades and protein turnover Overall, the revi

Biochemistry of Peroxynitrite and Protein Tyrosine Nitration.

A Chemical Aqueous Phase Radical Mechanism (CAPRAM) for modelling tropospheric multiphase chemistry is described. CAPRAM contains (1) a detailed treatment of the oxidation of organic compounds with one and two carbon atoms, (2) an explicit description of S(IV)-oxidation by radicals and iron(III), as well as by peroxides and ozone, (3) the reactions of OH, NO 3 ,C l 2 , Br 2 ,a nd CO 3 radicals, as well as reactions of the transition metal ions (TMI) iron, manganese and copper. A modelling study using a simple box model was performed for three different tropospheric conditions (marine, rural and urban) using CAPRAM coupled to the RADM2-mechanism (Stockwell et al., 1990) for liquid and gas phase chemistry, respectively. In the main calculations the droplets are assumed as monodispersed with a radius of 1m and a liquid water content of 0.3 g m 3 .I n the coupled mechanism the phase transfer of 34 substances is treated by the resistance model of Schwartz (1989). Results are presented for the concentration levels of the radicals in both phases under variation of cloud duration and droplet radius. The effects of the multiphase processes are shown in the loss fluxes of the radicals OH, NO 3 and HO2 into the cloud droplets. From calculations under urban conditions considering gas phase chemistry only the OH maximum concentration level is found to be 5:5 10 6 cm 3 . In the presence of the aqueous phase (r D 1 m, LWC D 0: 3g m 3 ) the phase transfer constitutes the most important sink (58%) reducing the OH level to 1:0 10 6 cm 3 . The significance of the phase transfer during night time is more important for the NO3 radical (90%). Its concentration level in the gas phase (1:9 10 9 cm 3 ) is reduced to 1:4 10 6 cm 3 with liquid water present. In the case of the HO2 radical the phase transfer from the gas phase is nearly the only sink (99.8%). The concentration levels calculated in the absence and presence of the liquid phase again differ by three orders of magnitude, 6 10 8 cm 3 and 4:9 10 5 cm 3 , respectively. Effects of smaller duration of cloud occurrence and of droplet size variation are assessed. Furthermore, in the present study a detailed description of a radical oxidation chain for sulfur is presented. The most important reaction chain is the oxidation of (hydrogen) sulphite by OH and the subsequent conversion of SO 3 to SO 5 followed by the interaction with TMI (notably Fe 2C )a nd chloride to produce sulphate. After 36 h of simulation ((H2O2U0 D 1 ppb; (SO2U0 D 10 ppb) the direct oxidation pathway from sulfur(IV) by H2O2 and ozone contributes only to 8% (2:9 10 10 M s 1 ) of the total loss flux of S(IV) (3 :7 10 9 Ms 1 ).

CAPRAM2.3: A chemical aqueous phase radical mechanism for tropospheric chemistry

Photochemical reactions contribute to the transformation of contaminants and biogeochemically important substrates in environmental aquatic systems. Recent research has demonstrated that halogen radicals (e.g., Cl•, Br•, Cl2•–, BrCl•–, Br2•–) impact photochemical processes in sunlit estuarine and coastal waters rich in halides (e.g., chloride, Cl–, and bromide, Br–). In addition, halogen radicals participate in contaminant degradation in some engineered processes, including chlorine photolysis for drinking water treatment and several radical-based processes for brine and wastewater treatment. Halogen radicals react selectively with substrates (with bimolecular rate constants spanning several orders of magnitude) and via several potential chemical mechanisms. Consequently, their role in photochemical processes remains challenging to assess. This review presents an integrative analysis of the chemistry of halogen radicals and their contribution to aquatic photochemistry in sunlit surface waters and engineer...

Halogen Radical Oxidants in Natural and Engineered Aquatic Systems.

Rate constants have been measured by pulse radiolysis for the reactions of the carbonate radical, CO 3 .− , with a number of organic and inorganic reactants as a function of temperature, generally over the range 5 to 80 o C. The reactants include the substitution-inert cyano complexes of Fe II , Mo IV , and W IV , the simple inorganic anions SO 3 2− , ClO 2 − , NO 2 − , I − , and SCN-, several phenolates, ascorbate, tryptophan, cysteine, cystine, methionine, triethylamine, and allyl alcohol

Temperature dependence of the rate constants for reactions of the carbonate radical with organic and inorganic reductants

Rate Constants for Reactions of (Perhaloalkyl)peroxyl Radicals with Alkenes in Methanol

The Arrhenius parameters of the bimolecular rate constants for the decay of several phenoxyl radicals in aqueous solution were measured. The p-halophenoxyl radicals (F, Cl, and Br) decay in a diffusion controlled reaction as the activation energies are the same as that of diffusion of water (16 ± 1.5 kJ · mol−1). The A factors are 1012.2 ± 0.2. For alkyl and alkoxy substituted phenoxyl, slightly higher activation energies were found (19.5 − 21.9 kJ · mol−1). © 1993 John Wiley & Sons, Inc.

Temperature dependence of the self‐decay rate constants of some phenoxyl radicals in aqueous solution

Rate constants for the reactions of triplet decafluorobenzophenone (3DFB) with 30 alkenes have been measured, with values in the range 107−109 L mol-1 s-1. The rate constant increases upon substitu...

L. C. T. Shoute

Papers

Temperature dependence of the rate constants for reactions of the carbonate radical with organic and inorganic reductants

Rate Constants for Reactions of (Perhaloalkyl)peroxyl Radicals with Alkenes in Methanol

Temperature dependence of the self‐decay rate constants of some phenoxyl radicals in aqueous solution

Reactions of Triplet Decafluorobenzophenone with Alkenes. A Laser Flash Photolysis Study

Temperature Dependence of the Self-Decay Rate Constants of Some Phenoxyl Radicals in Aqueous Solution