Free Radicals and Environmental Considerations

Free radicals are highly reactive species, and their lifetimes depend on the presence of other radicals and target molecules or ions. For instance, in neutral aqueous solutions, SOT radicals derived from the photodissociation of S20 anions (4.23) decay mostly via recombination (fc25 = 1.1 x 109M 1s 1) that results in the regeneration of the parent S20 ion [33]  

In our laser flash photolysis experiments, the concentrations of SOI radicals after the actinic laser flash (around 60 mj/pulse/cm2) are typically approximately 10 gM and the lifetimes of the SOJ radicals are t25 = (2k25[SOJ ]0) 1 50 gs [54]. Decreasing the laser pulse energy to around 10mJ/pulse/cm2 results in the reduction of the initial sulfate radical concentration to [SOT ]0 1.7 gM and an increase in the lifetimes of SOT radicals, r25, to approximately 300 gs. 

In contrast, the decay of radicals in bimolecular reactions with other molecules (or ions), which are typically present at much higher concentrations than free radicals, does not depend on radical concentrations (or laser flash energy) and follows pseudo-first-order kinetics (4.16). An example of such a bimolecular reaction, is the one-electron oxidation (fc2S = 4.6 x 106M 1s 1) of bicarbonate anions by sulfate radicals [33]  

When the oxidizing species, an electron acceptor, and the electron donor are both embedded within a biological macromolecule (e.g., in a protein or DNA molecules), the reaction kinetics are entirely different from those in solution in which both species can diffuse freely and encounter one another in order to undergo chemical reaction. An example of such intramolecular processes is the one-electron oxidation of guanine (G) by a 2AP neutral radical, both site-specifi-cally positioned within a DNA duplex [28]. Here, both reaction partners are fixed within a DNA helix and the bimolecular reaction model is not suitable for describing the reaction kinetics (4.16). Instead, the kinetics of oxidation of G by 2AP(-H) radicals in double-stranded DNA follow first-order kinetics with the magnitudes 

The slopes of these plots are a measure of the parameter [3, which is a function of the intervening bases between the G and 2AP(-H) residues. We found that in the case of intervening thymines, the parameter 3 = 0.4 A-1 while in the case of intervening adenines 3 = 0.12 A-1 in double-stranded DNA [28, 56]. 

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