Free radicals formation

Because the decomposition is first order, the rate of free-radical formation can be controlled by regulating the temperature equations relating half-life to temperature are provided in Table 7. These decomposition rates ate essentially independent of the solvent (73). 

Certain compounds, known as chelating agents (qv), react synergisticaHy with many antioxidants. It is beheved that these compounds improve the functional abiUties of antioxidants by complexing the metal ions that often initiate free-radical formation. Citric acid and ethylenediaminetetraacetic acid [60-00-4] (EDTA), C2QH2gN20g, are the most common chelating agents used (22). 

Free-Radical Formation. Hydrogen peroxide can form free radicals by homolytic cleavage of either an O—H or the O—O bond. 

There are numerous synthetic and natural compounds called antioxidants which regulate or block oxidative reactions by quenching free radicals or by preventing free-radical formation. Vitamins A, C, and E and the mineral selenium are common antioxidants occurring naturally in foods (104,105). A broad range of flavonoid or phenoHc compounds have been found to be functional antioxidants in numerous test systems (106—108). The antioxidant properties of tea flavonoids have been characterized using models of chemical and biological oxidation reactions. 

The reactive species that iaitiate free-radical oxidatioa are preseat ia trace amouats. Exteasive studies (11) of the autoxidatioa mechanism have clearly estabUshed that the most reactive materials are thiols and disulfides, heterocycHc nitrogen compounds, diolefins, furans, and certain aromatic-olefin compounds. Because free-radical formation is accelerated by metal ions of copper, cobalt, and even iron (12), the presence of metals further compHcates the control of oxidation. It is difficult to avoid some metals, particularly iron, ia fuel systems. 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

Such free-radical formation may be generally indicated as I—I------->21—... 

Coadministration of dexrazoxane Chelation of iron in the heart, correction of iron dysregulation or mitigation of free radical formation Approved for use in patients who continue DOX above 300 mg/m2 or require another anthracycline after a prior exposure to 300 mg of DOX/m2... 

DOX, as EPI seems to form fewer amounts of ROS and secondary alcohol metabolite, (ii) encapsulation of anthracyclines in uncoated or pegylated liposomes that ensure a good drug delivery to the tumor but not to the heart, (iii) conjugation of anthracyclines with chemical moieties that are selectively recognized by the tumor cells, (iv) coadministration of dexrazoxane, an iron chelator that diminishes the disturbances of iron metabolism and free radical formation in the heart, and (v) administration of anthracyclines by slow infusion rather than 5-10 min bolus (Table 1). Pharmacological interventions with antioxidants have also been considered, but the available clinical studies do not attest to an efficacy of this strategy. 

The observation (Porter ef a ., 1972) that added BrCCla almost completely suppresses the polarization of the olefin, while leaving the polarization of trans-4 unalfected, points to the secondary radical pair as the principal immediate precursor of a-methylstyrene. A rate constant for the decomposition of thediazenyl radical of 10 -10 sec has been estimated. Cage collapse and free-radical formation are also thought to occur and appropriately polarized products have been identified (see above). 

The oxidation of polymers is most commonly depicted in terms of the kinetic scheme developed by BoUand [14]. The scheme is summarized in Figure 15.1. The key to the process is the initial formation of a free-radical species. At high temperatures and at large shear forces, it is likely that free-radical formation takes place by cleavage of C-C and C-H bonds. 

The coupling of two DIT molecules to form T4—or of an MIT and DIT to form T3—occurs within the thyroglobulin molecule. A separate coupfing enzyme has not been found, and since this is an oxidative process it is assumed that the same thyroperoxidase catalyzes this reaction by stimulating free radical formation of iodotyrosine. This hypothesis is supported by the observation that the same drugs which inhibit H oxidation also inhibit coupfing. The formed thyroid hor-... 

A discussion of ligand exchange reactions of organometallic compounds associated with oxidation-reduction processes leading to free-radical formation will be found in Volume 14 (Free-radical polymerization). 

Renchrona, S., Hauge, H.N. andSiesjo, B.K. (1989). Enhancement of iron-catalyzed free radical formation by acidosis in brain homogenates difference in effect by lactic acid and CO2. J. Cereb. Blood Flow Metab. 9, 65-71. 

Norrins, A.L. (1962). Free radical formation in the skin following exposure to ultraviolet light. J. Invest. Dermatol. 39, 445-448. 

Reoxygenation following transient anoxia increases oxygen free radical formation by isolated rat hepatocytes. Hepatology 16, 159A. 

Riesz P, Kondo T (1992) Free radical formation induced by ultrasound and its biological implications. Free Radic Biol Med 13 247-270... 

---