Stable free radicals, origin

Free radicals react most efficiently with other free radicals, and the chemicals that have relatively stable free radicals, such as 2,2-diphenyl-1-pricylhydrazyl (DPPH), could be used, in principle, to quantitatively estimate the amount of free radicals (DPPH method). The peroxide formed on the surface that derived from free radical could be quantitatively analyzed by the determination of iodine liberated from KI solution, which could be used to calculate the amount of the original free radical from that peroxide was derived (iodine method). 

Amino acids are quickly deaminated by L-ascorbic acid, leading to browning reactions (66). In the presence of oxygen, iron, and ascorbic acid or DHA, the amino acids gave ammonia, carbon dioxide, and an aldehyde with one carbon less than the original acid (67,68). The aldehydes are isolated as dimedone derivatives and are useful for identification of the amino acids. In the presence of copper and UV light, the deamination is increased. The red color 69-73) formed upon reaction of DHA with amino acids was used for their detection. Recent studies (74-78) of the reaction of DHA with amino acids led to the isolation of a product that changes readily to a novel, stable, free radical species... 

Altschul et al. (1, 2) originally discovered that cytochrome c peroxidase reacts with a stoichiometric amount of hydroperoxide to form a red peroxide compound, which will be referred to hereafter as Compound ES. It has a distinct absorption spectrum, as shown in Fig. 2. The formation of Compound ES from the enzyme and hydroperoxides is very rapid (fci > 10 10 sec"M. No intermediate, which precedes Compound ES, has been thus far detected. In the absence of reductants, or S2, Compound ES is highly stable. The rate constant of its spontaneous decay is of the order of 10 sec 22). The primary peroxide compound (Compound I) of horseradish peroxidase decays much faster at a rate of 10 sec (6). This unusual stability of Compound ES allows one to determine various physical and chemical parameters quantitatively and reliably. Titrations of Compound ES with reductants such as ferrocjHio-chrome c Iff, 20) and ferrocyanide 18, 34) have established that Compound ES is two oxidizing equivalents above the original ferric nnzyme. The absorption spectrum of Compound ES is essentially identical to that of Compound II of horseradish peroxidase which contains one oxidizing equivalent per mole in the form of Fe(IV). In addition, EPR examinations have revealed that Compound ES contains a stable free radical, the spin concentration of which is approximately one equivalent per mole (Fig. 3). Therefore, it is reasonable to conclude that two oxidiz-... 

Other teams worked on the functionalization of the aminoxyl group situated at the co position. For instance, the method of Ding et al. [342] is original for the synthesis of a novel series of poly(sodium styrenesulfonate) (PSSNa) macromonomers (compound 3 in Scheme 74) based on stable free radical polymerization in the presence of TEMPO. 

The nature, origin, and properties of the second group of humic materials are not yet fully understood (36). They comprise approximately 85-90% of the soil humus. The active portion of soil humus is made of various humic acids and small amounts of stable free radicals. 

However, antioxidant screening in complex mixtures of plant origin requires simple and rapid in vitro models for a possible combination with chromatographic techniques such as TLC, HPLC, HSCCC, or CPC. The reduction of the stable free-radical DPPH (l,l-diphenyl-2-picryUiydrazyl) by antioxidant substances is currently the most widely used chemical test for the screening of plant extracts. Only a few bioassay-guided fractionation processes have also evaluated the antimicrobial, antibacterial activity, or cytotoxicity of fractions enriched in specific flavonoids by using HSCCC or CPC liquid-liquid systems [30, 35]. 

The stratosphere contains, however, only small amounts--a few tenths of a ppb-of chlorine free radicals of natural origin. They are produced by the decomposition of methyl chloride, CH3Q. The nitrogen oxides (NO and NO2) are more abundant and are produced in the stratosphere by the decomposition of nitrous oxide, N2O. Both CH3CI and N2O are of biological origin these compounds, released at the Earth s surface, are sufficiently stable to reach the stratosphere in significant amounts. 

The reactions between free radicals and flavonoids (or polyphenols) are assumed to form aroxyl radicals (PO) (reaction 8). The stability of these secondary radical species is an important element to be considered in their antioxidant actions. Flavonoids with similar reduction potentials can originate radicals with very different reactivity toward other molecules present in biological systems. While a stable and relatively nonreactive PO is also nonreactive to propagate the chain reaction, a high reactive PO would propagate rather than interrupt a chain reaction. 

R-NO are formed. The behavior of propylene is presumably to form the fairly stable radical R—CH2—CH—CH3, which may dimerize. These methods are, for a number of reasons, only partially successful in elucidating the mechanism of the original reaction. In the first instance there is always the possibility that the added inhibitor may play a role in altering the original reaction. This is certainly evidenced in the case of NO in some instances it may even accelerate the reaction.A second difficulty is that the inhibition or capture of free radicals is incomplete, i.e., the radicals may react with other substances either more rapidly or rapidly enough to make the data ambiguous. Finally, there are always the problems of back reactions and of further decomposition of the radical-inhibitor products, found in the case of reactions of CH3CO with V and also for products RNO. These same difficulties appear in the mirror techniques. In brief, while these methods are valuable in certain instances, their use must be circumscribed by a careful consideration of the reaction studied. 

---