1.1- Diphenyl-2-picrylhydrazyl radical

The reduction of the stable 1,1 -diphenyl-2-picrylhydrazyl radical (DPPH) has been used to assess the efficiency of antioxidants in beverages (Larrauri et al, 1999 Porto et al, 2000), vegetable oils (Espin et al, 2000) and of pure phenolic compounds (Madsen et al, 2000), reaction [16.17] ... 

Popular EPR-based assays of antioxidant activity include the DPPH assay, in which the ability of compounds to quench (by H-atom transfer) the 1,1-diphenyl-2-picrylhydrazyl radical is used to rank their antioxidant activity. This method has been applied widely to the analysis of dietary antioxidants and extracts from medicinal plants.213 219 Extensive use has also been made of assays based on the competition between spin traps and test compounds for reaction with enzymatically-generated 02 and, in the presence of a metal catalyst, the OH rad-... 

DNAP, DNA-dependent DNA polymerase DNAS, DNA synthesis DPPH, 1,1 -diphenyl-2-picrylhydrazyl radical... 

Radical scavenging effects of tannins and related polyphenols were also evidenced by ESR studies for the effects on the superoxide anion radical generated in the hypoxanthine-xanthine oxidase system and on the 1,1-diphenyl-2-picrylhydrazyl radical which is often used as a model of free radicals in lipid peroxidation [100]. Investigation of structure-activity... 

Figure 2.5 Consumption of diphenyl picrylhydrazyl radicals in relation to surface area of...

The retarder may be a free radical which is too unreactive to initiate a Polymer chain (e.g. triphenyl methyl or diphenyl-picrylhydrazyl). The mechanism of retardation is simply the combination or disproportionation of radicals. 

Figure 15.1 The stable 2,2-diphenyl-1-picrylhydrazyl radical is used as a radical trap in a large number of kinetic studies.

The method used by Bawn and Mellish relies on the presence of a radical trap in the reaction mixture, that is, a compound that reacts very fast with the acyl radicals produced, thus preventing their recombination. This substance was the vivid colored 2,2-diphenyl-1-picrylhydrazyl radical (figure 15.1). When these nitrogen-centered radicals, herein abbreviated by P, react with an acyl radical (reaction 15.6), the solution color change can be monitored with a spectrophotometer. 

Radical Generation. The ESR spectrometer, flow system, and general procedure have been described (46). The apparatus was calibrated with freshly prepared diphenyl picrylhydrazyl (DPPH) solutions. The peroxy radical concentrations were determined by double integration of derivative spectra. A standard coal sample in the dual cavity allowed corrections to be made for changes in cavity Q. The rates of decay of the less reactive radicals were determined by stopped-flow techniques with manually or electrically operated valves. The decay was recorded... 

Evidence for such stable free radicals has been obtained from electron spin resonance measurements. A sample of the 77-23 PVC-styrene, which had been exposed to 0.8 megarad of gamma-radiation at room temperature, displayed resonance peaks comparable to 3 X 10 8 mole per gram of free radicals (compared with a diphenyl picrylhydrazyl standard). When an identical sample was heated for 10 minutes at 75°C. following irradiation, the free radical population had fallen below detection limits. Heating evidently destroyed or decreased the free radical content by reaction or termination. 

A more fruitful approach to the irradiation of pure liquids would appear to be that being undertaken by Schuler (S2) in this country and Chapiro (C2) in France. In these studies solutes (iodine and diphenyl-picrylhydrazyl) are added to organic liquids prior to irradiation and the disappearance of these solutes is taken as a measure of the number of free radicals produced by the radiation. The experimental results using the two scavengers mentioned yield values in good agreement for the number of free radicals produced in the radiolysis of organic liquids. 

Nanjo F, Goto K, Seto R, Suzuki M, Sakai M, Hara Y. 1996. Scavenging effects of tea catechins and their derivatives on l,l-diphenyl-2-picrylhydrazyl radical. Free Radic Biol Med 21 895-902. 

Garlic has been shown to prevent atherosclerosis and cancer and to retard the aging process. Alliin from garlic can scavenge OH, and garlic powder has been shown to scavenge both OH and l,l-diphenyl-2-picrylhydrazyl radicals. 

Karki SB, Treemaneekam V, Kaufman MJ. Oxidation of HMG-CoA reductase inhibitors by tert-butoxyl and l,l-diphenyl-2-picrylhydrazyl radicals model compounds reactions for predicting oxidatively sensitive compounds during preformulatiom. J Pharm Sci 2000 89 1518-1524. 

The enthalpy of activation for the decomposition of perlauric acid is given in Table 128 the Arrhenius activation energy, determined with the use of diphenyl-picrylhydrazyl, was found to be 19 + 2 kcal.mole . In the aromatic series, the activation energy for perbenzoic acid decomposition is reported to be 30 kcal. mole in benzene solvent . The measurements were made by following the rate of disappearance of the free radical scavenger, diphenylpicrylhydrazyl. The activation energies appear to be low and the difficulty may be due to induced decomposition. Additional rate data are given in Table 129 for various aliphatic peracids. 

A photometric method of analysis was used to determine the rate of thermal dissociation of tetraphenylhydrazine" in benzene at 65 to 85 °C, based on the fast reaction between the (CgH5)2N radical and l,l-diphenyl-2-picrylhydrazine forming the intensely coloured l,l-diphenyl-2-picrylhydrazyl radical. The first order rate constant for decomposition hk = 6.15x 10 exp(—28,040/Jir)sec" . In an independent study of the same reaction the activation energy and heat of dissociation for the above dissociation have been determined by an optical method to be 29.2 kcal.mole and 23.5+2 kcal respectively. 

Famesylhydroquinone 335, which exhibited DPPH (1, l-diphenyl-2-picrylhydrazyl) radical scavenging activity, was separated from Penicillium sp. marine fiingi and the method of the preparation of this compound was described. 

Lavandulylkaempferol (96) of a lavandulylated flavonoid was isolated from the roots of Sophora flavescens AITON. 8-Lavandulylkaempferol (96) might be a scavenger of both l,l-diphenyl-2-picrylhydrazyl radicals and ONOCT (Fig. 24) [25],... 

Yoshida, T., Mori, K., Hatano, T., Okumura, T., Uehara, I., Komogoe, K., Fujita, Y., and Okuda, T. 1989. Studies on inhibition mechanism of autoxidation by tannins and flavonoids. V. Radical-scavenging effects of tannins and related polyphenols on l,l-diphenyl-2-picrylhydrazyl radical. Chem. Pharmacol. Bull. 37(7) 1919—1921. 

Radicals Azobisisobutyronitrile. a,T-Bis(diphenylene)-d-phenylallyL r-Butyl perbenzoate. 7-Butyrolactone. Dibenzoyl peroxide. Di-r-butyl nitroxidc. Di-r-butyl peroxide. Diphenyl-picrylhydrazyl. Galvinoxyl. Hydrogen peroxide-iron salts. Lead dioxide. N-Phenyl-N -benzoyldiimide. Potassium nitrosodisulfonate. Thiolacetic acid. Trichloromethanesulfonyl chloride. 

It is well known from chemical history that the discoveries of the first stable organic radicals, such as triphenylmethyl, diphenyl-picrylhydrazyl, tri-tert-butylphenoxyl, and nitroxides are very significant contributions to theoretical chemistry. The relative stabilities of these radicals were attributed by chemists to the participation of an unpaired electron in conjugated ir-electron systems. Classical stable radicals can thus be thought of as a superposition of many resonance structures with different localizations of an unpaired electron. The first stable radical obtained by Pilotti and Schwerin in 1901 in the pure state can be described by a variety of tautomeric and resonance structures as shown in Scheme 1. 

The N-centred radical 42a participates in a reversible N-N coupling/dissoci-ation process [3], Tetraphenylhydrazine 43 was formed in the yield of 40% from 42a generated from DPA with a, a-diphenyl-/ -picrylhydrazyl. Various dimeric C-N and C-C coupling products, e.g. 44, 45 were isolated [3-5,53]. 

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