Antioxidant stoichiometric factor

In our experiments, limited protection of AFM-induced injury to cucumber cotyledons was obtained with BHA and BHT. These two compounds showed some synergistic characteristics. However, the concentrations necessary for protection were high (400 yM) and caused some injury to the controls. The concentrations of BHA and BHT needed to protect the tissue should be higher than for a-T. BHA and BHT each have an antioxidant stoichiometric factor (n) equal to 1, whereas for a-T, n can be equal to 2 (67). An n of 2 for a-T means this molecule has the ability to quench up to 2 radicals before being destroyed. DPPD has been reported to have antioxidant activity (68). However, in our tests DPPD was toxic at high concentrations (400 yM) and ineffective at lower concentrations (e.g., 50 yM),... 

In the water-like solvent tert-butyl alcohol, a-tocopherol was found to prevent lipid oxidation, showing a distinct lag-phase for oxygen consumption. This was in contrast to quercetin or epicatechin, which were only weak retarders of lipid oxidation without any clear antioxidative effect. Quercetin or epicatechin, when combined with a-tocopherol, increased the lag-phase for oxygen consumption as seen for a-tocopherol alone. The stoichiometric factor for a-tocopherol, a-TOH, as chain-breaking antioxidant has the value n = 2 according to the well-established mechanism ... 

Antioxidant activity and stoichiometric factor—H-atom transfer and... 

The factor n in equations 12-14 represents the number of peroxyl radicals trapped by the antioxidant in reactions 10 and 11 the stoichiometric factor. This value is expected to approximate 2 for those phenols, which are efficient antioxidants. 

Consequently, the retarder may be consumed slowly while oxygen uptake is only reduced slightly, but the effect occurs well past the time at which two peroxyl radicals have been generated from the initiator for every molecule of retarder. Under these conditions, a retarder may appear to react with more than two peroxyl radicals. This situation is quite often observed and causes misinterpretation of results concerning inhibition efficiency, unless a reliable method is used to determine the stoichiometric factor and antioxidant activity (See Section II.A.)... 

Several sulfur analogs (XI, XII) were also synthesized and their reactivities measured during inhibition of styrene autoxidation. The stoichiometric factors, n, were less than 2 for these compounds, so their antioxidant activities were reported as n x values. Compounds XII are compared with a-Toc and hydroxychromans in Table 4. It is seen that in all cases the activities of the sulfur analogs are lower than the vitamin E class. 

It has been demonstrated that the nitroxyl radical, X, reacts with a secondary alkyl radical to form XI which, under high-temperature conditions (>120°C), regenerates the original diphenylamine molecule, Reaction (4.36). In essence, this group of stabilisers acts catalytically by scavenging alternately peroxy (ROO-) and alkyl radicals (R-). As stated earlier, sterically hindered phenols deactivate only two peroxy radicals per phenol molecule. Hence, under high-temperature conditions, aromatic amines are far superior to their phenolic counterparts. As shown in Table 4.3, the stoichiometric factor of the diphenylamines depends on the substituents in the para position [33]. The efficacy of the diphenylamine antioxidant is improved by alkylating the para positions. The stabilisation mechanism for phenyl-a-naphthylamines. Reaction sequence (4.37) [34], is described as follows ... 

Reactivity with ROO was also used for explanation of the nitroxide regeneration from 65 and, consequently, of the high stoichiometric factor of DPA in autoxida-tion of hydrocarbons [66]. Thermolysis of NOR 65 at temperatures exceeding 100 °C was proposed as another process regenerating the antioxidant activity [65]. Hydroxylamines 66, strong CB AO, are formed. NOH 66 are reoxidized to 56 either by scavenging ROO (Scheme 7) or by oxidation with ROOH via CTC pN +OH, HO, RO ]. 

The rate of initiation is generally measured by an induction period method using an antioxidant (AH) that has a known stoichiometric factor n, defined as the number of radicals trapped by each molecule of antioxidant. Because a-tocopherol is known to have an n value of 2, a known concentration is used to determine the induction period, T, during which the oxidation is inhibited. 

As the XRF technique only provides elemental concentration data, it is a non-specific method (i.e. the same phosphorus calibration curve can be employed for a variety of phosphite antioxidants). Once the phosphorus content of an unknown sample has been obtained, the value is converted to the antioxidant concentration using the appropriate stoichiometric factor (e.g. Ultranox 626, a phosphite antioxidant, has a phosphorus content of 10.2%). Knowledge of which type of antioxidant is formulated into the sample is therefore required. As a result, XRF analysis is more suited for QC type applications, although it can be employed as an effective screening tool to determine the presence of secondary antioxidants in unknown samples. 

The reaction scheme (Fig. 3.35) shows that one antioxidant molecule combines with two radicals. Therefore, the maximum achievable stoichiometric factor is n = 2. In practice, the value of n is between 1 and 2 for the antioxidants used. Antioxidants, in addition to their main role as radical scavengers, can also partially reduce hydroperex-ides to hydroxy compounds. 

Quinones and, in particular, naphthoquinone derivatives are industrially valuable products for further processing and for direct use due to their pronounced bioactivity [la,b], 2-Methyl-1,4-naphthoquinone, vitamin K3 ( menadione ), is the basis of the vitamin K group (coagulation vitamins). The skeleton of 2-methyl-1,4-naphthoquinone is common to all fat-soluble K vitamins. Derivatives of vitamin K promote the formation of prothrombin and other blood coagulation factors. They are used on an industrial scale as supplement in animal feed, but are also employed in the treatment of Melaena neonatorum in newborn babies. Trimethyl-p-benzoquinone is a key compound for the synthesis of vitamin E, active as antioxidant agent. As an example, the current method of the production of trimethyl-p-benzoquinone on an industrial scale is p-sulfo-nation of 2,3,6-trimethylphenol followed by stoichiometric oxidation using Mn02 [Ic]. 

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