Hydroxyl scavengers

Transformation of parent contaminants into secondary products may occur during the processes of atmospheric diffusion and transport as a result of physical, chemicjd, and photochemical processes (22). Chemical conversion within the atmosphere may also change the physico-chemical characteristics of contaminants, dramatically altering their atmospheric residence times and fates from those of the parent contaminants. The complex reactions within the atmosphere that are driven by chemical processes such as hydroxyl scavenging... 

Klein, S.M., Cohen, G., Lieber, C.S. and Cederbaum, A.l. (1983). Increased microsomal oxidation of hydroxyl scavenging agents and ethanol after chronic consumption of ethanol. Arch. Biochem. Biophys. 223, 425-432. 

Another pathway that can lead to the formation of peroxynitrous acid is the reaction between OH and NO2, provided that no hydroxyl scavengers are present in solution. Another unstable species, peroxynitrate (02N00-), can also form upon reaction between 0(3P) and nitrate [22-24,27,29,33,34] ... 

Phenol can be nitrated upon nitrate irradiation, yielding 2- and 4-nitrophenol [54,58,79,99,100]. The generation of OH + NO2 upon nitrate photolysis (reactions 1 and 2) would suggest the possibility that phenol nitration might follow an OH-mediated pathway as in the gas phase [80,81]. Furthermore, hydroxyl-mediated nitration in aqueous solution has been described in the case of benzene [107]. However, the addition of hydroxyl scavengers to the system (formate [79], 2-propanol [58]) favours the formation of ni-trophenols, while an OH-mediated nitration would be inhibited by hydroxyl consumption. The positive effect of the scavengers can be accounted for in the hypothesis that phenol nitration takes place upon reaction with nitrogen... 

In the presence of nitrate under irradiation, catechol yields 4-nitrocatechol and hydroxybenzoquinone. The effect of hydroxyl scavengers indicates that the formation of 4-nitrocatechol is due to nitrogen dioxide, whereas hydroxybenzoquinone forms upon reaction of catechol with hydroxyl [78,79]. 

Last but not least, the transformation of 4-hydroxybenzoate in the presence of nitrate under irradiation is quite interesting from a mechanistic point of view. The substrate undergoes nitration to 4-hydroxy-3-nitrobenzoate, and nitration is inhibited by addition of hydroxyl scavengers [143]. This marks a difference from all the other cases reported so far. Whenever the effect of OH scavengers on nitration upon nitrate photolysis has been studied, the typical result was an enhancement effect consistent with nitration by... 

For diols, aldehyde linkers of the general structure 47 have been used [107-109]. Cleavage can be effected with TFA mixtures containing hydroxylic scavengers (Figure 14.15). 

Wijewickreme, A.N., Krejpcio, Z., and Kitts, D.D. Hydroxyl scavenging activity of glucose, fructose, and ribose-lysine model MaUlard products, J. Food Sci., 64,457, 1999. 

With aldehydes, primary alcohols readily form acetals, RCH(OR )2. Acetone also forms acetals (often called ketals), (CH2)2C(OR)2, in an exothermic reaction, but the equiUbrium concentration is small at ambient temperature. However, the methyl acetal of acetone, 2,2-dimethoxypropane [77-76-9] was once made commercially by reaction with methanol at low temperature for use as a gasoline additive (5). Isopropenyl methyl ether [116-11-OJ, useful as a hydroxyl blocking agent in urethane and epoxy polymer chemistry (6), is obtained in good yield by thermal pyrolysis of 2,2-dimethoxypropane. With other primary, secondary, and tertiary alcohols, the equiUbrium is progressively less favorable to the formation of ketals, in that order. However, acetals of acetone with other primary and secondary alcohols, and of other ketones, can be made from 2,2-dimethoxypropane by transacetalation procedures (7,8). Because they hydroly2e extensively, ketals of primary and especially secondary alcohols are effective water scavengers. 

The composition of I, and possibly its structure, may be deduced by identifying Q. Certain examples from peroxide chemistry will illustrate the scope of the method. The reactions of ferrous(nitriloacetate) and ferrous(ethylenediamine-N,N -diacetate) with hydrogen peroxide are complicated processes.1 A particular scavenger T did indeed divert the reaction at high concentrations of T. The required levels of T were, however, much higher than those that would have been needed to trap the hydroxyl radical, HO. It is thereby ruled out. With this and with spectroscopic evidence, a reactive hypervalent iron complex was suggested as the intermediate. 

Solutions containing HO as the only important energetic species can be prepared by scavenging e with H30+ [Eq. (11-58)] or by using N20-saturated solutions. The latter, an effective scavenger of e, also doubles the yield of the hydroxyl radical ... 

Irradiation of dilute aqueous solutions results in the interaction ofthe ionizing radiation with water molecules. The radiolysis of water produces hydrated electrons (eaq ", G = 2.8), hydrogen atoms (G = 0.6) and hydroxyl radicals (G = 2.8) which react with the molecules of the solutes. The use of special scavengers can convert one species to another, e.g. 

Fig. 3. a) First order plot of oxygen uptake in the Methylene-blue (MB)-sensitized photooxidation of GA 8.4 pM and 1.3 mM histidine (control) in phosphate buffer pH 7. b) Percentage radical scavenging activity for the control molecule Trolox and GA at pH 7.4 in phosphate buffer 10 mM (hydroxyl radical) and pH 10 in sodium carbonate buffer 50 mM (anion superoxide radical). 

As strong antioxidants and scavengers of superoxide, hydroxyl and peroxyl radicals, tea flavonoids can suppress radical chain reactions and terminate lipid peroxidation (Kumamoto and Sonda, 1998, Yang and Wang, 1993). 

The antioxidant property of ferulic acid and related compounds from rice bran was reported by Kikuzaki et al, (2002). Their results indicated that these compounds elicit their antioxidant function through radical scavenging activity and their affinity with lipid substrates. Another recent study reported by Butterfield et al, (2002) demonstrated that ferulic acid offers antioxidant protection against hydroxyl and peroxyl radical oxidation in synaptosomal and neuronal cell culture systems in vitro. The effect of ferulic acid on blood pressure (BP) was investigated in spontaneously hypertensive rats (SHR). After oral administration of ferulic acid the systolic blood pressure (SBP) decreased in a dose-dependent manner. There was a significant correlation between plasma ferulic acid and changes in the SBP of the tail artery, suggesting... 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

Brezonik PL, J Fulkerson-Brekken (1998) Nitrate-induced photoysis in natural waters controls on concentrations of hydroxyl radical photo-intermediates by natural scavenging agents. Environ Sci Technol 32 3004-3010. 

If an aromatic compound reacts with an OH radical to form a specific set of hydroxylated products that can be accurately identified and quantified in biological samples, and one or more of these products are not identical to naturally occurring hydroxylated species, i.e. not produced by normal metabolic processes, then the identification of these unnatural products can be used to monitor OH radical activity therein. This is likely to be the case if the aromatic detector molecule is present at the sites of OH radical generation at concentrations sufficient to compete with any other molecules that might scavenge OH radical. 

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