Hydroxyl radicals, determination

The ion-pair complex formed by the interaction of hydroxobis(8-quinolyloxo) vanadium (V) [VOQ2OH] and /i-butyl amine is also effective in photoinitiation of polymerization of MMA in bulk and in solution [40]. The quantum yield of initiation and polymerization determined are equal to 0.166 and 35.0, respectively. Hydroxyl radical ( OH) is reported to be the initiating radical and the following photoreaction is suggested ... 

Hydroxyl radicals. The acid ionization constant of the short-lived HO transient is difficult to determine by conventional methods but an estimate can be made because HO, but not its conjugate base, O -, oxidizes ferrocyanide ions HO + Fe(CN) — OH- + Fe(CN)g . Use the following kinetic data26 for the apparent second-order rate constant as a function of pH to estimate Ka for the acid dissociation equilibrium HO + H20 =... 

Trace-gas Lifetimes. The time scales for tropospheric chemical reactivity depend upon the hydroxyl radical concentration [HO ] and upon the rate of the HO/trace gas reaction, which generally represents the slowest or rate-determining chemical step in the removal of an individual, insoluble, molecular species. These rates are determined by the rate constant, e,g. k2s for the fundamental reaction with HO, a quantity that in general must be determined experimentally. The average lifetime of a trace gas T removed solely by its reaction with HO,... 

The dominant transformation process for trichloroethylene in the atmosphere is reaction with photochemically produced hydroxyl radicals (Singh et al. 1982). Using the recommended rate constant for this reaction at 25 °C (2.36x10 cm /molecule-second) and a typical atmospheric hydroxyl radical concentration (5x10 molecules/cm ) (Atkinson 1985), the half-life can be estimated to be 6.8 days. Class and Ballschmiter (1986) state it as between 3 and 7 days. It should be noted that the half-lives determined by assuming first-order kinetics represent the calculated time for loss of the first 50% of trichloroethylene the time required for the loss of the remaining 50% may be substantially longer. 

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. 

Rothmel, RK, DL Shinbarger, MR Parsek, TL Aldrich, AM Chakrabarty (1991) Functional analysis of the Pseudomonas putida regulatory protein CatR transcriptional studies and determination of the CatR DNA-binding site by hydroxyl-radical footprinting. J Bacterial 173 4717-4724. 

The mobile charge carrier species may either recombine or reach the semiconductor surface, where they can be trapped by the surface adsorbates or other sites. The lifetime of electron-hole (e /h+) pairs that are generated is important in determining the reaction yield. The holes are mainly trapped by water molecules or hydroxyl ions, giving rise to very reactive hydroxyl radicals ... 

Iron-mediated generation of hydroxyl radical ( 0H) was monitored by the hypoxanthine-xanthine oxidase method as previously described (28). Formaldehyde produced by reaction of 0H with DMSO was determined spectrophotometrically by the Hantzsch reaction (29). 

Very recently, rate constants for scavenging of hydroxyl radicals by DMPO, and by the nitrone [18c], have been determined (Marriott et al., 1980) (see Table 5). As might be expected, the figures are close to the diffusion-controlled limit. The report of this work includes a concise and informative discussion of some of the difficulties with, and limitations of, the spin trapping method, especially where these relate to reactions involving hydroxyl radicals. 

The generation of HO by pulse radiolysis provides a way for investigating the kinetics of hydroxyl spin adduct formation. For PBN and some of its 4-substituted derivatives (ranging from 4-MeO to 4-N02), rate constants in the range of (5-9) X 109 dm3 mol 1 s 1 were determined (Greenstock and Wiebe, 1982). A study of the reaction of the water-soluble 2-, 3- and 4-PyBN[23] and HO showed that most of the hydroxyl radicals became attached to the heteroaromatic ring (Neta et al., 1980 Sridhar et al., 1986). Similar findings... 

Three C60 derivatives with two to four malonic acid groups (DMA C60, TMA C60, and QMA C60) were prepared and the phototoxicity of these compounds against HeLa cells was determined by MTT assay and cell cycle analysis (Yang et al., 2002). The relative phototoxicity of these compounds was DMA C60 > TMA C60 > QMA C60. Hydroxyl radical quencher mannitol (lOmM) was not able to prevent cells from the damage induced by irradiated DMA C60. DMA C60, together with irradiation, was found to decrease the number of G(l) cells from 63% to 42% and increase G(2) + M cells from 6% to 26%. 

Just as the fate of H radicals is crucial in determining the rate of the H2—02 reaction sequence in any hydrogen-containing combustion system, the concentration of hydroxyl radicals is also important in the rate of CO oxidation. Again, as in the H2—02 reaction, the rate data reveal that reaction (3.44) is slower than the reaction between hydroxyl radicals and typical hydrocarbon species thus one can conclude—correctly—that hydrocarbons inhibit the oxidation of CO (see Table 3.1). 

Here one notices that the structure of the 90° (COO) bonding determines the intermediate ketone, aldehyde, and hydroxyl radicals that form. 

Reaction 2-6 is sufficiently fast to be important in the atmosphere. For a carbon monoxide concentration of 5 ppm, the average lifetime of a hydroxyl radical is about 0.01 s (see Reaction 2-6 other reactions may decrease the lifetime even further). Reaction 2-7 is a three-body recombination and is known to be fast at atmospheric pressures. The rate constant for Reaction 2-8 is not well established, although several experimental studies support its occurrence. On the basis of the most recently reported value for the rate constant of Reaction 2-8, which is an indirect determination, the average lifetime of a hydroperoxy radical is about 2 s for a nitric oxide concentration of 0.05 ppm. Reaction 2-8 is the pivotal reaction for this cycle, and it deserves more direct experimental study. 

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

Chemical/Physical. Chemical oxidation of mono-, di-, and trichlorophenols using Fenton s reagent were investigated by Barbeni et al. (1987). To a 70-mL aqueous solution containing 4-chlorophenol thermostated at 25.0 °C was added ferrous sulfate and hydrogen peroxide solution (i.e., hydroxyl radicals). Concentrations of 4-chlorophenol were periodically determined with... 

The mechanism of cytochrome P450 catalysis is probably constant across the system. It is determined by the ability of a high valent formal (FeO) species to carry out one-electron oxidations through the abstraction of hydrogen atoms or electrons. The resultant substrate radical can then recombine with the newly created hydroxyl radical (oxygen rebound) to form the oxidized metabolite. Where a heteroatom is the (rich) source of the electron more than one product is possible. There can be direct recombination to yield the heteroatom oxide or radical relocalization within the... 

The hydroxyl radical can also abstract a single electron from dG to generate the base radical cation (G ). In duplex DNA, the G " " will be stabilized by its delocalization into adjacent bases. Both calculations and kinetic measurements " indicate that GG sequences have a lower oxidation potential than an isolated G. Nucleo-bases on the 3 -side of G determine the extent of G formation, and here purines are more effective than pyrimidines at lowering the oxidation potential of G, which accounts for the GG effect and that GA sites are also reactive. ... 

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