HO* radical

Aqueous Phase. In pure water, the decomposition of ozone at 20°C iavolves a complex radical chain mechanism, initiated by OH and propagated by O2 radical ions and HO radicals (25). O3 is a radical ion. 

The two main termination steps for neutral solutions are HO + HO — H2O2 + 2 O3 and HO + HO3 — H2O2 + O3 + O2. An alternative mechanism has been proposed that does not involve HO and HO but has a different initiation step (26). Three ozone molecules are destroyed for each primary event. In the presence of excess HO radical scavengers, ie, bicarbonate, the pseudo-first-order rate constant at 20°C for the initiation step is 175 X. This yields an ozone half-hfe of 66 min at pH 8. In distilled water = 50 mmol/L), the half-hfe is significantly lower, ie, 7 min. 

In natural water, the half-hves fall between these extremes. For example, the half-life of Lake Zbrich water (pH 8, 1.5 meq/LHCO ) is 10 min (27). The decomposition in natural water also can be initiated by trace metal ions, eg, Fe , promoted by impurities such as organic matter, and inhibited by HO radical scavengers, eg, HCO3, COg , HPO (25,28). 

Now, we will consider the major reactions of peroxynitrite with biomolecules. It was found that peroxynitrite reacts with many biomolecules belonging to various chemical classes, with the bimolecular rate constants from 10-3 to 10s 1 mol 1 s 1 (Table 21.2). Reactions of peroxynitrite with phenols were studied most thoroughly due to the important role of peroxynitrite in the in vivo nitration and oxidation of free tyrosine and tyrosine residues in proteins. In 1992, Beckman et al. [112] have showed that peroxynitrite efficiently nitrates 4-hydroxyphenylacetate at pH 7.5. van der Vliet et al. [113] found that the reactions of peroxynitrite with tyrosine and phenylalanine resulted in the formation of both hydroxylated and nitrated products. In authors opinion the formation of these products was mediated by N02 and HO radicals. Studying peroxynitrite reactions with phenol, tyrosine, and salicylate, Ramezanian et al. [114] showed that these reactions are of first-order in peroxynitrite and zero-order in phenolic compounds. These authors supposed that there should be two different intermediates responsible for the nitration and hydroxylation of phenols but rejected the most probable proposal that these intermediates should be NO2 and HO. ... 

Homolysis of H2O2 to HO" radicals, which react with hydroperoxo-Ti species... 

Formation of HO radicals by decomposition of H2O2 on contact with titanium silicates increases with temperature. At 77 K, this decomposition is less probable. 

The HO radicals, generated from the decomposition of H202, perhaps cause the hydroperoxo/peroxo to superoxo conversion. The superoxo species (with the 0-0 stretching absorption near 1120-1150 cm- ) could not be seen in the FTIR spectmm (63), perhaps because of the dominant stretching and bending modes of water in the same region. 

Metal-texaphyrin complexes such as 55 selectively accumulate in tumor cells (240) (see Section III). Complex 55 readily undergoes aone-electron reduction (Ei/2 = 0.08 V vs NHE), forming a free radical which is capable of damaging DNA. Because of the high electron affinity of 55, it may prolong the lifetime of HO- radicals formed by radiolysis of water. Complex 55 is now in phase II clinical trials for the treatment of brain tumors and lung, head, neck, and pancreatic cancer. 

A method of grafting vinyl monomers to substrates of cellulose xanthate was invented by Faessinger and Conte-. The initiation is a reaction of ferrated (12) or acidic (13) cellulose xanthate with hydrogen peroxide according to the following scheme (HO-OH and Fe + give HO- radicals) ... 

The hydroxyl radicals formed may abstract hydrogen from the cellulose fiber substrate which gives grafting sites and subsequently grafted polymer with monomer present. The HO- radicals may also initiate homopolymerization. This means that reaction (17) is not specific for initiation of grafting. Another disadvantage is that the Fe + ions formed - if not carefully removed -may cause discoloration of the resulting product. 

Hiibner, G. and Roduner, E. 1999. EPR investigation of HO radical initiated degradation reactions of sulfonated aromatics as model compounds for fuel cell proton conducting membranes. Journal of Materials Chemistry 9 409- 18. 

The chemical details of the reaction sequence following HO radical addition and abstraction from olefins should be explored fully— ... 

A report considers the reactions of 1-butoxy and 1-pentoxy radicals with oxygen (eqs 1 and 2) and of their isomerizations by 1,5-H-shift (eqs 3 and 4) using direct and time-resolved monitoring of the formation of NO2 and HO radicals in the laser flash-initiated oxidation of 1-butyl and 1-pentyl radicals. ... 

The pair TF/Tl is also used widely for one-electron oxidation in aqueous acidic solutions of organic substrate under pulse radiolysis. The aqueous solutions must be saturated with NjO. The latter takes part in the reaction and the reaction leads to HO radical e + NO2 + H2O —> Nj + HO + HO-. This HO radical generates the desired TF from TT HO + TT + H+ TF + HjO. 

Free radicals are involved in a wide variety of reactions, due to their reactivity and versatility understanding their behavior as it relates to combustion is a goal of many experimentalists and theoreticians. Two specific classes of radicals of interest to combustion processes are peroxy (ROO ) radicals and oxy (RO ) radicals along with hydroxyl (HO ) radical, many of these radicals are important members in the general class referred to as reactive oxygen species (ROS). ... 

Oxidation in the atmosphere begins photolytically with radiation from the sun rather than thermolytically thus, atmospheric chemistry differs between day and night. In the daytime, the most common initiation step for VOC degradation involves photolysis of ozone by the sun s ultraviolet light, leading to hydroxyl (HO ) radical generation ... 

At night, when the sun s radiation is minimal, the dominant VOC oxidant is nitrate radical (NO3 ). The chemistry initiated by NO3 differs from that initiated by HO radical in that NO3 prefers to react with unsaturated compounds via addition to one of the carbons of the 7t-system, rather than by hydrogen atom abstraction ... 

As shown, NO3 radical leads to different chemistry than does HO radical the peroxy radical can decompose to yield several products, including acetaldehyde, formaldehyde, 1,2-propanediol dinitrate (PDDN), nitroxyperoxypropyl nitrate (NPPN), and a-nitrooxyacetone. The reactions of the peroxy radicals with NO , species can lead to highly functionalized (and oxidized) organic compounds. 

Fig. 7 Reactions of toluene with HO radical. HO can abstract a benzylic hydrogen atom (a) or add to the aromatic ring at the ipso (b), ortho (c), meta (d), and para (e) positions relative to the methyl group. Each resultant radical can decompose by various pathways, depending on temperature and pressure.

The initial ionization of a water molecule produces an electron and the water radical cation. The water radical cation is a strong acid and rapidly loses a proton to the nearest available water molecule to produce an HO radical and HsO. The electron will lose energy by causing further ionizations and excitations until it solvates (to produce the solvated electron Ca ). In addition to the two radical species HO and e q, a smaller quantity of H-atoms, H2O2, and H2 are also produced. 

About 20% of the HO radicals interact with the sugar phosphate by H-atom abstraction and about 80% react by addition to the nucleobases. In model sugar compounds, the H abstraction would occur evenly between the hydrogens on Cl, C2, C3, C4, and C5. In DNA, H abstraction occurs mainly at C4 since the C4 -H is in the minor groove and to some extent with the C5 -H2. 

The HO radical is electrophilic and can interact by addition with the unsaturated bonds of the nucleobases. For the pyrimidines, this would be the C5=C6 double bond [8,9]. For the purines, this would include predominately C4 and C8 addition, with a minor amount ofC5 addition [10,11]. 

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