Hydrated electron hydroxyl radical reactions

We have seen that there are examples of metal complexes in all widely accepted mechanisms of radiation modification, and metal complexes in general must be considered as excellent potential for further in vitro and in vivo studies. A review paralleling this chapter has appeared [103]. Many of the effects of ionizing radiation on DNA with respect to modification have also been reviewed in a comprehensive volume [104]. Systematic in vitro studies to correlate activity with chemical parameters e.g. reaction with the hydrated electron, hydroxyl radical etc. are clearly required. The fact that some metal complexes show a fine line between sensitization and protection is indicative of competing processes. In this respect, the point has been made that these processes are the two sides of a coin . The basic mechanism for both reactions is closely related and we can consider protection as a reductive process or repair and implying donation to a particular molecule, while sensitization can be thought of as abstracting electrons, an oxidative process [105]. 

Michael BD, Hart EJ. The rate constants of hydrated electron, hydrogen atom, and hydroxyl radical reactions with benzene, 1,3-cyclohexadiene, 1,4-cyclo-hexadiene, and cyclohexene. J Phys Chem 1970 74 2878-2884. 

For studies of hydroxyl radical reactions, the hydrated electrons can be converted to hydroxyl radicals by saturating the reaction solution with nitrous oxide. N2O is substantially soluble in water with a Henry s law constant (mole fraction scale) at 25 °C of 0.182 X 10, it has a concentration of about 26 mmol dm at 25 °C. Solvated electrons are rapidly scavenged by the N2O (reaction... 

Anbar, M. and Neta, P. (1967). A compilation of specific biomolecular rate constants for the reaction of hydrated electrons, hydrogen atoms and hydroxyl radicals with inorganic and organic compounds in aqueous solutions. Int. J. Appl. Radiat. Isot. 18, 493-497. 

Buxton, G. V., Greenstock, G. L., Helman, W. P., Ross, A. B. (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solutions. J. Phys. Chem. Ref. Data 17, 513-886. 

Ionizing radiations (a, ft and y) react unselectively with all molecules and hence in the case of solutions they react mainly with the solvent. The changes induced in the solute due to radiolysis are consequences of the reactions of the solute with the intermediates formed by the radiolysis of the solvent. Radiolysis of water leads to formation of stable molecules H2 and H2O2, which mostly do not take part in further reactions, and to very reactive radicals the hydrated electron eaq, hydrogen atom H" and the hydroxyl radical OH" (equation 2). The first two radicals are reductants while the third one is an oxidant. However there are some reactions in which H atom reacts similarly to OH radical rather than to eaq, as e.g. abstraction of an hydrogen atom from alcohols, addition to a benzene ring or to an olefinic double bond, etc. 

The irradiation of water is immediately followed by a period of fast chemistry, whose short-time kinetics reflects the competition between the relaxation of the nonhomogeneous spatial distributions of the radiation-induced reactants and their reactions. A variety of gamma and energetic electron experiments are available in the literature. Stochastic simulation methods have been used to model the observed short-time radiation chemical kinetics of water and the radiation chemistry of aqueous solutions of scavengers for the hydrated electron and the hydroxyl radical to provide fundamental information for use in the elucidation of more complex, complicated chemical, and biological systems found in real-world scenarios. 

Other than water, protein is the major constituent of meat averaging nearly 21% in heef or chicken meat, with fat varying fiom 4.6 to 11.0% in beef and fiom 2.7 to 12.6% in chickoi. The principal radiolytic reactions of aqueous solutions of aliphatic amino acids are reductive deamination and decarboxylation. Alanine yields NH3, pyruvic add, acetaldehyde, propionic acid, CO2, H2, and ethylamine (6). Sulfur-containing amino adds are espedally sensitive to ionizing radiation. Cysteine can be oxidized to cystine by the hydroxyl radical or it can react with the hydrated electron and produce... 

Transfer of radiation-induced electrons and holes (H20 ) from the hydration layer of DNA has been of considerable recent interest. Results from ESR experiments at low temperatures suggest that ionization of hydration water (reaction 4) results in hole transfer to the DNA (reaction 5) [4, 24-28]. Since the proton transfer reaction (reaction 6) to form the hydroxyl radical likely occurs on the timescale of a few molecular vibrations [29], it is competitive with and limits hole transfer to DNA [27]. 

SCHEME 4.1 Schematics of radiolysis and reducing species. As a result of ionization of the water molecule, hydroxyl radicals and hydrated electrons are formed. The final radiolytic yield depends on the secondary reactions in spurs and on the presence of other compounds. See Refs 25,26,190, and 191 for the detailed discussion and references. Solvated electrons are mobile enough to escape spurs and to react with the heme protein complexes even at 77K. All other reactive products of radiolysis are immobilized in the solid solvent matrix, or trapped by radical quenchers. 

---