Acids reaction + hydrated electron

In the presence of 10 pM peroxide, the yields of H2, H202, and of H + eh are about the same in neutral and 0.4 M acid solutions. Since H atoms produced by the reaction of acid with hydrated electrons have different reaction rates and sequences of reaction, a much greater difference of the... 

The formation of hydrated electrons by the photolysis of halide ions in solution may be envisaged in two steps. The first step is the CTTS absorption leading to (X -). The second step is a slow, thermal process releasing the electron in competition with degradation and recapture. In the presence of acid and alcohol, photolysis of halide solutions generates H2 with a yield that increases both with acid and alcohol concentrations (seejortner et al., 1962, 1963, 1964). At 25°, the limiting quantum yields are 0.98 for Cl- at 185 nm, 0.6 and 0.5 for Brat 185 and 229 nm, respectively, and 0.3 and 0.25 for I- at 254 and 229 nm, respectively. Since most of these yields are less than 1, the direct reaction of HsO and (Xaq-) is ruled out. Instead, it is proposed that eh is produced from the... 

The reaction H + OH— eh is undoubtedly responsible for the increase of G(eh) at high pH. Similarly, the reaction eh + H+—H must be responsible for the reduction of the hydrated electron yield in acid solution. The increase of total reducing yield and water decomposition yield at pH = 1.3 is not clearly understood, but it may also be due to secondary reactions. 

A. Shafferman and G. Stein, The effect of aromatic amino acids on the photochemistry of a disulfide Energy transfer and reaction with hydrated electrons, Photochem. Photobiol. 20, 399-406 (1974). 

The yields of these so-called primary species, present at the time when radical combination in, and diffusive escape from, the spurs is complete, were obtained by adding solutes to the water to capture the radicals and by measuring the stable identifying products. It was from a number of these studies that it became clear that the reducing radical must exist in two forms, which turned out to be the hydrogen atom and the hydrated electron (e q). For example, Hayon and Weiss [6] found that the yields of H2 and Cl produced by irradiating solutions of chloroacetic acid varied with pH in a manner that was consistent with the following reactions ... 

Nucleophilic addition to C=0 (contd.) ammonia derivs., 219 base catalysis, 204, 207, 212, 216, 226 benzoin condensation, 231 bisulphite anion, 207, 213 Cannizzaro reaction, 216 carbanions, 221-234 Claisen ester condensation, 229 Claisen-Schmidt reaction, 226 conjugate, 200, 213 cyanide ion, 212 Dieckmann reaction, 230 electronic effects in, 205, 208, 226 electrons, 217 Grignard reagents, 221, 235 halide ion, 214 hydration, 207 hydride ion, 214 hydrogen bonding in, 204, 209 in carboxylic derivs., 236-244 intermediates in, 50, 219 intramolecular, 217, 232 irreversible, 215, 222 Knoevenagel reaction, 228 Lewis acids in, 204, 222 Meerwein-Ponndorf reaction, 215 MejSiCN, 213 nitroalkanes, 226 Perkin reaction, 227 pH and, 204, 208, 219 protection, 211... 

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... 

The rate constants for 8 and 9 were determined by pulse radiolysis by adding known amounts of excess acid or H20-2 and measuring the pseudo-first order decay of hydrated electron absorption (15, 25). The rate constant for recombination of OH radicals (Reactions 19) was deter-... 

Water, H20 + and Bronsted Acids. The most important reagent in the chemistry of e aq is obviously the solvent, water. Were it not for the relatively low reactivity of elq with H20 most of our information on hydrated electrons would be merely hypothetical. Fortunately the rate of the eaq + H20 - H + OH - reaction is slow enough to enable one to examine the kinetic behavior of any solute reacting with e aq at a rate over 106 Af-1 sec.-1... 

Hydrated electrons react with many Bronsted acids. This reaction is not a proton transfer process but an incorporation of the electron into the acid to form an AH - ion radical, which may subsequently undergo decomposition. This decomposition may occasionally yield a hydrogen atom, but in many cases other pathways of dissociation have been observed. 

Re-examination of the radiolysis of aqueous solutions of alanine (absence of oxygen) shows that electrons react rapidly with the cationic form, less rapidly with the zwitterion, and much less rapidly with the anionic form. These conclusions have been confirmed by pulse radiolysis. Rate constants for amino acids, peptides, proteins, and numerous other substances have been obtained. Critical evaluation of these and correlation with molecular properties is now well under way. In living systems the reactions of the hydrated electron vary with the part of the cell concerned, with the developmental stage of the cell, and possibly with the nature of any experimentally added substances. 

The effect of ionic form on the reaction of the hydrated electron with amino acids has been examined. The cationic form could not be examined since appreciable amounts of H + would have to be present, and with currently available techniques the electron would disappear too rapidly. But by making the solutions alkaline it has been possible to study the anionic form. For glycine (Table I), and several other amino acids and peptides (7), it has been shown that the amino acids are less reactive in the anionic form, agreeing with the conclusion drawn by Garrison. The results for glycine however cannot be interpreted on the basis of the known pK together with assumed rate constants for zwitterion and anion. Other factors are evidently present, and further work is required. 

Therefore, in aqueous N20 saturated solutions, OH radicals are the predominant species (90%), while in acidic aqueous solution H and OH radicals exist, but not hydrated electrons. The small fraction ( ca 10%) of hydrogen atoms in N20 saturated aqueous solution does not interfere significantly with the measurements of the reactions of OH radicals. Frequently, H atoms and OH radicals react with solutes in a similar manner, e.g. by addition to a double bond or by hydrogen atom abstraction, however with different rate constants. 

The hydrated electron and the hydrogen atom can be considered the basic and acid forms of the reducing species produced in the irradiation of water. Interconversions are possible by reaction (2)... 

The different reactivity of hydrogen atoms and of hydrated electrons with organic substrates is best demonstrated by their reaction with chloroacetic acid (Eqs. 6-19 and 6-20). 

The reaction of amino acids with the hydrated electron occurs at a range of different rates depending upon the side chain. Aliphatic amino acids tend to react very slowly with e q whereas aromatic amino acids react at a somewhat more rapid rate and cysteine has a very facile reaction to lose HS" see Table I. The caveat in these systems is that reaction (5), the reaction of e with oxygen, is diffusion controlled and oxygen is omnipresent in most systems at a relatively high concentration (250 pM in air and 1.2 mM in Oj-saturated water). 

Faraggi M, BetteUieim A. (1977) The reaction of the hydrated electron with amino acids, peptides, and proteins in aqueous solutions 111. Histidyl peptides. RadiatRes7 311-324. 

Braams R. (1966) Rate constants of hydrated electron reactions with amino acids. Radiat Res 27 319-329. 

The water radical cation, produced in reaction (3), is a very strong acid and immediately loses a proton to neighboring water molecules thereby forming an OH radical [reaction (5)]. The electron becomes hydrated by water [reaction (6)]. Electronically excited water can decompose into OH and H [reaction (7)]. Thus, three kinds of free radicals are formed side by side in the spurs, OH, e, and H. To match the charge of the electrons, an equivalent amount of H is also present. 

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