Reduction by hydrated electron

It is known that hydrated electrons react with the carbonyl groups of peptidic bonds. Consequences seem to be fragmentation and ammonia liberation in amino-acids. This cleavage is supposed to take place at the free radical level  

G-values of ammonia were measured after reduction of amino-acids and peptides (7, 9), and of some proteins such as histones that contain relatively low amount of sulfur residues and histidines (26 ). They may reach the G(e aq) value. The high reactivity of e aq toward peptidic bonds is also responsible for the progression of the rate constant with the number of carbonyl groups in small peptides (table 1). The deaminated radical was observed by pulse radiolysis (absorption maximum around 430 run, extinction coefficient at this wavelength ca. 1100 mol 1 cm (3). 

Rate constants of reactions of e aq with some aliphatic amino-acids and some peptides. 

Amino-acid or peptide Rate constant (mopl 1 s ) 

Data are taken from G. V. Buxton, C. L. Greenstock, W. P. Helman and A. B. Ross, J. Phys. Chem. Ref. Data 17 (1988) 513. 

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Mb. Subsequent application of this technique to reduction of various derivatives of reduced and oxidized myoglobin led to the observation that the rate of reduction by hydrated electrons depends primarily on the net charge of the protein and the dissociation constant for formation of ligand bound derivatives of metMb. 

Fig. 14. A schematic mode) of the dissociation of earthworm hemoglobin induced by the reduction by hydrated electron. The molecule consists of 10 subunits with the total molecular weight of 4000000 [84]...

Pin S, Hickel B, Alpert B, Ferradini C. (1989) Parameters controlling the kinetics of ferric and ferrous hemeproteins reduction by hydrated electrons. Biochim Biophys Acta 994 47-51. 

In a pure water matrix, a species having a similar spectrum was obtained (Figure 7, Curve I). However, in neutral 0.01M chloride (not containing alcohol), a double humped spectrum forms (Figure 7, Curve II). The peak near 410 n.m. corresponds to the one in pure water or alkali owing to reduction by hydrated electrons and that near 325 n.m. to oxidation by OH radicals. 

In conclusion, structurally balanced fullerene derivatives were successfully incorporated into various vesicular matrices. Truly monomeric immersion of amphiphilic C6o(C4HioN ) and C6o[C(OCH2CH2)3CH3]2 derivatives into the polar head group anchored the fullerene core still close enough to the lipid bilayer interface to facilitate very efficient reductions by hydrated electrons and CH2OH radicals. 

Pulse radiolysis has also been used to study the reduction of various Mb derivatives by hydrated electrons 151-154). With this technique, it was possible to study reduction of ligand-bound forms of metMb at cryogenic temperature and thereby identify reduced, ligand-bound forms of the protein 152) and to reduce oxyMb to produce ferryl (Fe(IV)=0)... 

Photolysis of ion pairs of cobalt(III) complexes with iodide ions leads to oxidation of iodide and reduction of the complex.55,63-86 Under the normal experimental conditions, however, most of the light is absorbed by free iodide and the reduction of the complex is effected by hydrated electrons produced as in reaction (36).86... 

The rate constants for the reduction of aquo cations by hydrated electrons and for the oxidation of aquo cations by hydroxyl radicals at 25°C are given in Tables 7.25b and 7.25c, respectively. 

Second-order rate constants (10-8 A2 (dm3 mol-1 s l)) for reduction of aquo cations by hydrated electrons at... 

The alkali metal cations Li, Na, K, Rb and Cs cannot be reduced by hydrated electrons because they have a more negative reduction potential. 

The (US.. SU)" radical anion was generated via the one-electron reduction of USSU, initiated by hydrated electrons [analogous to reaction (37)], and is characterized by a broad optical absorption band with X = 450 nm, = 6000 M" cm" and a half-width of 1.0 eV. It exists in equilibrium with the conjugated thiyl radical (US ) and the corresponding thiolate ion (US") [reaction (41)]... 

The irradiation of the phenoxazin-3-one dye, resazurin (21), in the presence of tertiary amines results in its efficient deoxygenation via the triplet state, forming the dye resorufin (22). The radiolysis/photolysis of aqueous solutions of pyrimidines produces radicals which are the result, in general, of 1-electron reduction involving hydrated electrons. The structures of the radicals were assigned on the basis of their by EPR spectra. The study included 2-hydroxypyrimidine... 

Results. (1) Reduction of Au1 by Hydrated Electrons. Irradiations were carried out either in neutral or alkaline matrices, in the presence or absence of CH3OH (used as an OH scavenger). The spectrum... 

The rate constants for the reduction of C6o(C4HioN )/DODAB and C6o[C(OCH2CH2)3CH3]2/DODAB systems by hydrated electrons amount to 3.2 x 10 °... 

The actual electronic structure of (RSSR) is especially interesting. Key feature is a 2o/10 bond between the two sulfur atoms, rendering [RS.. SR]-an even more appropriate and informative notation.55,56 While further details on this three-electron bond will be dealt with in the odd electron bonds section vide infra), the following is of immediate interest. The combined effect of the two bonding o-electrons and the one antibonding a electron affords a formal bond order of 1/2. This, in turn, provides the rationale for the above equilibrium and relative ease of redissociation of the newly formed sulfur-sulfur bond. The same [RS. . SR] species is, incidentally, formed in the reduction of disulfides by hydrated electrons. Thiyl radicals and disulfide radical anions thus are two conjugate forms of the one-electron redox intermediate between thiols and disulfides. 

A limited number of radiation chemical results pertains to sulfuranyl radicals. The simplest of these three-coordinate sulfur-centered radicals, namely, R3S are very short-lived (tj/2 < 1 M-s) and so far escaped direct detection. Their transitory formation can, however, be deduced from a fast, close to diffusion controlled, reduction of the corresponding sulfonium salts by hydrated electrons and scavenging of the radicals cleaved as an overall result of reaction sequence 35. 

The reduction of other mercuric halides or pseudohalides HgXj (X = Br, T, CN , or SCN, respectively) by hydrated electrons or alcohol radicals yields as well transient HgX monomers. . They dimerize except HgCN which disproportionate. It was found that the HgjXj absorption spectra are at shorter wavelengths that those of HgX (Table 6). The monomer are strong electron donor towards O2, tetranitromethane and p-benzoquinone. On the contrary, in diluted solutions of HgClj, added with more concentrated salts such as Cd, ... 

The reduction of ferricytochrome c by hydrated electrons and by several free radicals has been studied by pulse radiolysis. The reduction of oxidized cytochrome c by [Fe(edta)] - follows first-order kinetics for both protein and reductant, with a rate constant of 2.57 x 10 1 mol" s" at pH 7 and activation enthalpy and entropy of 6.0 kcal mol" and —18 cal K" mol", respectively. These values are comparable to those for outer-sphere cytochrome c reductions and redox reactions involving simple iron complexes, and are compatible with outer-sphere attack of [Fe(edta)] " at the exposed haem edge, although the possibility of adjacent attack through the haem pocket is not ruled out. The rate data at pH 9 are consistent with [Fe(edta)] " reduction of two slowly interconverting forms of the protein, native kt = 2.05 X10 1 mol" S" ) and high-pH kt = 2.67 x 10 1 mol" s" ) isomers. A possible route for the transfer of the electron from Cr + to ferricytochrome c has been suggested as a result of the chemical analysis of the chromium(m) product. The reduction by Cr + of the native protein and of ferricytochrome c carboxy-methylated at the haem-linked methionine (residue 80) has been studied kinetically. At pH 6.5 the former process is simple and corresponds to a second-order rate constant of 1.21 x 10 1 mol" s". The latter, however, is complex - two chromium-... 

The reduction of cytochrome c oxidase by hydrated electrons has been studied in the absence and in the presence of cytochrome c. In the former case the haem group of the enzyme is not readily reduced, confirming a previous suggestion that haem a is not directly exposed to the solvent. In the presence of cytochrome c the latter is first reduced by eaq (A = 4x 10 1 mol s at 22 C and pH 7.2) and then transfers electrons to the cytochrome c oxidase (k — 6 x 10 1 mol s. It was found that two equivalents of cytochrome c are oxidized initially per equivalent of haem a reduced, showing that one electron is accepted by a second group, probably one of the copper atoms in the enzyme. 

Aquo-metal ions in unusual oxidation states are readily produced by reduction of stable metallic ions by hydrated electrons e.g. ... 

Reductive generation of thiyl radicals from disulfides may not only be initiated by hydrated electrons but also by many other reducing species like CO2 ", a-amino, or a-hydroxyl radicals. Certain complications may, however, arise. For example, reduction of RSSR by a-hydroxyl radicals may proceed via a reversible adduct formation [31] ... 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

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