Disulfides radicals from

Selected entries from Methods in Enzymology [vol, page(s)] Acetylthiocholine as substrate, 251, 101-102 assay by ESR, 251, 102-105 inhibitors, 251, 103 modification by symmetrical disulfide radical, 251, 100 thioester substrate, 248, 16 transition state and multisubstrate analogues, 249, 305 enzyme receptor, similarity to collagen, 245, 3. 

The importance of ionization of the metal-sulfur bond in the electron-transfer process accounts for inhibitory properties being observed only for compounds of types I and II. Support for this mechanism was also obtained by the isolation of the disulfide (IV) from a reaction between peroxy radicals and zinc diisopropyl dithiophosphate. 

Before proceeding to describe the mechanistic features of disulfide reduction, it is useful to first describe the effect of substituents on the stability of thiyl radicals and disulfide radical anions as well as the consequences, from a theoretical viewpoint, of the unpaired electron on both the bond energy and the S—S bond length of disulfides. 

The thiol-dependent release of nitric oxide may well involve metal catalysis from trace contamination of copper or iron in buffers. The disulfide radical may also reduce another nitrosothiol to produce more nitric oxide. 

Thiyl radicals from irradiated bis(2,2 -carboxyphenyl) disulfide failed to abstract hydrogen from acetic acid or an electron from acetate ion,... 

An H-transfer may also occurs intramolecularly such as in DDT via a five-membered transition state [reaction (33) Akhlaq and von Sonntag 1986]. In the given case, an H2S forming chain reaction is induced [cf. reaction (34) followed by the (slow) H-abstraction of the thus-formed radical from DTT] which comes to a halt when the thiyl radical is complexed as the disulfide radical anion at higher pH values [cf. reaction (40)]. 

Table 7.4. Absorption maxima and molar absorption coefficients of cyclic disulfide radical anions and their protonated forms. According to von Sonntag (1990).The lipoic acid value has been taken from Hoffman and Hayon (1972). Note that the eguilibria (cf. apparent pKa values) include the ring-open forms which are not detectable...

Bonifacic M, Schafer K, Mockel H, Asmus K-D (1975b) Primary steps in the reactions of organic disulfides with hydroxyl radicals in aqueous solution. J Phys Chem 79 1496-1502 Bonifacic M, Armstrong DA, Carmichael I, Asmus K-D (2000a) p-Fragmentation and other reactions involving aminyl radicals from amino acids. J Phys Chem B 104 643-649 Bonifacic M, Hug GL, Schoneich C (2000b) Kinetics of the reactions between sulfide radical cation complexes,[S.. S]+ and [S. N]+, and superoxide or carbon dioxide radical anions. J Phys Chem A 104 1240-1245... 

Fang X, Wu J, Wei G, Schuchmann H-P, von Sonntag C (1995) Generation and reactions of the disulfide radical anion derived from metallothionein a pulse radiolytic study. Int J Radiat Biol 68 459-466... 

In support of the intermediate formation of the acetate radical from N-nitrosoacetanilide is the evolution of carbon dioxide during the reaction. Moreover, metals such as copper, zinc, lead, antimony, and iron are attacked when N-nitrosoacetanilide is allowed to decompose in a non-polar solvent such as carbon disulfide in the presence of the metals, a behavior similar to that of metals in contact with active free radicals in the Paneth test. 

To study their properties, the easiest way is to form them by one-electron reduction of disulfide bonds. Among radicals from water radiolysis, hydrated electron is the most powerful reductant. It reacts with almost all amino acids and especially with the disulfide groups. Using less powerful reductants such as COO radicals, some selectivity in the attack appears. An example is displayed in Figure 3. 

Figure 5. Time-resolved absorption changes, induced by reaction of CO2" radicals, due to intramolecular ET from the internal disulfide radical anion to Cu(ll) in the C3/C26A-N42C azurin dimer measured at 625 nm. Protein concentration was 20pAf, where T = 299K pH 7.0 0.1 Af formate 10 mM phosphate N2O saturated pulse width 1.5 ps optical path 3 cm. Time is in seconds the left panel shows the faster phase, while the right one shows the reaction taking place at the slower phase. The lower panels show residuals of the fits to the data.

AU three T1 centers are well protected from the solvent by the protein matrix. Thus, it is not surprising that only indirect copper reduction by radicals via the exposed disulfide groups is observed (72). In fact, this behavior is reminiscent of results of our studies of intramolecular ET in azurins where, in addition to the direct, bimolecular reduction of both the disulfide group and the T1 site, LRET takes place from the disulfide radical to Tl(Cu ) over a distance of 2.6 nm... 

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