Rate relative, from radiolysis studies

Rhus vernicifera stellacyanin has been labeled with asRu + at both exposed histidines, His-32 and 100 (38, 39). A pulse radiolysis study was carried out, and both the Ru(III) and Cu(II) sites were reduced by formate radicals on the order of 1 x 10 A/ s", in 55 and 45% proportions. A slower reduction of Cu(II) also was found this was attributed to Ru(II) to Cu(II) ET, with a rate constant of 0.05 s. Since a crystal structure of stellacyanin is not available, the ET distances were estimated from a computer model to be —16.1 A (39). The relatively low ET rate is in line with the slow ET reactions observed for azurin and plastocyanin. 

Pulse radiolysis studies show that the rapid reduction of the type 1 centers is followed by a partial reoxidation as an electron is transferred to the type 2 Cu centers. The extent of reoxidation is determined by the relative redox potentials of the two centers. In the presence of nitrite the reaction (Equation (4)) proceeds to completion and the type 1 center becomes fully reoxidized. Typical values for Atet for both green (AcNiR) and blue (AxNiR) at pH 7 are 1.4 x 10 s , which decreases by 50% in the presence of nitrite. The rapid rate of reduction of the type 1 center was unaffected by pH but Atet showed a marked pH dependence and at pH 6 the rate was 2 X 10 s and was unchanged by nitrite. Curves for the pH Atet dependence were essentially the same as the pH activity curve suggesting that two proton dissociations with values of 5 and 7 play an important role in both processes. Studies with AxNiR mutants Asp98Ala and His255Ala showed that these residues in wild-type NiR control the rate of electron transfer from the type 1 to the type 2 center by the formation of an H-bonding network ... 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

The results of this study show (99) the involvement of fragments such as Cr(CO) , (3 < jc < 6) which react with CO molecules which come from any of several sources fragmentation of the original molecules, bulk radiolysis of the compound, application of an external atmosphere, or perhaps from intermolecular exchange. It was concluded from the data that diffusion processes are involved and that the relative rates of reaction and of diffusion away are important in determining the height of the annealing plateaus. 

Interaction of the solute with radicals from the water is the first of a sequence of reactions which finally leads to stable products. Kinetic studies of the type cited give valuable information about the primary radical species and their relative reaction rates with molecules of different types. When sufficient data have been accumulated, it should be possible to predict the course of radiolysis in complex molecules. From the nature and yields of the products and by observing the effects on them of various factors such as concentration, pH, 02, and specific radical scavengers, it is often possible to speculate about the mechanisms by which products are formed. More often than not, this is a difficult problem because the products, even from relatively simple compounds, prove to be complex. Furthermore, it is often possible to produce more than one mechanism to fit the experimental data. The proteins are particularly difficult because of their complex structures. They contain approximately 20 different amino acids with an average of more than three carbon atoms in the side chains, which vary considerably in their structure hence, the possible number of products is large. For this reason, model compounds such as peptides and polyamino acids have been studied because they contain the peptide linkage but are free from the complications which arise from the diversity of the amino acid residues in a protein. A further practical difficulty which applies to chem-... 

Coleman (11) has used a sample of the relatively long-lived (7951 yr.) Am to study the disproportionation of Am(V) without the excessive radiolysis which made earlier results difficult to interpret (98). In 2M perchlorate solutions the rate was found to be second power in Am(V) and approximately 2.5 power in acid concentration. This latter dependence diflFers from those of the analogous reactions of U(V) (59) and Pu(V) (66) which are predominantly first power in hydrogen ion concentration. The temperature dependence was determined at a single hydrogen ion concentration so activation parameters for the individual rate determining reactions cannot be estimated with reasonable precision without making further assumptions. 

Studies of intramolecular ET in oxidases provide interesting examples of how pulse radiolysis is employed to obtain insights into both (1) these enzymes respective mechanisms of action and (2) electron transfer along protein polypeptide matrices that were most probably selected by evolution (9,10, 30-32). Thus, early attempts to study the electron uptake mechanism by the blue oxidase, ceruloplasmin, showed that a diffusion-controlled decay process of the eaq in solutions of this protein is paralleled by the formation of transient optical absorptions due to electron adducts of protein residues, primarily of cystine disulfide bonds (30). The monomolecular decay of the latter absorption was found to have the same rate constant as that at which the type 1 Cu(II) absorption band was reduced. These results were interpreted as being the combined result of the high reactivity of the e q and the relatively inaccessible type 1 Cu(II) site, yielding an indirect, intramolecular electron transfer pathway from surface-exposed residues (30). 

Since it is known that OH does not in fact attack co-ordinated ammonia, a rapid dissociation of the Co ammine complex followed by attack by this radical on the ammonia released would then account for the formation of NHa radicals. The reaction products are NH4+, Ng, N2O, and a relatively small amount of Oj. The yields of N2 and NjO are close to those expected if NH2OH were the intermediate giving rise to these gases. An interesting overlap on these studies is provided by data on the rate of detachment of NH3 from cobaIt(in) complexes reduced by pulse radiolysis. The rate constant Atq = 8 x lO 1 moI s for the process... 

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