Spectrophotometric pulse radiolysis

K.M. Morehouse and P. Neta, Redox Reactions of Manganese Porphyrins in Aqueous Solutions. Steady-State and Pulse Radiolysis Spectrophotometric Studies, J. Phys. Chem., 88 (1984) 1575. 

Neta and Evans (214) studied the anion radicals of some bianthronylidenes. The anion radical was prepared in the Ar form by pulse radiolysis in 2-propanol, and its conversion to BT was followed by fast spectrophotometric technique. The rate was determined at 21°C for 129c, its 3,3 -dimethyl, 3,3 -dimethoxy, and 1,1 -dimethyl analogs, and free energy barriers of 10.6, 10.6, 11.5, and 13.1 kcal/mole, respectively, were found, which were considerably lower than that for the B - A conversion in the neutral molecules. 

Let us consider the data on the dependence of the kinetics of et decay at 77 K on the radiation dose. As seen from Fig. 11, over the dose range 3 x 1019 - 3.6 x 102° eV cm 3, the kinetics of et decay is virtually independent of the dose. At the same time, at lower doses, the decay of et is significantly slowed down. For example, for a dose of 1019 eV cm-3, the change in optical density of y-irradiated samples at the maximum (585 nm) of the et absorption spectrum with time is also described by eqn. (5), but the slope of the kinetic curve the coefficient M in eqn. (5)] is smaller by almost a factor of two [28] than for the curve of Fig. 11. Further investigations by pulse radiolysis technique with spectrophotometric recording of et showed that, at a still lower dose (6 x 1017 eV cm"3) no decay of et in water-alkaline matrices is observed at all [43] while at high doses (5 x 1021 eV cm"3) for the same samples, the decay of efr does occur [43]. A decrease in the rate of etr decay via the reaction with O at small doses was also reported in ref. 44. This behaviour of the kinetic curves seems to reflect special features of the spatial distribution of etr and 0 particles in samples irradiated with different doses. 

The pulse radiolysis method has been described in detail in some of the early papers (22, 22), in a brief review of the subject (23), and in a current comprehensive review (14). It is, in brief, a fast reaction method in which the external perturbation applied to the system is a microsecond pulse of electrons. The current is sufficiently high to produce an instantaneous concentration of transient species high enough to be observed by fast measurement of the optical absorption. Spectra may be recorded either photographically or spectrophotometrically. The kinetics are studied by fast spectrophotometry. Since a perturbing pulse as short as 0.4 /xsec. has been used, the time resolution has approached 10-7 sec. The flash photolysis method used in some of the other studies (27, 15) has been reviewed in detail (24). 

The products of reaction of e aq with olefins are carbanions which react with the solvent within less than 10-n sec. (47) to give the corresponding hydrogen adducts >C = C< + e aq(> C = C <)- + H + - > C — CH <. In the case of acetone, the adduct (CH8)2CHO was observed spectrophotometrically by pulse radiolysis and was found to be identical to the radical obtained from isopropyl alcohol by hydrogen abstraction (69). (CH8)2CO- was also detected by electron spin reso-... 

Studies of proton transfers from carbon with f1/2 less than one microsecond are quite rare. The ionization of hydrogen cyanide in aqueous alkaline solution has been studied [20] using the ultrasonic stationary method which is applicable to reactions with tl/2 in the range 10 5 to 10 9 sec. Several reactions of benzyl carbanion having f 1/2 in the range 10-6—5 x 10-8 sec have been studied in tetrahydrofuran. The carbanion was generated by pulse radiolysis of solutions of dibenzyl mercury and its subsequent reaction with water and alcohols was followed spectrophotometrically [21]. 

Table 2. Comparison between time-resolved spectrophotometric detection set-ups of several laser-photocathode electron accelerator facilities for picosecond pulse radiolysis. 

The scheme of Mn2+ ion extrusion from corresponding sarcophaginate has been deduced using UV-vis spectrophotometry, EPR and cyclic voltammetry [179]. The extrusion of the copper ion after its reduction to copper(I) ion is too fast to be examined by cyclic voltammetry, and the kinetics of the reduction of [Cu(sar)]2+ cation to copper(II) [Cu(sar)] complex has been followed by pulse radiolysis over a shorter time using the hydrated electron to probe for extrusion of the metal ion. This reduction process has been registered spectrophotometrically to yield a second-order rate constant. The... 

Absorption spectra of phenoxyl radicals derived from biologically important molecules were recorded in numerous cases. The tyrosyl radical was studied by many investigators and its spectrum was used to detect tyrosine oxidation in a protein and to follow intramolecular electron transfer from tyrosine to the tryptophan radical in dipeptides and polypeptides . A number of catecholamines, such as adrenaline and dopa, were also studied by kinetic spectrophotometric pulse radiolysis " ". The absorption spectra of most of these substituted o-semiquinone anion were similar to those of the unsubstituted... 

The dissociation constants for semiquinone radicals, measured by spectrophotometric pulse radiolysis, were reviewed before and are summarized in Table 8. Protonation of semiquinone anion radicals takes place in most cases with p/sTa 4-5 with several higher and lower values. The simple benzosemiquinones show a strong effect of the relative positions of the oxygens on the pX a, i-e. 4 for 1,4-benzosemiquinone, 5 for the 1,2-isomer and ca 7 for the 1,3-isomer. The relatively high pX a for m-benzosemiquinone is clearly related to the lower spin density on the oxygens of this radical. 

Figure 1. Absorption spectra of aromatic anions in ethyl alcohol, formed by electron attachment in pulse radiolysis, (a) Diphenyl in ethyl alcohol. The solid curve was obtained during the pulse. The dashed curve was obtained after a delay of 12 sec. and is thought to be the adduct radical Ci2Hu. (b) Anthracene in ethyl alcohol. This band of the anthracenide ion was observed spectrophotometrically at about 0.2 yjsec. after the pulse, (c) p-Terphenylide ion in solutions of p-terphenyl in ethyl alcohol. Spectra are...

All these reactions are rapid and a maximum concentration of O2 is reached within 5 xsec of the radiation pulse. The decay of O2 is followed spectrophotometrically at around 250 nm. The time resolution of pulse radiolysis is very high, and reaction times as short as 2 X 10 sec can be easily followed. In common with the direct assays that utilize the ultraviolet absorption of O2, the problem of sample absorption in this region arises. Also the maximum single pulse yield of O2 (ca. 200 xM) is less than that obtained from a solution of potassium superoxide. However, the technique has proved extremely useful for working with pure enzymes. The mechanisms decribed in Section I have all been obtained by this technique. 

Silica cells, which gave the possibility of working up to 25 °C., were used in experiments with liquid ammonia and ammonium solutions. A thermostated jacket for investigating shortlived intermediates by pulse radiolysis with fast spectrophotometric recording is shown in Figure 1. 

Whereas much of the underlying mechanisms for the effects of radiation on materials were outlined using steady state radiation sources, the advent of pulse radiolysis on the heels of flash photolysis opened a window into direct observation of the intermediates. One of the early discoveries utilizing pulse radiolysis was the spectrophotometric detection of the hydrated electron by Boag and Hart (35,36). Since then thousands of rate constants, absorption spectra, one-electron redox potentials and radical yields have been collected using the pulse radiolysis technique. The Radiation Chemistry Data Center at the University of Notre Dame accumulates this information and posts it (at www.rcdc.nd.edu/) for the scientific community to use. They cover the reactions of the primary radicals of water and many organic radicals and inorganic intermediates. 

Most of the techniques used in the analysis of intermediates are utilized today in pulse radiolysis as well. These include spectrophotometric, emission, near-IR, conductivity, resonance-enhanced Raman, and ESR techniques. Early attempts to observe surface-enhanced Raman scattering (SERS) from adsorbed radicals were inconclusive (44) but they led to the realization that the sensitivity... 

We recognized the need for methodology to measure SOD activity directly that would be more accessible to the bench-top scientist than is the method of pulse radiolysis, another direct measure. Consequently, we developed methodology to measure the catalytic dismutation of superoxide by stopped-flow kinetic analysis.By this technique, we directly monitor the decay of superoxide spectrophotometrically in the presence or absence of a putative SOD mimic at a given pH. Kinetic analysis of this decay can determine whether the complex is a SOD mimic (decay of superoxide becomes first-order in superoxide and first-order in complex see equations 1 and 2), or is inactive (decay of superoxide remains second-order for its self-dismutation see equation 3). At least a tenfold excess of superoxide over the putative SOD mimic is used in the stopped-flow assay, to eliminate contributions due to a stoichiometric reaction of the complex with superoxide. A catalytic rate constant for the dismutation of superoxide by the complex can be determined from the observed rate constants of superoxide decay as a function of catalyst concentration. ... 

Steady-state pulse radiolysis of methylene chloride solutions containing ferric deute-roporphyrin or the chemical reduction of ferric deuteroporphyrin solutions containing methyl iodide led to iron methyl o-bonded species which were spectrophotometrically characterized ... 

A phenylbutazone radical has been identified spectrophotometrically using pulse radiolysis during which a solution of phenylbutazone was bombarded with hydroxyl radicals (Evans and Aruoma, unpublished work). It is therefore likely that ferryl systems oxidise the drug to free radical forms which cause oxidative damage to arachidonic acid and ai-antiproteinase. 

Computerisation for control and automation in pulse radiolysis [4,hJ]. The great volume of experimental results obtained by pulse radiolysis may be attributed to several factors. The radiation sources work reliably and continuously over long periods. The detection techniques mostly allow rapid replication. Digitization is routinely available for times down to about one nanosecond. One consequence is that, with a dedicated computer, the raw data can be quickly analysed on-line, and validity checks can be made on each set of data [4,h]. Another is that the operation of pulse-radiation apparatus can be automated [4,j] for example, a series of experiments at different temperatures over different periods of time, with different radiation doses, and with spectrophotometric detection at a series of... 

Anion radicals of aromatic hydrocarbons are known to undergo protonation in protic media (1-5). The spectrophotometric pulse radiolysis technique has been used to demonstrate this process in alcoholic solutions of several aromatic compounds ( ). In such experiments, the anion radicals are produced by the reaction of the hydrocarbon with the solvated electron k. 

In Table 14.4, the most stable states are shown in bold type and the most unstable states are indicated by parentheses. (Oxidation states that have been claimed to exist, but not independently substantiated, are indicated with question marks.) The most unstable oxidation states have only been observed in solid compounds, or produced as transient species in solution by pulse radiolysis [17-20]. In this very interesting technique, a beam of electrons is injected into an aqueous solution of the ion under investigation. These have been mainly the 3 + actinide ions. When N2O is present in the reaction mixture, the hydrated electrons formed by the injection of the electrons into water are converted into OH radicals, which are strong oxidants. If t-butanol is present in place of nitrous oxide, the OH radicals are scavenged, and only the hydrated electron, e (aq), a powerful reducing agent, is formed. The reactions of these reagents with actinide ions is followed spectrophotometrically. Reaction of the in ions in 0.1 m perchloric add with e (aq) forms Am(ii), Cm(ii), and Cf(ii). When OH radicals react, Am(iv) and Cm(iv), but no Cf(iv), are produced. All of the 2+ and 4 + spedes are transient. The ii spedes disappear with rate constants of about 10 s by what appears to be a first-order process. Am(ii), Cm(ii), and Cf(ii) have half-lives of the order of 5-20 /is Am(iv) appears to be appredably more stable and... 

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