Detection by pulse radiolysis

Pulse radiolysis has been seldom used in the detection of intermediates in macromolecular systems. Polystyrene and polymethylmethacrylates in the solid phase have been studied [188—191] while attention has been paid to the solution radiolysis of polyethylene oxide [192]. 

With polymethylmethacrylate, absorption is observed between 350 and 500 nm. The emitted fluorescence is weak and decays exponentially with a half-life of 40—50 ns. In the presence of biphenyl, T — T2 absorption of Ci2H10 at 370 nm is observed the anion of biphenyl absorbs at 410 and 590 nm. A third group of bands at 370—390 nm is assigned to a reaction product of diphenyl and polymethylmethacrylate. As with polystyrene, no correlation could be found between the decay of the excited singlet molecule and the intensity of the Tj—T2 absorption. Triplet state formation was thus also assigned to triplet energy transfer from the polymer host. 

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The hydrated electron is commonly detected by pulse radiolysis with electron beams. In the case of highly-structured track, this very reducing species reacts easily with oxidant like OH radical in its vicinity at earliest time after the ionization track is formed. That is the reason why it is a real challenge to detect this species yet with heavy ion irradiation. Giving a G-value remains delicate because the concentrations are lower than 10 M and dose must be measured with a high accuracy. As it is shown in Fig. 5, the time dependence of hydrated electron is typical of a track structure in space and time in the first 100 ns the concentration of initial hydrated electron is at least divided... 

The products of these reactions may be short lived, but they often have characteristic absorption spectra that can be detected by pulse radiolysis. Subsequent reactions, such as electron transfer and ligand labilization, can be followed kinetically with the appropriate detection technique. Reviews of the spectra, kinetics and mechanisms of complexes in unusual and unstable oxidation states are available A compilation of rate constants for the reactions of metal ions in unusual valency states is available ... 

A large number of studies in the last decade have been concerned with primary processes in irradiated polymers. Unstable intermediates have been trapped and identified at low temperatures or detected by pulse radiolysis. In some cases, reactions of these intermediates have been identified and workers have attempted to correlate such reactions with the chemical effects of irradiation observed at room temperature. Some of the conclusions drawn will be summarized below. The elementary processes during irradiation at 77°K will be discussed first. The reactions of charged species, excited states and neutral radicals will be outlined successively. 

Reaction 7 contributes less than 4% to the photolytic decomposition at 1849 A. and about 14% at 1236 A. (18). The formation of NH at A < 1600 A. has been demonstrated by flash photolysis (2). There is no direct evidence for the postulated intermediates in the radiolysis except for the NH radical, which has been detected by pulsed radiolysis experiments (19). 

Trradiation of aqueous solutions of aromatic solutes is believed to lead initially to adding H atoms and OH radicals to the ring to form cyclo-hexadienyl radicals, which then react further to form the observed products (17, 21, 34). The hydrated electron can also react to form transient negatively-charged entities (8, 29, 40) which may either be protonated by reaction with the solvent molecules (8, 40, 41) or else dissociate to form a stable anion and a substituted phenyl radical (8). In some systems (12, 41) the lifetime of the electron adduct is sufficient for its detection by pulse radiolysis techniques, but in many cases only the protonated species is seen. The rates of reaction of H atoms, OH radicals and hydrated electrons with aromatic solutes vary widely, but some success... 

The products of the reactions depicted by Eqs. (20) and (21) are identical. These reactions constitute the early binding between the two metals within the same entity. Mixed species including two different metals have already been detected by pulse radiolysis, e.g. (TlAg)+ in solutions of monovalent-monovalent ionic precursors, and (CdAg) + and (CoAg) + in divalent-monovalent precursors. Further coalescence as in Eqs. (22) and (23) of the mixed species formed in Eqs. (20) and (21) yields alloyed clusters of higher nuclearity, (Ag,Au,)cN- this accounts for the increase in absorbance at 400 and 520 nm, at least up to 2 s. " ... 

Veltwisch and colleagues45 studied the reaction of OH with several sulfoxides by pulse radiolysis using electrical conductivity for the detection of formation or disappearance of ions. Pulse radio lysis of N20-saturated aqueous solution of DMSO (10-3m) leads to a decrease in conductivity at basic pH (pH = 9.0) and an increase in conductivity at acidic pH (pH = 4.4). This is explained by the reactions... 

It is pertinent that S20g accepts an electron generated by pulse radiolysis of water to give optically detectable S04 within 1.5 x 10 sec . ... 

The radical cations of urazole-annelated azoalkanes 65 were generated by pulse radiolysis and the transients characterized spectrally and kinetically by time-resolved optical monitoring. The initial distonic 1,3 radical cations 66 were detected, and the methyl-substituted 66 further deprotonates to radical 67 (Scheme 1) <1997JA10673>. 

FDMR has also been used to detect the transient radical cations formed from secondary amines by pulse radiolysis. As mentioned earlier this technique has been used to study a variety of systems such as the radical cation of triethylamine. The radical cations of diethylamine, n-propyl amine and f-butylamine, have also been studied25. The results have shown that the FDMR signal is enhanced with increasing alkyl substitution of the amine as in the pyrrolidines (18) and the piperidines (19)25. 

V. Jagannadham and S. Steenken, J. Amer. Chem. Soc. 106, 6542 (1984). The reaction of RCHOH, generated by pulse radiolysis, was studied with p-substituted nitrobenzenes using time-resolved optical and conductance detection. The radical anion of the nitrobenzene is produced directly and indirectly. 

The differential absorption spectra obtained in the presence of these two nucleotides are indeed similar to those obtained after reduction electrolysis of the complex in the first reduction wave, and obtained by pulse radiolysis. The prerence of the deprotonated radical cation GMI —H) can also be detected by recording the transient absorption after reaction of the reduced complex with O2. 

The spectra of silver and gold nanoclusters are intense and distinct (Table 4). They are thus particularly suitable to detect the evolution of a cluster composition during the construction of a bimetallic cluster in mixed solution. The system studied by pulse radiolysis was the radiolytic reduction of a mixed solution of two monovalent ions, the cyano-silver and the cyano-gold ions Ag(CN)2 and Au(CN)2 (Fig- 7) [66]. Actually, the time-resolved observation demonstrated a two-step process. First, the atoms Ag and Au are readily formed after the pulse and coalesce into an alloyed oligomer. However, due to... 

Actually, the kinetic study of the cluster redox potential by pulse radiolysis [31] (Section 20.3.2) somewhat mimics the process of the black-and-white photographic development, except that clusters are free in the solution (not fixed on AgBr crystals), and that they are produced by ionizing radiation (as in radiography and not by visible photons but the last choice had been incompatible with the time-resolved optical detection in the visible. Beyond the critical nuclearity, they receive electrons without delay from the developer already present (actually, the photographic development is achieved in a delayed step). 

The lifetime of the RSSR radical anions is usually very short on the microsecond timescale in water. However, they have been detected and characterized by time-resolved optical methods. In one early study, y irradiation of matrices containing alkyl and aryl disulfides provided spectroscopic evidence for the formation of the corresponding radical anions. Subsequently, the formation of RSSR radical anions has been well documented, particularly by EPR, flash photolysis, and pulse radiolysis. In fact, 2a/ a three-electron bonded radical anion species, particularly from sulfur compounds, constitute significant and interesting intermediates. The RSSR radical anions may be obtained from different approaches. One is by one-electron reduction of disulfides (equation 75), such as by pulse radiolysis. However, the most common approach is by association of RS and RS (equation 79). ... 

They were found not to react with BESOD, the rate constant was estimated to be < 10 M s , if there was a reaction at all The reaction of BESOD was also investigated with several other radicals generated by pulse radiolysis. With the semiquinone of riboflavin 5 -phosphate no reaction was detected. The semiquinone of 9,10-anthraquinone-2-sulfonate and the radical anion of 4-nitroacetophenone converted the enzyme into an unreactive form... 

There is an extensive chemistry of the nickel(I) ion generated by pulse radiolysis which is beyond the scope of this review. Complexes with saturated amines such as 1,2-diaminoethane have been studied by this method and by the y radiolysis of aqueous glasses, but the species formed have no more than a transient existence. The imine ligands phen and bpy offer a more attractive environment for nickel(I) by allowing electron delocalization over the ligand n system (178,179). A number of complexes of these ligands have been reported in y-radiolysis studies. The EPR spectra indicate that reduction is primarily metal centered with a significant orbital contribution. Electrochemical reduction of [NiH(bpy)3]2+ in anhydrous acetonitrile results in [Ni (bpy)3] +, which can be detected by EPR methods. The reduction potential is reported to be —1.55 V but the complex is thermodynamically unstable with... 

The kinetics of this reaction have been monitored by pulse radiolysis, using conductivity detection. At pH 9.5, the half-life of 17 is <3.5 jus, because of the base-induced reaction 38, and the appear-... 

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-... 

X-ray ionization of o-vinylbenzaldehyde (136) in argon matrices leads to the quinoketene (137) via die radical cation, detected by IR spectroscopy.298 The product identity was confirmed by the independent preparation of (137) and (137+-) by the photo-stimulated ring-opening of 2-methylbenzocyclobutenone (138) (Scheme 21). The reactions of benzaldehyde, acetophenone, and benzophenone with OH, 0 and SC>4 have been studied by pulse radiolysis in aqueous solution.299 The addition of OH to the carbonyl moiety of benzaldehyde predominates over addition to the aromatic ring, whereas ring addition is predominant in the case of acetophenone. Disproportionation of the exocyclic OH adduct is proposed to explain the formation of benzoic acid, which is a major product in the reaction of benzaldehyde and OH or SO4T Rate constants for each reaction have been calculated. 

Ulanski P, von Sonntag C (2000) Stability constants and decay of aqua-copper(lll) - a study by pulse radiolysis with conductometric detection. Eur J Inorg Chem 1211-1217 Vaish SP, Tollin G (1971) Flash photolysis of flavins. V. Oxidation and disproportionation of flavin radicals. J Bioenergetics 2 61-72... 

Tripathi GNR (1998) Electron-transfer component in hydroxyl radical reactions observed by time resolved resonance Raman spectroscopy. J Am Chem Soc 120 4161-4166 TsaiT, Strauss R, Rosen GM (1999) Evaluation of various spin traps for the in vivo in situ detection of hydroxyl radical. J Chem Soc Perkin Trans 2 1759-1763 Tsay L-Y, Lee K-T, Liu T-Z (1998) Evidence for accelerated generation of OH radicals in experimental obstructive jaundice of rats. Free Rad Biol Med 24 732-737 Ulanski P, von Sonntag C (2000) Stability constants and decay of aqua-copper(lll) - a study by pulse radiolysis with conductometric detection. Eur J Inorg Chem 1211-1217 Veltwisch D, Janata E, Asmus K-D (1980) Primary processes in the reactions of OH radicals with sul-phoxides. J Chem Soc Perkin Trans 2 146-153... 

For the detection of free-radical-induced DNA damage and its repair, biochemical techniques are increasingly applied. It would exceed the allocated space to discuss these techniques here, and the reader is referred to the original literature. However, to assist biologists and biophysicists, some of the chemical techniques for measuring typical DNA lesions are discussed. Most of the kinetic data concerning the free-radical chemistry of DNA and its model systems has been obtained by pulse radiolysis. This technique is only available in a few laboratories worldwide. For this reason, it will be described in some detail here. 

Among the electron transfer induced reactions of cyclobutane systems, cycloreversions are the most prominent. These reactions are the reverse of the cycloadditions discussed in Sect. 4.1. The reactivity of the corresponding radical cations depends on their substitution pattern. We have mentioned the fast two-bond cycloreversion of quadicyclane radical cation as well as the ready ring closure of a tetracyclic system (3, Sect. 4.1). A related fragmentation of cis-, trans-, cis-1,2,3,4-tetraphenylcyclobutane (84) can be induced by pulse radiolysis of 1,2-dichloro-ethane solutions. This reaction produces the known spectrum of trans-stilbene radical cation (85) without a detectable intermediate and with a high degree of... 

As described above, recent advances in accelerator technology have enabled the production of very short electron pulses for the study of radiation-induced reaction kinetics. Typically, digitizer-based optical absorbance or conductivity methods are used to follow reactions by pulse radiolysis (Chap. 4). However, the time resolution afforded by picosecond accelerators exceeds the capability of real-time detection systems based on photodetectors (photomultiplier tubes, photodiodes, biplanar phototubes, etc.) and high-bandwidth oscilloscopes (Fig. 8). Faster experiments use streak cameras or various methods that use optical delay to encode high temporal resolution, taking advantage of the picosecond-synchronized laser beams that are available in photocathode accelerator installations. 

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