Pulse Radiolysis EPR

Although TR EPR with pulse radiolysis excitation became established many years ago, this experimental technique is only available in a few laboratories,FT EPR being the detection method most commonly used recently. The advantage of pulse radiolysis over pulse photolysis is the possibility to generate more different radical structures, but the experimental efforts are much higher. In recent years, only a few papers using this technique have been published. 

1 Reactions of H Atoms. - In the radiolysis of water, the hydrated electron Caq and the OH- and H- radicals are the primary radiolysis products. Whereas the reactivity of hydrated electrons and OH- radicals with scavenger molecules S can be easily studied by optical methods (pulse radiolysis), TR EPR is the unique method which can be used to study the reactivity of hydrogen atoms. Bartels et investigated the temperature dependence of the hydrogen 

Spin-polarized mobile hydrogen and deuterium atoms were detected in silica glass containing 1200 ppm of OH or OD groups by the time-resolved pulsed EPR technique.The EPR spectra of the H/D atoms indicate the occurrence of CIDEP in reactions of H/D atoms with radiolytically induced metastable spin centres. The CIDEP effects in silica glass exhibit unusual polarization patterns, temperature dependence and dependence on hfs constants. A Monte Carlo lattice model is introduced to simulate the RPM spin polarization in amorphous silica. Although the model does not explain all the features observed, it indicates where peculiarities of CIDEP in disordered solids may originate. 

Chemerisov et al. report on radiolytically generated H atoms in water/ice clusters in NaA, NaX, NaY and HY zeolites. By FT EPR two states of spin-polarized hydrogen atoms with slightly different hfs constants were detected in NaY and HY zeolites, whereas in NaX and NaA zeolites only one H atom 

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Janik I, Ulanski P, Flildenbrand K, Rosiak JM, von Sonntag C (2000) Hydroxyl-radical-induced reactions of poly(vinyl methyl ether) a pulse radiolysis, EPR and product study in deoxygenated and oxygenated aqueous solutions. J Chem Soc Perkin Trans 2 2041-2048 Jayson GG, Stirling DA, Swallow AJ (1971) Pulse- and X-radiolysis of 2-mercaptoethanol in aqueous solution. Int J Radiat Biol 19 143-156... 

Mercero JM, Matxain JM, Lopez X, Fowler JE, Ugalde JM. (2002) Methoxyphenols-antioxidant principles in food plants and spices Pulse radiolysis, EPR spectroscopy, and density functional theory calculations. Int J Quantum Chem 90 969-979. 

Time-resolved EPR Studies of Transient Organic Radicals 3 Pulse Radiolysis EPR... 

Other Radicals. - In recent years only a few papers deahng with pulse radiolysis EPR have been published on radicals other than hydrogen/deuterium atoms. 

Epr is most effective for detecting free radicals that may occur as intermediates in oxidation and reduction reactions involving transition metal ions. Since these transients are invariably quite labile, epr is combined with continuous flow, (more conveniently) stopped-flow, flash photolysis, and pulse radiolysis. 

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). ... 

It has been shown that Pulse Radiolysis is a powerful tool for the study of the properties of such short lived intermediates (1,2). The application of this technique is based on the capability of producing a large variety of aliphatic radicals (vide infra) within less than 1 pis in physically observable concentrations. Thus one can follow by different techniques the kinetics of disappearance of the initially formed radicals and the properties of unstable intermediates, if formed, in these reactions. The most common detection technique is the spec-trophotometric one, but changes in specific conductivity, EPR, resonance Raman, etc., can be applied and are often helpful in elucidating the nature of the short lived intermediate observed. 

The rapid-freezing EPR data obtained in pulse radiolysis experiments had been interpreted by half-site reactivity In the meantime preparations have, however, been obtained with a definitely higher specific activity (second-order rate constant... 

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 hydrated electron is characterized by its strong absorption at 720 nm (e = 1.9 x 104 dm3 mol-1 cm-1 (Hug 1981) the majority of the oscillator strength is derived from optical transitions from the equilibrated s state to the p-like excited state (cf. Kimura et al. 1994 Assel et al. 2000). The 720-nm absorption is used for the determination of its reaction rate constants by pulse radiolysis (for the dynamics of solvation see, e.g Silva et al. 1998 for its energetics see, e.g Zhan et al. 2003). IP only absorbs in the UV (Hug 1981), and rate constants have largely been determined by EPR (Neta et al. 1971 Neta and Schuler 1972 Mezyk and Bartels 1995) and competition techniques (for a compilation, see Buxton et al. 1988). In many aspects, H and eaq behave very similarly, which made their distinction and the identification of eaq" difficult (for early reviews, see Hart 1964 Eiben 1970 Hart and Anbar 1970), and final proof of the existence of the... 

Grierson L, Hildenbrand K, Bothe E (1992) Intramolecular transformation reaction of the glutathione thiyl radical into a non-sulphur-centered radical a pulse radiolysis and EPR study. Int J Radiat Biol 62 265-277... 

Wang D, Gyorgy G, Hildenbrand K, von Sonntag C (1994) Free-radical-induced oxidation of phloro-glucinol - a pulse radiolysis and EPR study. J Chem Soc Perkin Trans 2 45-55 Wang W-F, Schuchmann MN, Schuchmann H-P, Knolle W, von Sonntag J, von Sonntag C (1999) Radical cations in the OH-radical-induced oxidation of thiourea and tetramethylthiourea in aque-ous solutions. J Am Chem Soc 121 238-245... 

Han P, Bartels DM (1994) Encounters of H and D atoms with 02 in water relative diffusion and reaction rates. In Gauduel Y, Rossky P (eds) AIP conference proceedings 298. "Ultrafast reaction dynamics and solvent effects." AIP Press, New York, 72 pp Hasegawa K, Patterson LK (1978) Pulse radiolysis studies in model lipid systems formation and behavior of peroxy radicals in fatty acids. Photochem Photobiol 28 817-823 Herdener M, Heigold S, Saran M, Bauer G (2000) Target cell-derived superoxide anions cause efficiency and selectivity of intercellular induction of apoptosis. Free Rad Biol Med 29 1260-1271 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of peroxyl radicals of polyfacrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Howard JA (1978) Self-reactions of alkylperoxy radicals in solution (1). In Pryor WA(ed) Organic free radicals. ACS Symp Ser 69 413-432... 

Wang D, Gyorgy G, Hildenbrand K, von Sonntag C (1994) Free-radical-induced oxidation of phloro-glucinol - a pulse radiolysis and EPR study. J Chem Soc Perkin Trans 2 45-55... 

From a pulse radiolysis study on the S04 -induced reactions of Thd (Deeble et al. 1990), it has been concluded that the pKa of the Thd radical cation (deprotonation at N(3)) should be near 3.5, i.e. close to that at N( 1) in Thy. It is noted that also in the parent, Thy, the pKa values at N( ) and at N(3) are quite close. A Fourier-transform EPR study using photoexcited anthraquinone-2,5-disulfonic acid to oxidize Cyt and IMeCyt shows that the radical cation of the former de-protonates rapidly at N(l) while that of the latter deprotonates at the exocylic amino group (Geimer et al. 2000). The EPR evidence for the formation of heteroatom-centered radicals by S04 in its reactions with some other pyrimidines (Bansal and Fessenden 1978 Hildenbrand et al. 1989 Catterall et al. 1992) is in agreement with a marked acidity of such radical cations. It is re-emphasized that this conclusion does not require that radical cations are formed in the primary step. 

In pulse radiolysis studies of Urd and its derivatives (but not with dUrd), spectral changes are observed after the completion of the S04, reaction [k = 3 x 10s s 1 Bothe et al. 1990] that are not typical for S04 reactions with pyrimidines. On the basis of EPR experiments (Hildenbrand 1990 Catterall et al. 1992), these observations can be interpreted by an (overall) intramolecular H-transfer giving rise to a radical at the sugar moiety. This requires that considerable amounts of Ura are released which is indeed observed (Fujita et al. 1988 Aravindakumar et al. 2003 Table 10.4). Chain reactions occur as with the other pyrimidine/peroxodisulfate systems. This increases the Ura yield beyond that expected for a non-chain process, but when corrections are made for this by carrying out experiments at the very high dose rates of electron-beam irradiation, a... 

Under hypoxic conditions, cellular enzymes reduce the benzotriazine di-N-oxide [(reaction (68) P450 reductase Cahill and White 1990 and NADPH may be involved Walton et al. 1992 Wang et al. 1993]. Upon microsomal reduction of tirapazamine the radical formed in reaction (68) has been identified by EPR (Lloyd et al. 1991). Using the pulse radiolysis technique, it has been shown that this radical has a pKd of 6 (Laderoute et al. 1988), and it is the protonated form that undergoes the DNA damaging reaction (Wardman et al. 2003). The rate constants of the bimolecular decay of the radical [reaction (70)] has been found to be 2.7 x 107 dm3 mol-1 s 1. The reaction with its anion is somewhat faster (8.0 x 108 dm3 mol-1 s 1), while the deprotonated radicals do not react with one another at an appreciable rate. From another set of pulse radiolysis data, a first-order process has been extracted (k = 112 s 1) that has been attributed to the water elimination reaction (72), and the tirapazamine action on DNA [reaction (74)] has been considered to be due to the resulting radical (Anderson et al. 2003). 

Perkin Elmer MPF-3 spectrofluorometer. X- and Q-band measurements of EPR spectra were carried out at liquid nitrogen and liquid helium temperatures. Microcalorimetric measurements were performed on a LKB 10700 batch microcalorimeter. Temperature-jump relaxation kinetics were measured using a double beam instrument (18) with a cell adapted for anaerobic work. The relaxation signals were fed into an H.P. 2100 computer and analyzed as described in Ref. 7. The pulse radiolysis exepriments were carried out on the 5-MeV linear accelerator at the Hebrew University. Details of the system have been published previously (19). 

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