[en] In situ photolysis of aqueous solutions containing either 2'-deoxyguanosine-5'-phosphate or guanosine-5'-phosphate and K₂S₂O₈ at pH<4 resulted in a resolved EPR-spectrum consisting of 40 equidistant peaks with a separation of 0.068 mT centered at g = 2.0038. It originates from the guanosyl radical cation generated upon one-electron oxidation of the base moiety by SO₄^-.. In order to assist the assignment of the EPR-spectrum theoretical nitrogen and hydrogen hyperfine coupling constants were employed. They were obtained from spin density calculations based on the restricted open shell formalism at an ab initio level. For a conversion of calculated spin densities into coupling constants, calibration curves were constructed by applying a McConnell type approach to known nitrogen heterocyclic radical cations. For four different tautomers of the guanosyl radical cation theoretical spin densities and calibration curves were used to obtain coupling constants. They were applied in a simulation of the experimental EPR-spectrum. The simulation in combination with the calculated energies of the tautomers indicate that the EPR-spectrum originates from the guanosyl radical cation protonated at N1. (author)

[en] Reaction of photolytically produced radical OH radicals with polyuridylic acid [poly(U)] in neutral solutions resulted in the electron spin resonance (e.s.r.) spectrum of the C(5)-OH-6-yl (= 6-yl) radical 1b of the nucleobase. At pH ≤ 4 the spectrum of the base radical had disappeared and instead the cyclic 2'-oxo-3'-yl sugar radical 2a was observed. The assignment of the sugar radical was supported by model reactions with SO₄^-radical as the radical inducing agent. Time-resolved e.s.r. measurements showed that the rate of decay of the 6-yl base radical at neutral pH is virtually the same as that of the strand break (sb) formation. These results prove that: (i) in agreement with an earlier proposal the C(2') mechanism contributes to sb formation of poly(U), and (ii) the decay of the 6-yl radical is the rate-determining step in the reaction sequence leading to strand breakage. The change in the e.s.r. spectra at pH 4 is due to an increase in the rate of sb formation with increasing proton concentration. (author)

[en] The thiyl radical derived from glutathione (GSH) is shown to decay rapidly in aqueous solution by intramolecular rearrangement reactions into the non-sulphur-centred radical 1. The reaction is induced by OH^- with a rate constant of 5 x 10⁹ dm³ mol^-1 and is also observable at near-neutral conditions (at physiological pH values around 7.5 the rate of formation of 1 amounts to ∼ 1 x 10³ s^-1). The activation enthalpy and entropy at pH 8.4 and 20^oC were found to be 26.7 kJ mol^-1 and -77 J mol^-1 K^-1, respectively. Radical 1 was unequivocally identified by EPR as the α-amino radical at the glutamyl residue of GSH. It is relatively long-lived with typical biomolecular decay rate constants of the order of (2-20) x 10⁶dm³ mol^-1 s^-1. At higher GSH concentrations the formation of 1 is retarded but not inhibited. All radicals, sulphur- as well as non-sulphur-centred ones are connected via equilibria, partly under the action of 'repair' processes of GSH. These repair processes, however, are slow (k << 1.4 x 10⁵ dm³ mol^-1 S ^-1). (Author)

[en] Published in summary form only

No abstract available

No abstract available

[en] The authors have performed a measurement from which they can infer the spin-spin correlations in a deeply inelastic collision. They have exploited the fact that the angular distribution of fragments produced in the fission of a binary partner in deeply inelastic collision is highly anisotropic, and that the anisotropy can be used as a tool to probe the alignment and magnitude of the spin of the fissioning system. Here the authors use a reaction in which both beam and projectile are fissile nuclei. In this manner the bulk of the deep inelastic yield results in final states in which both deeply inelastically scattered fragments have fissioned. The angular correlation of this four-body final state then contain information which may be related to the relative orientation of the spins of the two fragments before they fissioned. The authors have used the ²⁰⁸Pb + ²³⁸U reaction at E/sub cm/ = 945 MeV at the GSI Unilac to make their measurements. The four fragments were detected in an array of six avalanche detectors. One array consisted of two detectors on one side of the beam axis. In this manner the two correlated fragments arising from the fission of one nucleus could be observed, thus defining the mass transfer, energy loss and reaction plane for the deep inelastic collision. In addition, they sampled the mass distribution independently using three surface barrier detectors as time-of-flight telescopes

[en] Short communication. 2 refs

No abstract available

No abstract available