Hydroxyl radical-induced reactions in polyadenylic acid as studied by pulse radiolysis. Part I. Transformation reactions of two isomeric OH-adducts.
The absorption spectra of polyadenylic acid (polyA) radicals in N2O saturated aqueous solution have been measured as a function of time (up to 15 s) following an 0.4 microsecond electron pulse. The spectra and their changes were analysed by comparison with those from monomeric adenine derivatives (nucleosides and nucleotides) which had been studied by Steenken. The reaction of OH. radicals with the adenine moiety in polyA results in the formation of two hydroxyl adducts at the positions C-4 [polyA4OH.] and C-8 [polyA8OH.]. Each OH-adduct undergoes a unimolecular transformation reaction before any bimolecular or other unimolecular decay occurs. These reactions are characterized by different rate constants and pH dependencies. The polyA4OH. adduct undergoes a dehydration reaction to yield a neutral N6 centered radical (rate constant kdeh = 1.4 x 10(4)s-1 at pH 7.3). This reaction is strongly inhibited by H+. In comparison with the analogous reactions in adenosine phosphates, the kinetic pK value for its inhibition is two pH units higher. This shift is the result of the counter ion condensation or double-strand formation. The polyA8OH. adduct undergoes an imidazole ring opening reaction to yield an enol type of formamidopyrimidine radical with the resulting base damage (kr.o. = 3.5 x 10(4)s-1 at pH 7.3). This reaction in contrast is strongly catalysed by H+ and OH-, similar as for adenosine but different compared to the nucleotides.
Hankiewicz
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Max-Planck-Institut für Strahlenchemie, Mülheim an der Ruhr, Germany.
Bothe
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Schulte-Frohlinde
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D055598:Chemical Phenomena; D002627:Chemistry, Physical; D005609:Free Radicals; D006863:Hydrogen-Ion Concentration; D006878:Hydroxides / Q000737:chemistry*; D017665:Hydroxyl Radical; D007536:Isomerism; D011061:Poly A / Q000737:chemistry*; D011675:Pulse Radiolysis
D005609:Free Radicals; D006878:Hydroxides; D011061:Poly A; D017665:Hydroxyl Radical