Model Reactions for the Degradation of DNA-4′ Radicals in Aqueous Solution. Fast Hydrolysis of α-Alkoxyalkyl Radicals with a Leaving Group in β-Position Followed by Radical Rearrangement and Elimination Reactions

Abstract

Abstract α-Alkoxyalkyl radicals with a leaving group L = Cl or OCOCH3 in β-position are produced by H-abstraction from the corresponding saturated substrates by ·OH, SO· 4 - or (CH3)3CO· radicals. From ESR spectroscopic observations it is concluded that in aqueous solution at pH 5 -9 the following fast hydrolysis reactions take place: The rate constants of these reactions and for the hydrolysis of CH3O-ĊH-CH2Cl are k ≥ 106 s-1, whereas the rate constant for CH3O-ĊH-CH2OCOCH3 was determined to be ≈ 2 × 103 s-1 at room temperature. The radicals with L = Cl cannot be scavenged by O2 which fact leads to a value of k ≥ 2 × 10-7 s-1. α-Alkoxyalkene radical cations are assumed as intermediates in the hydrolysis reactions. The radicals with L = OCOCH3 and the radical CH3O--ĊH-CH2Cl are observable in acetone solution ESR spectroscopically. In aqueous solution at pH below 3 proton catalyzed reactions are observed by ESR spectroscopy: Radicals resulting from H-abstraction at the CH3O-groups of the substrates or at the 5-positions of the cyclic ethers are also observed. The ESR parameters and the pH-ranges of existence of the above radicals are given. Support of the reported reactions comes from quantitative analysis of stable products such as H+, Cl- or CH3OH after 60Co-γ-irradiation of N2O saturated aqueous solutions of the substrates. The behaviour of the radicals is used as a model to describe a modified version of the degradation of DNA-4′ radicals in aqueous solution in the absence of oxygen.