An EPR study of photoionised thymine and its derivatives at 77 K

Abstract

The effect of high-energy ionising radiation on thymine and a wide range of its derivatives has been widely studied. Liquid-phase studies on aqueous solutions using pulse radiolysis and UV-photoionisation methods have been reported. Our aim was to use low temperature matrices in combination with laser photolysis and EPR spectroscopy in order to learn more about the double quantum event leading to UV-photoionisation. For thymine, electron-loss results in the normal π-radical cation but this rapidly undergoes proton loss, probably from the N¹–H group. The EPR spectrum is well defined and is characteristic of this radical. This species was detected in perchlorate matrices and there was no evidence of electron capture by thymine. However, in pure aqueous systems, TH˙ radicals, possibly formed by electron capture followed by protonation, were detected.

In contrast, for N¹ substituted species such as TMP, the π-radical cations were not detected by EPR spectroscopy. Instead, there were comparable yields of TCH₂˙ radicals, in which one of the methyl protons has been lost. These give rise to a characteristic quartet splitting from hyperfine coupling to the two –CH₂˙ protons, together with the C⁶–H proton which gives about the same splitting. Only in the case of alkaline perchlorate glasses, when the N³–H proton is removed, was the electron lost from the π system to give the neutral π-radical.

When frozen aqueous systems containing TCH₂˙ radicals were annealed the quartet signals were lost, and quintets grew in. Similar quintets have been previously assigned to TOH˙ radicals formed by the addition of water to TCH₂˙. However, use of D₂O did not modify the signals, so we propose that this species is due to a dimer radical and/or a cyclic radical. The former is formed by the addition of the TCH₂˙ unit to C⁶ of another molecule and the latter arises from intramolecular hydrogen-atom transfer from the C⁵′ hydrogen followed by cyclisation.

Use of 5-ethyl-2′-deoxyuridine, systems that gave the TCH₂· radicals for thymine derivatives, gave UĊHCH₃ radicals. Reasons for the formation of these species by biphotonic photoionisation are discussed. It is difficult to understand why the primary π-radical cations of the N¹-substituted thymine derivatives should prefer to lose a proton from the carbon rather than the >N₃–H groups, since the latter, but not the former, has an adjacent proton-acceptor. An alternative is considered, in which an excited state of the primary electron-loss centre is initially formed. This state has the local structure > CO˙⁺, the SOMO being the in-plane n-orbital on oxygen. By analogy with, for example, ester radical cations, it is suggested that hydrogen-atom transfer from the adjacent methyl group occurs more rapidly than the switch from this state to the π-ground state, thereby giving the required TCH₂˙ centre, with a proton on the adjacent carbonyl group.