The protonation of the 5-thymyl radical in single crystals of thymine derivatives: e.s.r. and INDO evidence

For the 5-thymyl radical, minor couplings additional to β-protons bonded to C6 and C7 were observed in single crystals of X-irradiated 1-methylthymine. Hyperfine coupling tensors are given of the N3 nitrogen (+3.5/0.0/0.0 G) and of the exchangeable proton (-4.1/-2.5/-0.9 G) bonded to it. In single crystals of thymine anhydrate thymine monohydrate and 5,6-dihydrothymine these additional couplings were unresolved, but were also present. INDO-calculations were performed to reproduce consistently both β-coupling constants and additional couplings from N3 and H(N3). Comparison of experimental and calculated values led to the conclusion that the 5-thymyl radical is protonated at 04 in all single crystals of thymine derivatives investigated. (author)