Contrasts between uracil and thymine in reaction with hydrogen isotopes in water

Using the level crossing resonance technique, it has been possible to identify the measure the relative yields of free radicals formed by addition of muonium across the C double-bond C group at carbon atoms 5 and 6 in uracil, thymine, and 6-methyluracil. The C(5) and C(6) adduct ratio is about 1 in uracil but reduces ∼ 0.15 in thymine due to presence of -CH3 on C(5). This considerable steric effect shown by muonium contrasts the situation with ordinary H atoms and underlines the importance of zero-point vibrational motion in determining stereospecific directional effects, even for efficient reactions. Proton hyperfine coupling constants of the free radicals formed from addition across the C(5)double-bond C(6) double bonds in uracil have been determined. For thiouracil, no addition was evident at C(5)double-bond C(6), suggesting that attack by muonium occurs at the C of C double-bond S to produce a thiyl radical