Two tautomers of guanyl radical have been identified experimentally.The most familiar tautomer 2(see Scheme 1)is produced by one-electron oxidation of guanine derivatives 1 using oxidants,such as SO_4~(·-),Br_2~(·-),or CO_3~(·-) whereas the tautomer 3 derives by protonation of the electron adduct of 8-bromoguanine derivatives 4.These two tautomers 2 and 3 showed different visible spectra around 610 nm,which allows the Arrhenius parameters to be measured for the tautomerization 3->2,that is,log(A/s~(-1))= 8.7 ± 0.4 and E_a = 23.0 ± 2.5 kJ mol~(-1).Kinetic isotope effect,phosphate catalysis,and theoretical work support a water-assisted tautomerization.
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