Influence of One-Electron Oxidation and One-Electron Reduction on the Tautomeric Preferences for Purine

Quantum-chemical calculations {DFT(B3LYP)/6-311+G**} were performed for all possible nine tautomers of neutral purine (P) and its charged radicals, cation (P+•) and anion (P-•), which may be formed in the presence of oxidizing or reducing agents. Comparison of the stabilities orders for the neutral and charged forms shows evidently that one-electron oxidation (P ? e ? P+•) has no important effect on the tautomeric preferences. The NH tautomer containing the moving proton at the N9 atom predominates for P+• (99.3%) as for P (99.8%). Dramatical changes take place for P-•. One-electron reduction (P + e ? P-•) favors the C atoms for the moving proton, particularly at the 6- (72.8%), 8- (26.2%) and 2-position (0.1%), and also the N3 atom (0.9%). Preference of the CH tautomers in the tautomeric mixture of P-• may partially explain synthesis and degradation reactions of purine derivatives.