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In lumazines, deazalumazines and pyrimidines, there are extremely low-energy "rare" tautomers (<2.3 kcal/mol), this fact perfectly explains the observed mobility of usually "non-labile" protons of methyl groups in such systems. In general, the dependence of tautomeric preference on structure correlates well with experimental findings. Thus, the activity of alkyl groups during biological transformations may be due to the presence of the corresponding thermodynamically stable tautomers., (Copyright © 2021 Elsevier Inc. All rights reserved.)