Tautomerism of pyrimidines and purines in the gas phase and in low‐temperature matrices, and some biological implications

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Abstract
Infrared spectra in the NH, OH, and CO stretching regions are reviewed for various methylated and halogenated 2(4)‐oxopyrimidines and uracils, as well as other nucleic acid derivatives, in the vapor phase and in low‐temperature matrices. The 2‐oxopyrimidines are predominantly in the enol form both in the vapor phase and in low‐temperature matrices. By contrast, the 4‐oxopyrimidines exhibit comparable populations of the keto and enol forms, with KT ≈ 1–2, and a difference in chemical binding energy between the two forms in the gaseous phase of the order of 1–2 kcal/mol. The observed tautomeric equilibria in the gas phase point to the need for a drastic revision of interpretations of theoretical methods, and simultaneously provide the appropriate quantitative data necessary for testing the results of quantum mechanical calculations. In sharp contrast to other heterocyclic systems, several of the bases found in natural nucleic acids were found to exist predominantly in the keto or amino forms, as in the solution phase. In particular, uracils exist predominantly in the keto form. This has made possible, for this class of compounds, to evaluate the heats of sublimation and vaporization, and to relate these data to hydrogen bonding and stacking interactions in the condensed phases. Examples are presented of base analogs which do exhibit appreciable tautomerism in solution. Some biological implications of the foregoing are presented in relation to the types of heterocyclic bases found in natural nucleic acids, and to concepts of spontaneous and induced mutagenesis.