Rotational Spectral Signatures of Four Tautomers of Guanine

摘要

Many efforts have been devoted to the identification of the preferred tautomeric forms of the nucleobase guanine. Theoretical calculations predict the existence of four low-energy tautomers with stabilities in the range 0-400 cm~(-1) (see Table 1), whereby the amino-oxo (keto) tautomers with a hydrogen atom at N7 or N9 are the most stable. The analysis of gas-phase UV and IR spectra on the basis of ab initio calculations has led to discordant identification of the tautomers. By using two-photon ionization (R2PI) and IR-UV double-resonance spectroscopy, de Vries and co-workers found spectral features that they assigned to the N9H keto, N7H keto, and N9H enol (cis or trans) forms. However, the most intense band assigned to the N9H enol was later attributed by Mons and co-workers to a higher-energy form of the N7H enol tautomer. Furthermore, they observed a fourth band, which they assigned to the N9H enol cis form (see Table 1). Choi and Miller studied guanine molecules embedded in He droplets and assigned the IR spectroscopic data to a mixture of the four more stable tautomeric forms: keto N7H, keto N9H, and enol N9H cis and trans.