The Guanine Tautomer Puzzle:  Quantum Chemical Investigation of Ground and Excited States

Abstract

Combined density functional and multireference configuration interaction methods have been employed to explore the ground and low-lying electronically excited states of the most important tautomeric and rotameric forms of guanine with the purpose of resolving the conflicting assignments of IR-UV bands found in the literature. The calculations predict sharp 1(pi-->pi*) origin transitions for the RN1 rotamer of the 7H-amino-hydroxy species and the RN7 rotamer of the 9H-amino-hydroxy species. The other 9H-amino-hydroxy rotamer, RN1, undergoes ultrafast nonradiative decay and is thus missing in the UV spectra. Because of its very small Franck-Condon factor and the presence of a conical intersection close by, it appears questionable, whether the 1(pi-->pi*) origin transition of 9H-amino-oxo-guanine can be observed experimentally. Vibrational overlap is more favorable for the 1(pi-->pi*) origin transition of the 7H- amino-oxo form, but also this tautomer is predicted to undergo ultrafast nonradiative decay of the 1(pi-->pi*) population. The good agreement of calculated IR frequencies of the amino-oxo species with recent IR spectra in He droplets and their mismatch with peaks observed in IR-UV spectra indicate that none of the bands stem from 7H- or 9H-amino-oxo guanine. Instead, our results suggest that these bands originate from 7H-imino-oxo guanine tautomers. In the excited-state dynamics of the biologically relevant 9H-amino-oxo tautomer, a diffuse charge transfer state is predicted to play a significant role.

Electronic Structure and Spectroscopy of Nucleic Acid Bases: Ionization Energies, Ionization-Induced Structural Changes, and Photoelectron Spectra

“…It should be noted that results of R2PI studies depend strongly on the corresponding excited state lifetime and relative absorption cross sections of different tautomeric forms. Marian [63] suggested that the bands corresponding to the oxo-forms in the IR-UV experiments [62] originate from the imino-oxo-isomers, whereas the amino-oxo forms do not appear in the spectra due to poor FCFs and a conical intersection located in the vicinity of Franck-Condon region of vertical π-π * excitation, which results in the short excited-state lifetime. Assignment of the IR spectra for guanine in He nanodroplets pointed to the presence of G7K, G9K, G9Es and G9Ea tautomers [64].…”

Gas-Phase IR Spectroscopy of Nucleobases

“…However, the B,C assignment to the keto form turned out to be incorrect. Surprisingly, these are imino forms, even though these tautomers are significantly higher in energy, and the three lowest energy tautomers are thus absent in the REMPI spectra (27,29). This absence is explained by short, sub-picosecond, excited state lifetimes, to be discussed below, which render nanosecond timescale REMPI blind for these tautomers.…”

“…Figure 1 shows IR frequencies for guanine as calculated by Marian (27). The enol forms are characterized by the OH stretch at about 3600 cm -1 , marked in red, and a red shift of the symmetric and antisymmetric NH 2 stretches, marked in blue and purple, respectively.…”

“…This elegant approach provides vibrational transition moment angles as additional data for comparison with ab initio theory. (27). Modes are color-coded as shown in the structures on the left.…”

“…Therefore these features can only occur in regions of the potential energy landscape that represent a deformation of the molecular frame from the ground state equilibrium geom-etry. For example, Figure 11 shows the geometry at the conical intersection for the keto tautomer of guanine, calculated by Marian (27). The CI involves strong out of plane bending of the C2 coordinates and different tautomeric arrangements lead to different potential surfaces along those coordinates.…”