The guanine tautomer puzzle: quantum chemical investigation of ground and excited states.
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.
Marian
Christel M
CM
Institute of Theoretical and Computational Chemistry, Heinrich-Heine-University Düsseldorf, 40225 Düsseldorf, Germany. Christel.Maian@uni-duesseldorf.de
D006147:Guanine / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D011789:Quantum Theory; D012680:Sensitivity and Specificity; D013055:Spectrophotometry, Infrared / Q000379:methods; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D014732:Vibration
D006147:Guanine