Guanine tautomerism revealed by UV-UV and IR-UV hole burning spectroscopy

The vibronic spectrum of laser desorbed and jet cooled guanine consists of bands from three different tautomers of guanine as revealed by UV-UV and IR -UV double resonance spectroscopy. 1-methylguanine, in which the Keto-Enol tautomerism is blocked, shows hole burning spectra from the 9H-and 7H-Keto form. A comparison of the vibronic pattern of the different tautomers demon strates that the vibronic spectrum built on the redmost guanine band at 32 870 cm(-1) (electronic origin 0) can be traced back to the 9H-Enol tautomer , while the spectra built on the origins at 0+404 cm(-1) and 0+1044 cm(-1) stem from the two Keto tautomers. The IR-UV double resonance spectra of the OH-and NH-stretch vibrations of the different tautomers support this assig nment. The UV and IR spectra can be partly assigned by comparison with ab i nitio calculated vibrational frequencies and with the help of deuteration e xperiments. (C) 2001 American Institute of Physics.