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 demonstrates that the vibronic spectrum built on the redmost guanine band at 32 870 cm Ϫ1 ͑electronic origin 0͒ can be traced back to the 9H-Enol tautomer, while the spectra built on the origins at 0ϩ404 cm Ϫ1 and 0ϩ1044 cm Ϫ1 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 assignment. The UV and IR spectra can be partly assigned by comparison with ab initio calculated vibrational frequencies and with the help of deuteration experiments.

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Highlights d CLK kinases are thermo-sensors reactive to subtle bodytemperature changes d Body temperature globally controls splicing and gene expression through CLKs d CLK homologs are evolutionarily adapted to diverse body/ growth temperatures d Wide functionality of CLKs: from TSD in reptiles to circadian biology in mammals

In this paper we present detailed R2PI spectra with IR-UV and UV-UV double resonance measurements of the guanine dimer (GG) and its methylated derivatives. We show that there are two isomers of GG in the investigated wavelength range from 32565 to 33600 cm À1 . We were able to assign the two isomers to specific structures based on comparison of the intermolecular vibronic patterns of the dimers with and without methylation, on analysis of the IR spectra in the range of the OH and NH stretching vibrations and on comparison with ab initio calculated dimer stabilities and vibrational frequencies. In both structures both guanine moieties are in the keto tautomeric form, even though the enol tautomers are also present in the beam. One isomer exhibits nonsymmetric hydrogen bonding with HNHÁ Á ÁN, NHÁ Á ÁN and C=OÁ Á ÁHNH interactions (K9K7-2). The other isomer has a symmetrical hydrogen bond arrangement with C=OÁ Á ÁNH=NHÁ Á ÁO=C bonding (K9K7-1). The most stable guanine dimer forms C=OÁ Á ÁNH=NHÁ Á ÁO=C hydrogen bonds and has C 2h symmetry (K9K9-1). Due to its strong exciton splitting the allowed S 0 -S 2 transition is outside the investigated spectral range. We did not observe any keto-enol or enol-enol dimers in the investigated wavelength region. The calculations predict these dimers to be considerably less stable.

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