Pairing of the nucleobases guanine and cytosine in the gas phase studied by IR-UV double-resonance spectroscopy and ab initio calculations

We present R2PI, IR-UV and UV-UV double resonance measurements of the guanine-cytosine (G-C) dimer formed in a supersonic jet. We show that there is only one isomer of G-C in the investigated wavelength range from 33200 to 34100 cm^-1. We assigned the observed G-C isomer to a specific structure, based on comparisons of the IR spectra of the G and C monomers with the G-C dimer in the range of the OH and NH stretching vibrations and ab initio-calculated vibrational frequencies and dimer stabilities. The cluster exhibits an HNH···O/NH···N/C=O ···HNH bonding similar to the Watson-Crick G-C base pair bonding but with C as the enol tautomer. We did not observe any keto-keto or enol-enol G-C dimers in the investigated wavelength region.