Pairing of the nucleobase adenine studied by IR-UV double-resonance spectroscopy and ab initio calculations

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Abstract
The vibronic spectrum of the adenine–adenine (A–A) base pair was obtained by one color resonant two photon ionisation (R2PI) spectroscopy of adenine thermally evaporated in a free jet at conditions favorable for formation of small clusters. The onset of the spectrum is at 35040 cm−1 exhibiting a large red shift relative to the π–π* origin of 9H-adenine at 36105 cm−1. The IR-UV spectrum fits to cluster structures with HNHN/NHN hydrogen bonds based on the comparison with the monomer 9H-adenine IR spectrum and with ab initio calculated vibrational spectra of the double hydrogen bound A–A isomers. The vibrational spectrum of the most stable A–A structure with symmetrical NHN/NNH hydrogen bonds does not fit the experimental spectrum. The vibronic spectrum of A–A could also be detected on the A+H mass exhibiting hydrogen transfer and fragmentation upon laser excitation of A–A. Experiments with methylated adenine were performed to further support structure assignment. 7-methyl-adenine (7MA)–adenine shows a rich vibronic spectrum close to the A–A spectrum. Its IR-UV spectrum points to a structure with HNH(7MA)N3(A)/N1(7MA)HN9(A) interaction. 9-methyl-adenine (9MA)–adenine absorbs near to A–A and 7MA–A but exhibits less vibronic bands and very efficient H transfer upon laser excitation. Its IR-UV spectrum points to a stacked structure with one free NH2 group and one weakly interacting NH2 and N9H group. 9MA–9MA shows only broad vibronic structure and is probably symmetrically hydrogen bound via the NH2 groups.