Non-standard base pairing and stacked structures in methyl xanthine clusters.

We present resonant two-photon ionization and IR-UV double resonance spectra of methylated xanthine derivatives including 7-methylxanthine dimer and theobromine dimer seeded in a supersonic jet by laser desorption. For 7-methylxanthine, theophylline and theobromine monomer we assign the lowest energy tautomer based on comparison with IR-UV double resonance spectra and calculated IR frequencies. For the 7-methylxanthine dimer, we observe hydrogen bonding on the N3H position suggesting 3 possible combinations, one that is reverse Watson-Crick type and two that are reverse Hoogsteen type. For the theobromine dimer, we observe a stacked structure. For trimethylxanthine dimers we infer a stacked structure as well.