Hydrogen bonding and structure of uracil-water and thymine-water complexes

The complexes uracil-water and thymine-water have been investigated using laser ablation molecular-beam Fourier transform microwave (LA-MBFTMW) spectroscopy. Only one conformer of each complex has been observed. The spectra of the parent and several isotopically enriched species with N-15 at uracil moiety and D or O-18 at water have been measured. For uracil-water the spectra are characterized by the hyperfine structure of two N-14 nuclei. For thymine-water only the parent and O-18-water enriched species have been observed. Their spectra are characterized by the hyperfine structure of two N-14 nuclei with an additional doubling due to the A and E states of the torsion of the methyl group. The barrier to internal rotation of the methyl group has been determined from the analysis of the spectrum. Investigation of the structure of the adducts from the rotational constants of the different isotopologues shows that the observed conformers correspond to the most stable forms in which water closes a cycle with the nucleic acid bases forming N-H-NB center dot center dot center dot O-w and H-W center dot center dot center dot O=C-2 hydrogen bonds.