Infrared cavity ringdown spectroscopy of jet-cooled nucleotide base clusters and water complexes

We present the first direct infrared absorption measurements of gas phase nucleotide base clusters and complexes with water. Spectra in the NH stretching region indicate the presence of several doubly H-bonded isomers of both thymine and uracil dieters, as well as both larger base clusters and uracil-water complexes, but no singly H-bonded species are observed. Three NH stretching bands are assigned to three different uracil-water dieter isomers. Bands due to larger complexes are also observed, but due to spectral congestion, assignment to specific clusters is tentative. The IR cavity ringdown laser absorption spectroscopy results show unusually large bonded OH stretch red shifts for uracil-water clusters, providing evidence of the extremely strong hydrogen bonds formed between these molecules predicted in ab initio calculations.</p

A DFT study of uracil and 5-bromouracil in nanodroplets

The canonical (keto) and rare (enol) tautomers of uracil and 5-bromouracil in clusters comprising 50 and 100 water molecules (nanodroplets) were studied using density functional theory. The geometries of the various complexes were optimized at two different levels of theory, BLYP/6-31G(d,p) and B3LYP/6-31G(d,p). Tautomerization energies were computed using the BLYP, B3LYP and M05-2X density functionals. The gas-phase tautomerization energies of uracil and 5-bromouracil are very similar, favoring the keto tautomer. However, in the hydrated phase, the tautomeric preference of 5-bromouracil is reversed. This result is obtained for all four sets of clusters (BLYP or B3LYP optimized, containing 50 or 100 water clusters) and at all levels of theory employed, and indicates that a bromine atom in the 5-position considerably increases the proportion of the hydroxyl group present in uracil.</p