Structures of Hydrated Li~+-Thymine and Li~+-Uracil Complexes by IRMPD Spectroscopy in the N-H/O-H Stretching Region

摘要

The interaction of lithium ions with two pyrimidine nucleobases, thymine and uracil, as well as the solvation of various complexes by one and two water molecules, has been studied in the gas phase. IRMPD spectra are reported for each of B—Li~+—(H2O)_n (n = 1—2) and B2—Li—(H2O)_m (m = 0—1) for B = thymine, uracil over the 2500—4000 cm~(-1) region. Calculations were performed using the B3LYP density functional in conjunction with the 6-31+G(d,p) basis set to model the vibrational spectra as well as MP2/6-311++G(2d,p) theory to model the thermochemistry of potential structures. Experimental and theoretical results were used in combination to determine structures of each complex, which are reported here. The lithium cation in all complexes was found to bond to the 04 oxygen in both thymine and uracil, and the first two water molecules of solvation were found to bond to Li~+. The experimental spectra obtained for BLi~+(H2O)_n (n= 1—2) and B2Li~+ for thymine and uracil clearly resemble one another, suggesting similar structural features in terms of bonding between the base and Li~+, as well as for solvation. This was confirmed through theoretical work. The addition of water to the lithium ion-bound DNA base dimers has been shown to induce a significant change in structure of the dimer to a hydrogen-bonded system similar to base pairing in the Watson—Crick model of DNA.