Alkali metal cation binding affinities of cytosine in the gas phase: revisited†
Binding of metal cations to the nucleobases can influence base pairing, base stacking and nucleobase tautomerism. Gas-phase condensation of dc discharge generated alkali metal cations and thermally vaporized cytosine (DC/FT) has been found to produce kinetically trapped excited tautomeric conformations of the M+(cytosine) complexes, which influences the threshold collision-induced dissociation (TCID) behavior. In order to elucidate the effects of the size of alkali metal cation on the strength of binding to the canonical form of cytosine, the binding affinities of Na+ and K+ to cytosine are re-examined here, and studies are extended to include Rb+ and Cs+ again using TCID techniques. The M+(cytosine) complexes are generated in an electrospray ionization source, which has been shown to produce ground-state tautomeric conformations of M+(cytosine). The energy-dependent cross sections are interpreted to yield bond dissociation energies (BDEs) using an analysis that includes consideration of unimolecular decay rates, the kinetic and internal energy distributions of the reactants, and multiple M+(cytosine)–Xe collisions. Revised BDEs for the Na+(cytosine) and K+(cytosine) complexes exceed those previously measured by 31.9 and 25.5 kJ mol−1, respectively, consistent with the hypothesis proposed by Yang and Rodgers that excited tautomeric conformations are accessed when the complexes are generated by DC/FT ionization. Experimentally measured BDEs are compared to theoretical values calculated at the B3LYP and MP2(full) levels of theory using the 6-311+G(2d,2p)_HW* and def2-TZVPPD basis sets. The B3LYP/def2-TZVPPD level of theory is found to provide the best agreement with the measured BDEs, suggesting that this level of theory can be employed to provide reliable energetics for similar metal–ligand systems.