PNNL

Abstract

Anionic states of guanine, which is the only nucleic acid base the anions of which have not yet been studieded in either photoelectron spectroscopic (PES) or Rydberg electron transfer (RET) experiments, have been characterized for the four most stable tautomers of neutral guanine using a broad spectrum of electronic structure methods from density functional theory with the B3LYP exchange-correlation functional to coupled cluster method with single, double, and perturbative triple excitations. Both valence and dipole-bound anionic states were addressed. We identified some of the difficulties facing future PES or RET experiments on the anion of guanine. Even if guanine is successfully transferred to the gas phase without thermal decomposition, it is critical to have the canonical amiono-oxo, and both amino-hydroxy tautomers in the beam, not only the most stable, a non-canonical, amino-oxo tautomer, as the latter does not support a bound anionic state. We also suggested a scheme for enrichment of gas phase guanine with the canonical tautomer, which is not the most stable in the gas phase, but which is of main interest due to its biological relevance. Only the tautomers GN7H and GHN7H support vertically bound valence anionic states with the CCSD(T) value of vertical detachment energy of +0.21 and +0.39 eV, respectively. These anionic states are, however, adiabatically unbound. The vertical electronic stability of these valence anionic states is accompanied by serious “buckling” of the molecular skeleton, in particular in the region of the 5-member ring, where the excess electron is primarily localized. The G and GHN7H tautomers support dipole-bound states with the CCSD(T) values of adiabatic electron affinity of 65 and 36 meV, respectively. A contribution from higher-then-second-order correlation terms represents, respectively, 48 and 68% of the total vertical electron detachment energy determined at the CCSD(T) level.