Intermolecular proton transfer induced by excess electron attachment to adenine(formic acid)_n (n = 2, 3) hydrogen-bonded complexes

The propensity of the neutral complexes between both adenine and 9-methyladenine (A/MA) with formic acid (FA) in 1:2 and 1:3 stoichiometries to bind an excess electron was studied using photoelectron spectroscopy and quantum chemistry computational methods. Although an isolated canonical adenine does not support bound valence anions, solvation by one formic acid molecule stabilizes the excess electron on adenine. The adiabatic electron affinities of the A/MA(FA)_2,3 complexes span a range of 0.8-1.23 eV indicating that the anions of 1:2 and 1:3 stoichiometries are substantially more stable than the anionic A-FA dimer (EA = 0.67 eV), which we studied previously and an attachment of electron triggers double-BFPT, confirmed at the MPW1K level of theory, in all the considered systems. Hence, the simultaneous involvement of several molecules capable of forming cyclic hydrogen bonds with adenine remarkably increases its ability to bind an excess electron. The calculated vertical detachment energies for the most stable anions correspond well with those obtained using photoelectron spectroscopy. The possible biological significance of our findings is briefly discussed. © 2007 Elsevier B.V. All rights reserved.