Intrinsic acidity and redox properties of the adenine radical cation

Abstract

The intrinsic chemical properties of the gaseous adenine radical cation were examined by using dual cell Fourier transform ion cyclotron resonance mass spectrometry. The adiabatic recombination energy of the radical cation (ionization energy of neutral adenine) was found by bracketing experiments to be 8.55 ± 0.1 eV (at 298 K; earlier literature values range from 8.3 to 8.9 eV). Based on this value, the heat of formation (Δ H f 298) of the adenine radical cation is estimated to be 246 ± 3 kcal/mol. The acidity (Δ H acid 298) of the adenine radical cation was bracketed to be 221 ± 2 kcal/mol. These thermochemical values suggest that the adenine radical cation reacts with neutral guanine by electron abstraction or proton transfer, with neutral cytosine by proton transfer, and via neither pathway with neutral thymine, molecular water or a sugar moiety of DNA (modeled by tetrahydrofuran). Experimental examination of the gas-phase reactivity of the adenine radical cation revealed a slow deuterium atom abstraction from perdeuterated tetrahydrofuran. Hence, in the absence of a nearby guanine or cytosine, the adenine radical cation may be able to abstract a hydrogen atom from a sugar moiety of DNA.