Among all the DNA components, extremely redox-active guanine (G) and adenine (A) bases are subject to facile loss of an electron and form cation radicals (G+" and A+') when exposed to irradiation or radical oxidants. The subsequent deprotonation of G+' and A+' can invoke DNA damage or interrupt hole transfer in DNA. However, compared with intensive reports for G+, studies on the deprotonation of A+ are still limited at present. Herein, we investigate the deprotonation behavior of A+. by time-resolved laser flash photolysis. The deprotonation product of A(N6-H)' is observed and the deprotonation rate constant, (2.0±0.1)×10 7 s-1, is obtained at room temperature. Further, the deprotonation rate con- stants of A+. are measured at temperatures varying from 280 K to 300 K, from which the activation energy for the N6-H deprotonation is determined to be (17.1±1.0) kJ/mol by Arrhenius equation. In addition, by incorporating the aqueous solvent effect, we perform density functional theory calculations for A+ deprotonation in free base and in duplex DNA. Together with experimental results, the deprotonation mechanisms of A+ in free base and in duplex DNA are revealed, which are of fundamental importance for understanding the oxidative DNA damage and designing DNA-based electrochemical devices.
Superexcited states of NO molecule and their neutral dissociation processes have been studied both experimentally and theoretically. Neutral excited N^* and O^* atoms are detected by fluorescence spectroscopy for the NO molecule upon interaction with 800 nm intense laser radiation of duration 60 fs and intensity 0.2 PW/cm^2. Intense laser pulse causes neutral dissociation of superexcited NO molecule by way of multiphoton excitation, which is equivalent to single photon excitation in the extreme-ultraviolet region by synchrotron radiation. Potential energy curves (PECs) are also built using the calculated superexcited state of NO^+. In light of the PECs, direct dissociation and pre-dissociation mechanisms are proposed respectively for the neutral dissociation leading to excited fragments N^* and O^*.