Base-Dependent Electron Photodetachment from Negatively Charged DNA Strands upon 260-nm Laser Irradiation

DNA multiply charged anions stored in a quadrupole ion trap undergo one-photon electron ejection (oxidation) when subjected to laser irradiation at 260 nm (4.77 eV). Electron photodetachment is likely a fast process, given that photodetachment is able to compete with internal conversion or radiative relaxation to the ground state. The DNA [6-mer]3- ions studied here show a marked sequence dependence of electron photodetachment yield. Remarkably, the photodetachment yield (dG6 > dA6 > dC6 > dT6) is inversely correlated with the base ionization potentials (G < A < C < T). Sequences with guanine runs show increased photodetachment yield as the number of guanine increases, in line with the fact that positive holes are the most stable in guanine runs. This correlation between photodetachment yield and the stability of the base radical may be explained by tunneling of the electron through the repulsive Coulomb barrier. Theoretical calculations on dinucleotide monophosphates show that the HOMO and HOMO-1 orbitals are localized on the bases. The wavelength dependence of electron detachment yield was studied for dG63-. Maximum electron photodetachment is observed in the wavelength range corresponding to base absorption (260-270 nm). This demonstrates the feasibility of gas-phase UV spectroscopy on large DNA anions. The calculations and the wavelength dependence suggest that the electron photodetachment is initiated at the bases and not at the phosphates. This also indicates that, although direct photodetachment could also occur, autodetachment from excited states, presumably corresponding to base excitation, is the dominant process at 260 nm. Excited-state dynamics of large DNA strands still remains largely unexplored, and photo-oxidation studies on trapped DNA multiply charged anions can help in bridging the gap between gas-phase studies on isolated bases or base pairs and solution-phase studies on full DNA strands.