Electron Photodetachment of DNA Polyanions: Photoelectron Spectroscopy and UV Action Spectroscopy

DNA polyanions trapped in a mass spectrometer undergo electron detachment following irradiation with UV light [1-3]. Electron photodetachment is a 1-photon process, and its efficiency depends on:
- The nature of the DNA bases: guanine-containing strands are the most prone to electron photodetachment, followed by adenine, cytosine, and finally thymine.
- The excitation wavelength: electron detachment is maximum around 260 nm, corresponding to base excitation.
- The charge of the polyanion: higher charge state ions undergo more efficient electron detachment because of the Coulombic repulsion.
Here we will discuss the electron photodetachment mechanism in the light of the most recent experimental results. Because the base-dependence of electron photodetachment efficiency is correlated with the base ionization potential and is maximum at wavelengths corresponding to the base absorption, we initially proposed that electron photodetachment might occur directly from the base, and that the photodetachment yield was correlated with the electron binding energy to the base [2]. Photoelectron spectroscopy experiments were performed on DNA multiply charged anions with varying base composition to probe how the electron binding energies changes with the base composition.
Finally, the electron detachment channel was used to perform UV spectroscopy experiments on large DNA polyanions trapped in the gas phase. Gas-phase UV spectra of DNA duplexes and G-quadruplexes containing up to 24 bases (> 7000 Da) will be presented.

[1] V. Gabelica, T. Tabarin, R. Antoine, F. Rosu, I. Compagnon, M. Broyer, E. De Pauw, and P. Dugourd, Anal. Chem. 78, 6564 (2006).
[2] V. Gabelica, F. Rosu, T. Tabarin, C. Kinet, R. Antoine, M. Broyer, E. De Pauw, and P. Dugourd, J. Am. Chem. Soc. 129, 4706 (2007).
[3] V. Gabelica, F. Rosu, E. De Pauw, R. Antoine, T. Tabarin, M. Broyer, and P. Dugourd, J. Am. Soc. Mass Spectrom. 18, 1990 (2007).