Photodetachment and Photodissociation Mass Spectrometry of Biomolecule Ions

We recently explored the effects of irradiating DNA polyanions stored in a quadrupole ion trap (QIT) mass spectrometer with an optical parametric oscillator (OPO) laser between 250 nm and 285 nm. We studied DNA 6-mer to 20-mer single strands, and 12-base pair double strands. In all cases, laser irradiation causes electron detachment from the multiply charged DNA anions. Electron photo-detachment efficiency directly depends on the number of guanines in the strand, and maximum efficiency is observed between 260 and 275 nm. Collisional activation of the radical anions results in extensive fragmentation, which can be used to sequence the DNA strands. It has therefore important potential applications in analytical chemistry.
We also obtained preliminary results on laser irradiation of DNA coupled to other chromophores (covalently bound or noncovalently bound). Depending on the chromophore, three different behaviors are encountered: (1) the photon energy can be redistributed in the molecule by internal conversion, (2) electron photodetachment was observed for a few chromophores, and (3) specific photodissociation was observed for a porphyrin chromophore.
Here we will address the question of the mechanism of electron photodetachment. DNA multiply charged anions 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 and/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. 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 states dynamics of large DNA strands still remains largely unexplored, and photo-oxidation studies on trapped DNA multiply charged anions can help bridging the gap between gas-phase studies on isolated bases or base pairs and solution-phase studies on full DNA strands. Time-resolved measurements on photodetachment and photodissociation can be used to contribute elucidating the mechanisms at stake in these novel experiments.