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

Abstract

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.

Electron photodetachment dissociation of DNA anions with covalently or noncovalently bound chromophores

“…A cylindrical lens (f ϭ 500 mm), located ϳ500 mm from the center of the trap, is used to reduce the ellipticity of the laser beam. The standard electrospray source was operated as described previously [7,8]. To perform laser irradiation for a given number of laser pulses, we add an MS n step with activation amplitude of 0%, during which a shutter located on the laser beam is open.…”

“…Altogether, photodetachment of the DNA complexes upon chromophore excitation can be interpreted by the following mechanism: (1) ligands with sufficiently long excited-state lifetime undergo resonant two-photon excitation to reach the level of the DNA excited states, then (2) the excited-state must be coupled to the DNA excited states for photodetachment to occur. Our experiments also pave the way towards photodissociation probes of biomolecule conformation in the gas-phase by Since then, some major advances must be mentioned in electron activation methods [3][4][5] and in infrared photodissociation [6].We recently started exploring the gas-phase reaction pathways of multiply charged DNA single strands and double strands upon UV irradiation around 260 nm [7,8]. To our surprise, we found out that, instead of fragmentation, electron detachment was the major reaction pathway with strands containing guanines.…”

“…We recently started exploring the gas-phase reaction pathways of multiply charged DNA single strands and double strands upon UV irradiation around 260 nm [7,8]. To our surprise, we found out that, instead of fragmentation, electron detachment was the major reaction pathway with strands containing guanines.…”

“…We will briefly summarize our current understanding of the photodetachment mechanism [8]. The electron binding energy in multiply charged DNA anions depends on the balance between electron binding energy of the different DNA constituents (the phosphates, the sugars, and the bases) and the Coulombic repulsion between like charges [10 -12].…”

“…The underlying questions relate to the efficiency of internal energy redistribution from specific electronically excited to vibrationally excited states, and to the efficiency of intramolecular energy redistribution in large biomolecules including noncovalently bound partners, when the energy is initially located in one well-defined chromophore [18 -24]. We show here that for some of the chromophores, photodetachment is the major photochemical pathway, like for the nucleic bases [7,8]. We also found some evidence of internal energy redistribution over the whole biomolecule ions.…”