DNA photoionization

DNA photoionization is the phenomenon according to which ultraviolet radiation absorbed directly by a DNA system (mononucleotide, single or double strand, G-quadruplex…) induces the ejection of electrons, leaving electron holes on the nucleic acid.

The loss of an electron gives rise to a radical cation on the DNA. Radical cations are precursors to oxidative damage, ultimately leading to carcinogenic mutations and cell death. This aspect, detrimental to the health, is exploited in the germicidal equipments using far-UVC lamps. The electric charges photogenarated in DNA could potentially find applications in optoelectronic devices.

Two properties are crucial regarding photoionization. On the one hand, the ionization energy (also called ionization potential, IP), refers to the energy necessary to remove one electron from a molecule; the lowest IP, corresponding to the ejection of a first electron, is the most biologically relevant factor. On the other hand, the photoionization quantum yield Φ, that is the number of electrons that are ejected over the number of absorbed photons; Φ depends on the irradiation wavelength.

The mechanism underlying DNA ionization depends on the number of photons that provoke the ejection of one electron (one-photon or multiphoton, induced by intense laser pulses). And, in the case of one-photon process, it differs according to the photon energy (high-energy or low-energy). While one- and two-photon ionization in condensed phase (aqueous solutions, cells…) is mainly studied in respect with the UV-induced oxidative damage, multiphoton ionization in the gas phase, often coupled to mass spectroscopy, is used in various techniques in order to obtain broader spectroscopic, analytical, structural or therapeutic information.