DNA photoionization

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).

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,[5] analytical,[6] structural[7] or therapeutic[8] information.

Since the end of the 20th century, numerous theoretical studies, performed using various types of quantum chemistry methods, focus on the computation of the lowest IP of nucleobases.

[9][10][11] Particular effort is being dedicated to evaluate environmental effects, such as the presence of water molecules,[12][13] base-pairing,[14] base stacking[15] or base-sequence.

[19][20][21][22] The IP values measured for nucleosides/nucleotides (8.1, 8.1, 7.6 and 7.3 eV for thymidine monophosphate, cytosine, adenosine and guanosine, respectively) match those computed for vertical ionization.

Photoionization quantum yields are determined for DNA in aqueous solution by means of the transient absorption spectroscopy using as excitation source nanosecond laser pulses.

To that effect, specific protocols regarding the purity of the nucleic acids and the ingredients of the aqueous solution as well as the intensity of the exciting laser pulses were established.

[34] The detailed examination of the structural factors affecting the low-energy photoionization, combined to quantum chemical calculations, indicates that it occurs via a complex mechanism.

This ionization mode started to be used already from the 1980sin order to characterize chemically the final DNA lesions (single and double strand breaks, 8-oxo-7,8-dihydroguanine,..), stemming from this process.