Conduction-band-edge ionization thresholds of DNA components in aqueous solution.

Numerous investigations have focused on DNA damage induced by ionizing radiation; however, photoionization threshold energies of nucleic acid components in aqueous solution are not known. Herein, data from gas-phase photoelectron experiments have been combined with results from self-consistent field and post-self-consistent field molecular orbital calculations and with theoretical Gibbs free energies of hydration to describe aqueous ionization energies of 2'-deoxythymidine 5'-phosphate (5'-dTMP-) and 2'-deoxycytidine 5'-phosphate (5'-dCMP-). For the test molecules, indole and tryptophan, this approach yields aqueous ionization energies (4.46 and 4.58 eV, respectively) in agreement with experimental values (4. 35 and 4.45 eV). When uridine and 2'-deoxythymidine ionization energies are evaluated, the results agree with recent data from 193-nm laser measurements indicating that uridine ionization occurs via a one-photon event. For 5'-dCMP- and 5'-dTMP-, a comparison of aqueous ionization energies with gas-phase ionization potentials (IPs) indicates that hydration alters the relative energies of ionization events. In the gas phase, phosphate vertical IPs are approximately 1.3 eV smaller than base IPs. In aqueous solution, the base and phosphate ionization energies are more similar, and only differ by approximately 0.5 eV. For 5'-dCMP- and 5'-dTMP-, the increased favorableness of base ionization, which accompanies hydration, is consistent with experimental data indicating that, at 77 K in aqueous perchlorate glasses, the primary photoionization pathway involves base ionization followed by deprotonation.