Vacuum-Ultraviolet Photoionization Studies of the Microhydration of DNA Bases (Guanine, Cytosine, Adenine, and Thymine)

In this work, we report on a photoionization study of the microhydration of the four DNA bases. Gas-phase clusters of water with DNA bases [guanine (G), cytosine (C), adenine (A), and thymine (T)] are generated via thermal vaporization of the bases and expansion of the resultant vapor in a continuous supersonic jet expansion of water seeded in Ar. The resulting clusters are investigated by single-photon ionization with tunable vacuum-ultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Photoionization efficiency (PIE) curves are recorded for the DNA bases and the following water (W) clusters: G, GW n (n = 1−3); C, CW n (n = 1−3); A, AW n (n = 1,2); and T, TW n (n = 1−3). Appearance energies (AE) are derived from the onset of these PIE curves (all energies in eV): G (8.1 ± 0.1), GW (8.0 ± 0.1), GW2 (8.0 ± 0.1), and GW3 (8.0); C (8.65 ± 0.05), CW (8.45 ± 0.05), CW2 (8.4 ± 0.1), and CW3 (8.3 ± 0.1); A (8.30 ± 0.05), AW (8.20 ± 0.05), and AW2 (8.1 ± 0.1); T (8.90 ± 0.05); and TW (8.75 ± 0.05), TW2 (8.6 ± 0.1), and TW3 (8.6 ± 0.1). The AEs of the DNA bases decrease slightly with the addition of water molecules (up to three) but do not converge to values found for photoinduced electron removal from DNA bases in solution.