Ion desorption from DNA components irradiated with 0.5 keV ultrasoft X-ray photons.

Positive ion desorption from thin films of DNA components, 2-deoxy-d-ribose, thymine, thymidine (dThd), and thymidine 5'-monophosphate (dTMP) was investigated in the oxygen K- shell edge excitation region using synchrotron ultrasoft X rays (538 eV). A large number of molecular fragments, H(+), CH(x)(+), C(2)H(x)(+), CO(+), CH(x)O(+), C(3)H(x)(+), C(2)H(x)O(+) and C(3)H(x)O(+) (x = 1, 2 and 3), were observed as desorbed ions from 2-deoxy-d-ribose. Some of these ions are related to simultaneous bond scission at particular C-C and C-O (or C-C) bonds in the furanose ring structure in the 2-deoxy-d-ribose molecule, indicating that the impact of photons on the oxygen atom and the impact of ejected secondary electrons (e.g. Auger electrons) cause an intense destruction of the furanose ring structure. In thymine thin films, H(+), CH(x)(+), CO(+), CH(x)O(+), C(2)H(x)N(+) and CH(x)NO(+) (x = 1, 2 and 3) fragments were observed. The yields of these ions were smaller than the yields from 2-deoxy-d-ribose. The desorption of CH(3)(+) from thymine might induce a molecular conversion from thymine to uracil. The mass patterns of dThd and dTMP, and especially that of dTMP, were similar to that of 2-deoxy-d-ribose, indicating that a number of ions were generated at the sugar site, even in the nucleotide molecule. It is therefore predicted that the sugar moiety is more fragile than the thymine base.