Photoion mass spectrometry of adenine, thymine and uracil in the 6-22Â eV photon energy range

Using synchrotron radiation as excitation source in the 6-22 eV photon energy region, a photoionization mass spectrometry study of three nucleic acid bases, adenine, thymine and uracil, revealed VUV-induced degradation pathways of these important biological molecules. The fragmentation patterns, ionization energies and ion appearance energies (AE) are reported, many for the first time, and are compared with results of electron impact and other studies. AE values enabled heats of formation of parent and some fragment ions to be revised or determined for the first time. Thermochemical data, coupled with the observed AEs, were also useful in clarifying dissociative photoionization pathways. The main neutral loss species are HCN for adenine, HNCO and CO for thymine and uracil, but many subsequent and other fragmentation pathways, including some not suggested previously, are observed and discussed. The hyperconjugation properties of the methyl group make CO loss easier in thymine than in uracil. The astrophysically important fragment ion HCNH+ is shown to be formed by several fragmentation pathways in all three nucleobases. The relative importance of competitive fragmentation processes was determined in some cases. Some astrophysical implications concerning the prospects for observation and survival of these nucleic acid bases in the interstellar medium and in meteorites are briefly discussed.