Photofragmentation of guanine, cytosine, leucine and methionine

The photofragmentation spectra of guanine, cytosine, leucine and methionine were measured with noble gas resonance radiation at energies from 8.43 to 21.2 eV, and the spectra are related to the published valence band photoemission spectra. As expected, lower photon energies lead to “softer” ionization and reduced fragmentation. For the nucleobases guanine and cytosine, photoionization below 16.67 eV leads predominantly to the parent ion. The fragmentation pattern of leucine is similar to that of published spectra of other amino acids with aliphatic side chains, and whose outer orbitals are similar, with oxygen and nitrogen lone pair character. Methionine behaves very differently because the outer orbital has sulphur lone pair character, and reduced fragmentation is observed.