Elucidation of Electron Ionization Induced Fragmentations of Adenine by Semiempirical and Density Functional Molecular Dynamics

The
gas phase fragmentation pathways of the nucleobase adenine
upon 70 eV electron ionization are investigated by means of a combined
stochastic and first-principles based molecular dynamics approach.
We employ no preconceived fragmentation channels in our calculations,
which simulate standard electron ionization mass spectrometry (EI-MS)
conditions. The reactions observed compare well to a wealth of experimental
and theoretical data available for this important nucleic acid building
block. All significant peaks in the experimental mass spectrum of
adenine are reproduced. Additionally, the fragment ion connectivities
obtained from our simulations at least partially concur with results
from previous experimental studies on selectively isotope labeled
adenines. Moreover, we are able to assign noncyclic structures that
are entropically favored and have not been proposed in nondynamic
quantum chemical studies before to the decomposition products, which
result automatically from our molecular dynamics procedure. From simulations
under various conditions it is evident that most of the fragmentation
reactions even at low internal excess energy (<10 eV) occur very
fast within a few picoseconds.