Secondary electron-induced biomolecular fragmentation in fast heavy-ion irradiation of microdroplets of glycine solution.

The influence of secondary electrons on radiation damage of biomolecules in water was studied by fast heavy-ion irradiation of biomolecular solutions. Water microdroplets containing the amino acid glycine under vacuum were irradiated by fast carbon projectiles with energies of 0.8-8.0 MeV. A variety of fragments from the droplets were observed by time-of-flight secondary-ion mass spectrometry: methylene amine cation and formate anion originating from the cleavage of C-Cα bonds, cyanide anion generated by cleavage of multiple bonds, and protonated and deprotonated glycine. The dependence of the yield of each fragment on projectile energy was examined; different behavior was observed for positive and negative fragments. Considering that biomolecular fragmentation may be induced by secondary electrons ejected from the water molecules surrounding biomolecules, we calculated the cross section for ejection of secondary electrons from liquid water. We found that the formation of both positive and negative glycine fragment ions correlated with the predicted emission of secondary electrons at different projectile energies. The formation of [Gly-H]- fragments, typical for gas phase dissociative electron attachment to amino acids, is shown to be caused by electrons from the low-energy part of the secondary electron distribution.