Energy Transfer in Microhydrated Uracil, 5-Fluorouracil, and 5-Bromouracil.

Experiment and theory are combined to study the interaction of low energy electrons with microhydrated uracil and its halogenated analogues 5-fluorouracil and 5-bromouracil. We report electron ionization (EI) and electron attachment (EA) mass spectra for the uracils with different degrees of hydration. Both EI and EA lead to evaporation of water molecules. The number of evaporated molecules serves as a measure of the energy transferred to the solvent. Upon EI, the amount of energy transferred to neighboring water molecules is similar for all three studied species. On the other hand, the energy transferred upon EA rises significantly from uracil to 5-fluorouracil and 5-bromouracil. 5-Bromouracil is the only studied molecule that undergoes dissociative electron attachment after hydration at the studied energy of 1.2 eV. Theoretical modeling of the energetics for the electron attachment process allows for setting the energy transferred to the solvent on the absolute scale. We discuss the importance of this energy for the radiosensitization.