The
gas-phase acidity and proton affinity (PA) of 5-halouracils
(5-fluorouracil, 5-chlorouracil, 5-bromouracil, and 5-iodouracil)
have been examined using both theoretical and experimental methods.
This work represents a comprehensive study of the thermochemical properties
of these nucleobases. Other than 5-fluorouracil acidity, the intrinsic
acidity and PA of these halouracils have not been heretofore measured;
these new experimental data provide a benchmark for the computational
values. Furthermore, we examine these 5-halouracils in the context
of the enzyme thymine DNA glycosylase (TDG), which is an enzyme that
protects the genome by cleaving these substrates from DNA. Our gas-phase
results are compared and contrasted to TDG excision rates to afford
insights into the TDG mechanism.