Decay pathways of thymine and methyl-substituted uracil and thymine in the gas phase

We report observations of a dark state in the decay pathways of thymine, 1,3-dimethyl thymine, 1,3-dimethyl uracil, and I-methyl uracil in the gas phase. After initial excitation by a nanosecond laser, the excited molecules failed to return to the ground state but rather were trapped in a dark state for tens to hundreds of nanoseconds. This result contradicts those reported in water solutions. We therefore propose that the photochemistry of these pyrimidine bases is different in the gas phase from that in the liquid phase and that the dark state is effectively quenched in water solutions. Although we do not have a quantitative measure of the yield of this dark state, the fact that further ionization from this dark state has a high yield in the deep UV makes this pathway important in the chemistry of nucleic acid bases. This result further implies that the photostability of our genetic code,may not be an inherent property of the bases themselves.