Mechanism for ultrafast internal conversion of adenine

Abstract

The mechanism for the internal conversion of adenine was investigated by femtosecond pump–probe transient ionization time-of-flight mass spectrometry. In particular, we tried to determine which of the nπ* and πσ* states was responsible for the ultrafast decay of adenine by comparing the dynamics of adenine and its several derivatives with methyl or deuterium substitution. Adenine and all its derivatives studied here had more or less the same decay time of ∼1 ps at the excitation energy of 37 500 cm−1. These results provide compelling evidence that the nπ* state, rather than the πσ* state, plays the role of a doorway state for internal conversion via a conical intersection to the ground state.

Gas-Phase IR Spectroscopy of Nucleobases

“…For 4-aminopyrimidine, surface hopping calculations identified two conical intersections (5,33,(80)(81)(82)(83)(84)(85)(86)(87)(88)(89)(90)(91)(92): deformation at the C2 position leads to deactivation of the excited state with a lifetime, τ*, of 1 ps and deformation at the C5=C6 bond with a τ* of 400 fs. Immobilizing the latter with a 5 membered ring forms adenine with a conical intersection due to the C2 deformation, τ* of 1 ps and dynamics similar to that in guanine.…”

The Mid-IR Spectra of 9-Ethyl Guanine, Guanosine, and 2-Deoxyguanosine

“…A number of models suggest that the excited-state lifetimes in DNA bases are shortened by internal conversion to the ground state, via an intermediate state and mediated by conical intersections. [11][12][13][14][15][16][17][18] It has been proposed that this type of photochemistry is nature's defense mechanism against photochemical damage.…”

UV-Excitation from an Experimental Perspective: Frequency Resolved

“…These features can only occur in regions of the potential energy landscape that represent a deformation of the molecular frame from the ground state equilibrium geometry. The following case study of adenine derivatives serves as an example 18,[45][46][47][48][49][50][51][52][53][54][55][56][57] . For 4-aminopyrimidine (Figure 7 top left), surface hopping calculations identified two conical intersections 58 : deformation at the C2 position (red circle) leads to deactivation of the excited state with a lifetime, τ*, of 1 ps and deformation at the C5=C6 bond (blue rectangle) leads to deactivation with τ* of 400 fs.…”