<p>Before the development of an ozone
layer in the Archean atmosphere, the flux of UV radiation reaching Earth was
suggested to be several orders of magnitude higher than it is today. For the
emerging biomolecules, constant exposure to strong UV irradiation meant that
useful molecules had to be resistant to UV damage and harmful photochemical
reactions. From this prebiotic environment, the Watson–Crick structures of A·T
and G·C base pairs survived to encode genetic information—and the
photostability of these winning pairs in this specific arrangement is
astonishing. Upon UV irradiation, the photoexcited canonical base pairs undergo
proton-coupled electron transfer (PCET), followed by non-radiative decay, and
convert internally to the electronic ground state within picoseconds. But the
underlying reason why this process happens so efficiently has not been
explained. Here we show that efficient photodeactivation in isolated base pairs
are driven by antiaromaticity relief during PCET. According to computed nucleus
independent chemical shifts, the A·T and G·C base pairs are aromatic in the
electronic ground state, but the purines become highly antiaromatic in the
first <sup>1</sup>ππ* state, and PCET relieves this excited-state
antiaromaticity. We found especially pronounced antiaromaticity relief for the
major PCET pathway of isolated Watson–Crick A·T and G·C base pairs, when
compared to alternative proton transfer routes or to PCET reactions in
non-canonical pairs. Our findings suggest that excited-state deactivation of
isolated base pairs are tied to sudden changes in aromaticity and
antiaromaticity within the picoseconds that follow a strike of UV-light.</p>
How DNA Base Pairs Escape From the Excited-State: Antiaromaticity Relief in the Picoseconds