Chemists had a tidy explanation for why DNA survives daily exposure to sunlight. One fast molecular step sheds the absorbed energy before any bonds can rearrange.
That picture was never wrong – just incomplete.
A new study used atomic-scale simulations to reveal not one escape route but a web of pathways, all activating within the first quadrillionths of a second after UV light strikes.
A molecule under fire
Sunlight carries a band of energy that our eyes never see. This is UV radiation, the invisible stuff that reddens skin and, over years, drives most skin cancers.
When UV radiation lands on DNA, the molecule soaks it up – which makes it dangerous.
Left unchecked, it can rearrange the bonds holding the code together, scrambling instructions and seeding mutations. Yet DNA rarely lets that happen.
Chemists call the trait photostability – taking in light and shedding it without harm.
A team at the University of Surrey, working with collaborators in France, set out to catch the process in the act.
The study was led by Juliana Gonçalves de Abrantes, a postgraduate researcher at Surrey.
Energy that must vanish
The reason DNA rarely breaks from sunlight comes down to speed.
A single base – one chemical letter of the genetic alphabet – can absorb a packet of UV energy and dump it almost at once.
Lab experiments have clocked lone bases shedding that energy in well under a trillionth of a second. The molecule slips back to its calm resting state before anything can go wrong.
That clock runs in femtoseconds. One femtosecond is a quadrillionth of a second, a span so short the energy drains away before the molecule’s bonds can rearrange and snap.
Two bases, many exits
DNA spells out its instructions with four bases that pair off in a fixed way.
The team zoomed in on two of them, guanine and cytosine, which clasp together through a few chemical bonds.
For years, the picture held that an excited pair had one main door back to safety. The new work tells a busier story.
In the simulations, absorbing UV light causes an electron to shift from guanine toward cytosine.
That lopsided, charged state is itself unstable, and rather than returning by a single route, the energy fans out through a network of competing reactions. The diversity is the heart of the finding.
Watching the atoms move
Seeing this in a living cell is impossible. It’s too fast and too small for any camera. The team rebuilt the base pairs inside a computer instead.
There, computer equations tracked the motion atom by atom, following where every electron and proton went after the flash.
The approach trades the mess of a real cell for a clean, controlled view.
To test whether layout mattered, they built two short stacks of these pairs in slightly different orders.
Both produced the same dominant escape route, a hint the behavior is built into the chemistry itself – not a quirk of one arrangement.
The proton’s quick jump
The leading escape came down to a single proton – a hydrogen nucleus – hopping from guanine to cytosine across the central bond.
An earlier paper had flagged this kind of move as part of DNA’s defence. In the simulations, that one jump did most of the work.
Across one version of the stack it handled more than four out of five escapes; in the other, roughly seven in ten. The same move, dominant either way.
A less common pathway moved the proton from a different region of guanine to an oxygen atom on cytosine.
This route accounted for roughly a quarter of escapes in one stack and about one-sixth in the other. Transfers in the opposite direction were rare.
Coupled but not locked
Watching it unfold, the team noticed something about how the electron and proton traveled.
In the simulations, the two moved together, each nudging the other, but not bolted into a single rigid step.
That loose link surprised the researchers most. Were the two locked together, DNA would have one escape mechanism and little else.
Because they move in concert without being chained, the pair finds many slightly different paths home.
The guanine-cytosine pairing has drawn attention for decades. Until this study, no one had mapped the full crowd of routes running at once.
Beyond a single cell
DNA protecting itself was already known. The new finding is more precise.
The protection runs through a web of competing, ultrafast reactions rather than one tidy mechanism, with electron and proton motions linked yet free to vary.
Dr. Marco Sacchi is an associate professor of physical and computational chemistry at Surrey and the study’s senior author.
“We can now see the incredibly fast molecular processes that safely drain away the energy before damage has a chance to spread,” said Dr. Sacchi.
Knowing the exact molecular steps gives scientists a clearer picture of how mutations arise, how cells age, and how radiation affects living tissue.
DNA guards itself not with one trick but with many, firing in the first quadrillionths of a second after light strikes.
The study is published in The Journal of Physical Chemistry Letters.
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