HOW A DNA LETTER SURVIVES A HIT OF ULTRAVIOLET

The bases of DNA absorb ultraviolet light strongly, yet they are remarkably stable: they get rid of the energy within moments, without emitting light. That ability protects genetic material. Thymine matters especially, the paper explains, because its excitation by ultraviolet light can start the most common kind of UV damage to DNA, known as the cyclobutane pyrimidine dimer. That damage can form within moments, or later, through longer-lived triplet states, a different arrangement of the electrons’ spins.

So where does thymine’s energy go? Studied in water, about 80% of excited molecules drop straight back to their resting state in less than 100 femtoseconds (millionths of a billionth of a second). Isolated in a gas, thymine does not take that shortcut: all of the energy passes into another excited state. Beyond 100 picoseconds, a long-lived “dark state” appears, often attributed to a triplet — largely on the basis of how long it lasts.

Five atoms as five reporters

Xiaojun Wang, Markus Gühr and colleagues at the DESY laboratory in Hamburg, with theorists in South Korea (Kyungpook National University) and France (Aix-Marseille University and CNRS), used a technique they call time-resolved multi-center X-ray photoelectron spectroscopy.

The principle: an ultraviolet flash (263 nanometres, about 80 femtoseconds long) excites thymine molecules evaporated from an oven at 180 °C. A short pulse of soft X-rays from the FLASH free-electron laser then knocks an inner electron out of a carbon atom. The energy of that electron depends on the atom’s electronic surroundings, so each of thymine’s five carbon atoms — four in the ring, one in its small methyl group — reports on what is happening around it. Delays ran from a few tens of femtoseconds to one nanosecond, nearly six orders of magnitude, and the high pulse rate gave clean data in only two hours. Quantum-chemistry calculations of the expected spectra for each state served as a key to read them.

The route, step by step

A fit with four successive states gives:

1. The first excited state lasts 92 ± 18 femtoseconds.
2. A second, “dark” singlet state traps the molecule for 6.3 ± 0.2 picoseconds. Calculations show why: the way back to the resting state from there lies about 1.6 electronvolts uphill.
3. Then comes the long-lived dark state — and here is the surprise. For the first 240 picoseconds or so, its signal is not a pure triplet, but a mixture of a further-relaxed singlet state and the triplet.
4. Only after that does the population become a pure triplet, which lasts beyond the one-nanosecond window. A barrier of 1.33 electronvolts keeps it from returning to the resting state.

The calculations also show that the jump from singlet to triplet can go directly to the lowest triplet, through a crossing point only about 0.5 electronvolts above the trapped singlet, without stopping at an intermediate triplet. The mixture explains a puzzle, the authors note: an earlier study measured a triplet yield of about 86%, and the missing share matches the singlet branch they identify.

A ring that breathes

In the first 200 femtoseconds, the signal oscillates at about 730 cm⁻¹ — a vibration with a period of 46 femtoseconds, the “breathing” of thymine’s ring. It survives the first jump between excited states, and simulations suggest the vibration stays in step for up to about 750 femtoseconds. Because the X-rays look at several atoms, the team could see that different atoms respond in opposite phase: when the ring expands, the signal from one carbon grows while two others weaken.

The methyl group’s two jobs

The most striking finding concerns the methyl group, a carbon carrying three hydrogens attached to the ring. During the first, singlet steps, it takes no part electronically: it is an “inertia spectator” whose mass slows a twisting motion of the ring that would otherwise let the molecule drop straight back to its resting state. Once the triplet forms, its role flips. Its electrons couple with the ring through an effect called hyperconjugation, its X-ray signal shifts by about 2 electronvolts, and it becomes the most sensitive marker that the triplet has appeared.

A gas, not a cell

These measurements were made on isolated thymine molecules in a gas, which behave differently from thymine in water or inside DNA. The exact nature of the intermediate singlet state remains unsettled; the authors propose X-ray measurements at the oxygen atoms over long delays to isolate it. They also state that AI was used to improve the readability and language of the manuscript.