UV protection racket: X-ray revelations

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  • Published: Jul 1, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: UV protection racket: X-ray revelations

Auger augurs well for research

Ground state Auger spectrum. Comparison of experimental and theoretical ground-state Auger spectra for thymine. The green and yellow curves are data taken at the Advanced Light Source (ALS) synchrotron and the LCLS free-electron laser, respectively, and are offset in intensity from the theory.  Credit: McFarland et al/Nature Commun

The molecular building blocks of DNA absorb ultraviolet light strongly and so one might imagine that sunlight damages them all beyond repair, yet it does not. Now, scientists have used ultrafast X-ray Auger probing of photoexcited molecular dynamics to help them to understand the critical "relaxation response" that apparently protects DNA and the genetic information it encodes, from ultraviolet damage.

Experiments at the US Department of Energy’s SLAC National Accelerator Laboratory have focused on thymine, one of the four DNA bases, in this context. Writing in the journal Nature Communications, Stanford's Philip Bucksbaum and colleagues explain how they repeatedly exposed thymine to short bursts of ultraviolet light and used a powerful X-ray laser to observe the molecular response. Basically, they found that a single chemical bond absorbs the energy, undergoes a stretch and quickly relaxes back down within 200 femtoseconds. This sets off a wave of vibrations that then dissipate the destructive ultraviolet energy harmlessly as heat, this secondary process lasting for several nanoseconds.

Sun damage

Current knowledge regarding sun-induced damage to DNA is the focus of public health campaigns that warn people of the dangers of sunburn and tanning salons in terms of their increasing a person's risk of developing skin cancer. Intriguingly though, earlier research had demonstrated that thymine among the four DNA bases is resistant to damage from UV from sunlight. A single, plausible mechanism remained out of reach, however. Shape-shifting was assumed to be the underlying process, but the details remained elusive. The use of the Linac Coherent Light Source (LCLS), which offers ultrashort X-ray laser pulses for research, could illuminate the changes taking place during UV irradiation of thymine at the level of individual atoms on the femtosecond timescale.

“As soon as the thymine swallows the light, the energy is funnelled as quickly as possible into heat, rather than into making or breaking chemical bonds,” explains team member Markus Guehr. “It’s like a system of balls connected by springs; when you elongate that one bond between two atoms and let it loose, the whole molecule starts to tremble.”

The X-rays measure the relaxation response indirectly by ionising the atoms down to the innermost electrons and the Auger decay in which other electrons are ejected is measured. This allowed the researchers to pinpoint the rather rapid changes that are taking place in a single carbon-oxygen bond in the thymine molecule.

Bouncing bonds

“This is the first time we’ve been able to distinguish between two fundamental responses in the molecule - movements of the atomic nuclei and changes in the distribution of electrons - and time them within a few femtoseconds,” adds Brian McFarland formerly of SLAC and now at Los Alamos National Laboratory in New Mexico.

Bucksbaum suggests that while demonstrating this protective effect is fundamentally important to biochemistry, the results goes way beyond DNA chemistry. “The new tool the team developed for this study provides a new window on the motion of electrons that control all of chemistry,” he explains. “We think this will enhance the value and impact of X-ray free-electron lasers for important problems in biology, chemistry and physics.” As such, additional experiments are planned to explore the protective relaxation response in thymine and also to extend the new method, of time-resolved Auger spectroscopy.

In addition to scientists at the Stanford PULSE Institute, researchers from LCLS, Stanford, the University of Perugia in Italy, Lawrence Berkeley National Laboratory, the University of Connecticut, Western Michigan University, the University of Gothenburg in Sweden, and UNIST in South Korea were involved in the work.

Related Links

Nature Commun, 2014, 5, 4235: "Ultrafast X-ray Auger probing of photoexcited molecular dynamics"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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