Adenine pathways: UV protection racket

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Ezine

  • Published: Sep 1, 2011
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Adenine pathways: UV protection racket

Building pathways

A collaboration between teams in Germany and the US has shown that one of the building blocks of the nucleic acids DNA and RNA, adenine, has a previously unrecognised variable range of ionization energies along its reaction pathways. The finding could improve our understanding of experimental data regarding how adenine survives exposure to ultraviolet light, showing that the processes might be more complicated than previously thought. However, it also has potentially far-reaching implications for spectroscopic measurements of other heterocyclic compounds.

Before the ozone layer formed, the atmosphere of the early Earth provided little protection from solar ultraviolet rays. It is incredible that prebiotic molecules, bombarded by high-energy photons survived the onslaught to follow the inexorable path to the complex self-replicating entities that ultimately gave rise to life. Obviously, life on Earth is a fact, it is those prebiotic molecules that were photostable that thrived as the building blocks of life, among them the nucleic acid bases adenine, cytosine, guanine, thymine and uracil.

Now, Mario Barbatti of the Max-Planck-Institut für Kohlenforschung, in Mülheim, Germany and Susanne Ullrich of the Department of Physics and Astronomy, University of Georgia, Athens, Georgia, have published research that shows that one of this group do compounds, adenine, displays a rather unexpected variation in its range of ionization energies. The finding could help explain how the molecule survives exposure to ultraviolet light but also has implications for how the spectra of other heterocyclic compounds are interpreted.

Quantum protection

The team has carried out quantum-chemical calculations to create the first baseline on how time-resolved spectroscopic techniques that exploit photoionization can be most reliably used to study this class of molecules.

"Photoprotection relies on the conversion of potentially harmful ultraviolet radiation into heat and has to operate on ultrafast time scales to compete over pathways that lead to the destruction of the biomolecule," explains Ullrich, who is an assistant professor in physics at Georgia. "Disentangling these pathways and their time scales is challenging and requires a very close collaboration between experimentalists and theorists." Because of the significance of nucleic acid bases as the genetic coding material, research into the photochemistry and the photophysics of nucleic acid bases has been the focus of much theoretical and experimental work. Ulrich adds that importantly, "Photostable organic molecules participated in the complex molecular evolution that led to the formation of life."

The team used time-resolved photoionization with femtosecond resolution to unravel the mechanisms that protect adenine against ultraviolet damage. For the spectroscopic measurements, they used a femtosecond laser and custom-built photoelectron and photoion spectrometers with supersonic jet expansion of adenine. The work by Ulrich and Barbatti might explain inconsistencies observed in such photoionization and photoelectron spectroscopic studies and calculations on adenine.

Until now, theoretical calculations have departed from experimental results for adenine's ionization and in particular in defining the relaxation of the molecule following ultraviolet excitation. Previously, little was known about gaseous adenine's ionization potentials along the main reaction pathways of excitation. This "knowledge gap," is now filled, Barbatti and Ullrich say.

Surprising energy

"To our surprise, we found there were significant variations in the ionization energy between two different regions on this pathway," says Barbatti. "Due to the general character of the three pathways we studied, we believe the ionization potentials computed along them can be used as a general guide for helping with setup and analysis for further experiments, not only with adenine but other related compounds."


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

 A collaboration between teams in Germany and the US has shown that one of the building blocks of the nucleic acids DNA and RNA, adenine, has a previously unrecognised variable range of ionization energies along its reaction pathways. The finding could improve our understanding of experimental data regarding how adenine survives exposure to ultraviolet light

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