Skin deep damage: UV exposure

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  • Published: Nov 1, 2014
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Skin deep damage: UV exposure

Exposure

The initial UV excitation leading to photofragmentation CREDIT: Michael Kamrath

The most obvious effects of too much sun exposure are in the short term sunburn and in the long term wrinkled and rough skin. However, some problems that might arise are not just skin deep. Ultraviolet light damages DNA and cause proteins in the body to break down into smaller, sometimes harmful pieces that may also damage DNA, increasing the risk of skin cancer and cataracts. UV-IR double-resonance photofragment spectroscopy now reveals the details.

Aleksandra Zabuga, Michael Kamrath, Oleg Boyarkin and Thomas Rizzo of the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have pulled together evidence that shows some peptides degrade under ultraviolet light by first passing through an energetic triplet quantum state, a reactive arrangement that can cause greater damage than a straightforward fragmentation alone. They experimented with gas-phase peptides containing tyrosine or phenylalanine, both of which are light-absorbing amino acids found in proteins throughout our bodies. The UV-IR double-resonance photofragment spectroscopy of these systems gave them conformer-specific vibrational spectra.

Skin deep

Their findings show that the near-UV excitation of protonated tyrosine- or phenylalanine-containing peptides leads to intersystem crossing to produce the triplet species. The team outlines the details in The Journal of Chemical Physics and suggests that this improved understanding of the effects of UV on peptides might lead to the development of better UV-protection mechanisms.

Knowing that the peptides pass through a triplet state provides new insights into the subsequent behaviour of molecules initially damaged by UV, perhaps from sun exposure. "Triplet states are long-lived and can be involved in harmful chemical reactions," explains chemical physicist Zabuga. The term long-lived is, of course, relative, these species persist only for a matter of microseconds, or at best milliseconds. But, that is long enough for them to do further damage to other biomolecules. "During that time the triplet species may transfer their energy to nearby oxygen and produce highly reactive singlet oxygen or other free radicals," Zabuga says. "These radicals can in turn move around the cell and cause DNA damage that is much more dangerous than the fragmentation of peptides."

Chemical havoc

Scientists have long been interested in fragmentation reactions induced in biomolecules by ultraviolet radiation, indeed others have reported the existence of triplet states in the reaction chains studied. However, it was assumed that peptides were less likely to fragment in such an environment because they would interact with the other molecules that surround them and be deactivated through alternative mechanisms, which ought to mediate any putative damage. Moreover, pigment molecules, including melanin in our skin and kynurenine in our eyes help to reduce the amount of UV radiation that reaches cells and so should also reduce potential damage. "It is interesting to consider the fact that all of these protection mechanisms are external to the peptide. In other words, peptides do not seem to have very efficient means of protecting themselves," Zabuga says. The new UV-IR spectroscopic data show that peptides are perhaps not as protected as we might hope and can ultimately wreak chemical havoc through the formation of this triplet state.

The team now hopes to examine the impact of the local environment on light-induced fragmentation. There is, for instance, a hypothesis that local water molecules or other amino acids on the same peptide chain might interact with the newly formed triplet state and change the ultimate fragmentation pathway, which could have implication for understanding more of the details of the biochemical changes that can occur in our skin when we are exposed to sunlight.

Related Links

J Chem Phys, 2014, online: "Fragmentation mechanism of UV-excited peptides in the gas phase"

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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