For the first time, American and German scientists have observed DNA being damaged by UV light. But what is really amazing is that it only takes one picosecond to DNA to get 'sunburned.' Please note that a picosecond is just one millionth of one millionth of a second (or 10-12 seconds). It was previously thought that DNA only could be damaged by much longer exposures to UV energy. "Now it seems more likely that short-lived states cause the most common chemical damage to DNA," says one of the researchers. As DNA damages can create mutations that lead to diseases such as skin cancer, these experiments will certainly influence future research about this disease. Anyway, be careful if you want to get a good suntan: protect yourself!
This research has been partially conducted by Bern Kohler, an associate professor of chemistry at Ohio State University and his colleagues in his research group.
Before going further, here is an illustration showing how DNA can be damaged by ultraviolet (UV) light. "An undamaged stack of thymine bases is colored green in the DNA double helix on the left. In the double helix on the right, UV light has caused two new chemical bonds to form, fusing two thymine bases together." (Graphic by Bern Kohler, courtesy of Ohio State University) Here is a link to a larger version of this image.
Here are some general comments about this study.
UV light excites the DNA molecule by adding energy, said Bern Kohler. Some excited energy states last a long time, and others a short time. The energy often decays away harmlessly, but occasionally it triggers a chemical reaction that alters the DNA's molecular structure. Previously, scientists believed that the longer a DNA molecule was excited by UV energy, the greater the chance that it would sustain damage. So long-lived excited states were thought to be more dangerous than short-lived ones. But this study shows that the most common UV damage is caused by a very short-lived excited state.
What kind of technique was used to see DNA get 'sunburned' in about a picosecond?
For this study, the chemists used a technique called transient absorption to observe the DNA damage. Transient absorption is based on the idea that molecules absorb light at specific wavelengths, and it allows researchers to study events that happen in less than a picosecond. They took specially designed strands of DNA -- ones made solely of thymine bases, in order to boost the chance of observing a reaction between adjacent thymines -- and exposed them to UV light. Then they timed the reactions that caused the new thymine bonds to form.
For your information, this research work has been published by Science under the name "Thymine dimerization in DNA is an ultrafast photoreaction" (Volume 315, Number 5812, Pages 625-629, February 2, 2007). Here is a link to the abstract. And if you want to know what is thymine, here is a link to a Wikipedia page about it.
Sources: Ohio State University news release, via EurekAlert!, February 1, 2007; and various other websites
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