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ULTRAFAST LASER PULSES REVEAL DNA'S NATURAL "SUNSCREEN" COLUMBUS -- Researchers at Ohio State University have glimpsed for the first time how DNA protects itself from the sun's harmful radiation. Through a natural process that lasts less than one trillionth of a second, light-absorbing parts of DNA convert ultraviolet (UV) radiation from the sun into heat. The speed of this process is key to DNA's natural ability to protect itself from the sun's radiation, said Bern Kohler, assistant professor of chemistry at Ohio State. UV is harmful because it can cause DNA to mutate and cause conditions including skin cancer and premature aging, Kohler explained. But our DNA doesn't mutate every time we're exposed to sunlight. "From what we've seen, DNA functions somewhat like its own sunscreen," said Kohler. "The process doesn't work perfectly every time, and that's when mutations take place -- so don't stop wearing your sunscreen. "For the first time, we've been able to see just how fast DNA dissipates UV energy, and that will help us better understand how light damages DNA," he continued. Previously, scientists had only indirect evidence that DNA rids itself of UV energy very quickly, because the processes involved take place much too quickly to be viewed using conventional instruments. The results appear in the September 27, 2000, issue of the Journal of the American Chemical Society. Kohler and two of his graduate students, Jean-Marc Pecourt and Jorge Peon, took portions of the DNA molecule called nucleosides, placed them in water, and bombarded them with an ultrashort pulse of UV light from a laser. The laser pulse lasted only 150 femtoseconds, or 150 quadrillionths of a second. The pulse deposited energy in the electrons of the nucleosides much as sunlight would, putting the nucleosides into an excited state. Because the laser pulses were so short, the researchers were able to study the extremely rapid process by which DNA changes energy from UV radiation into heat. In a fraction of a trillionth of a second, the electronic energy was transformed into increased vibrational motion of the nucleoside's atoms -- heat. The nucleoside's temperature jumped to over 1300°C. It cooled down again by transferring energy to the much cooler water molecules that surrounded it. The researchers found that the cooling process finished in only few trillionths of a second. Kohler said that while our DNA may reach a high temperature during this reaction, the timing is so brief, we can't feel it. He speculates that the speed of the process is critical for protecting DNA. "The longer the excited state energy remains localized in DNA, the greater the chance for permanent damage," he said. This method of protecting DNA is probably universal to life on earth, Kohler said. The nucleosides he and his students used in the lab are the same building blocks found in the genetic material of all plants and animals. The water surrounding the nucleosides in the lab simulated the water found in plant and animal cells. "We had to look at a component of DNA to start, but eventually it will be important for us to look at the whole DNA molecule," he added. In addition to their importance for cancer research, the findings may help researchers design better sunscreens for people and photostabilizers for paints. Photostabilizers are compounds that protect paint, plastics and other materials from damage by UV light. To Kohler, the results also say something about how life evolved on this planet. The early Earth had a much different atmosphere, he explained, so early life would have had to withstand much more UV radiation than we are exposed to today. "Today, ozone in the stratosphere protects us from most UV radiation, so scientists have wondered how life could have withstood that early onslaught. The fact that DNA functions very much like its own sunscreen may have been a critical factor in the evolution of life," he said. This work was supported by Ohio State. Peon will join the research staff of Ahmed Zewail, professor of chemistry at Caltech next year, and Pecourt is about to join the staff of the American Chemical Society's Chemical Abstracts Service. Zewail won the 1999 Nobel Prize in chemistry for the application of femtosecond laser techniques to chemistry. # Contact: Bern Kohler, (614) 688-3944; Kohler.40@osu.edu Written by Pam Frost, (614) 292-9475; Frost.18@osu.edu