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UV-A sunlight is not so sunny Most photochemists would pronounce that UV-A radiation, UV-B's milder cousin, damages DNA primarily through easily repaired oxidative damage. Yet in a full inspection of UV-A's damage to nucleic acids, biochemist Evelyne Sage at the Curie Institute in Orsay, France, and coworkers have found that UV-A irradiation of Chinese hamster ovary cells actually results in more cyclobutane pyrimidine dimers than oxidative lesions [Biochemistry, published online July 12, http://dx.doi.org/10.1021/bi034593c]. Pyrimidine dimers take longer to repair and are considered an initial step toward cancer. UV-A likely dimerizes hamster DNA indirectly, through a triplet energy transfer from a nearby excited chromophore, rather than directly, as UV-B does. Whether human cells contain a similar photosensitizer--and what it might be--is still entirely unclear, Sage says. Yet the results indicate a need for sunbathers to be more wary of UV-A, she says, because not all sunscreens protect against it. UV-A radiation, with a wavelength range of 320 to 440 nm, makes up about 95% of the sunlight that reaches Earth. |
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