Ultraviolet Photobiology*

Biology, for the most part, happens in an aqueous environment. Thus the optical properties of water are critical to studies of the effects of electromagnetic radiation on biological systems. Water does not absorb hard x-rays or long wavelength radio waves very well, but these regions of the spectrum are not of interest in this discussion. The regions of the spectrum that are important here are the two "windows" where water is reasonably transparent. The major "window" in the absorption spectrum of water extends from below 200 nm in the ultraviolet (UV) to about 1,000 nm in the near infrared. The "visible" region of the spectrum (380 to 800 nm) is in the center of this window and all of the biological processes that are included in the dicipline of photobiology (vision, photosynthesis, photomovement, photomorphogenesis, bioluminescence, chronobiology, photosensitization and ultraviolet photobiology) involve wavelengths in some part of the 200-1000 water window. In evaluating potential roles for UV free electron lasers (FELs) we are interested in wavelengths less than about 400 nm. In this part of the spectrum most biological effects are detrimental to biological materials and fall under the subdicipline refered to as UV photobiology. In addition to the biological effects of visible and UV light, these wavelength are used in spectroscopic experiments that probe biological structure and function. UV FELs may prove particularly useful in circular dichroism-, fluorescence- and Raman spectroscopy.