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The search terms you are looking for are "solvated electron" and "color center" (sometimes F-center). The former term is used to describe an unbound electron trapped in between a bunch of water molecules (which rotate their positive poles toward it to stabilize the species), and the latter term is used to describe a similar phenomenon in gemology (which gives amethyst its color). Solvated electrons in water are known to be blue, but they don't last very long. Usually only a few microseconds at room temperature, but it is not unreasonable that they would last longer at cryo temperatures. Water molecules can still rotate at 77 K. I first read about "frozen" solvated electrons in the EPR literature (Box, H. R. (1977). "Radiation Effects: ESR and ENDOR Analysis." London: Academic Press.), but the "discovery" of them by dissolving alkali metals in ammonia was apparently done more than 200 years ago, so there is probably no single and clear reference describing a proof of the structure of the species. The "Radiation Chemistry Data Center" at the NDRL, however is a good place to start looking: http://www.rad.nd.edu/ I have personally noticed that the blue color only appears when the pH of the hyperquenched solution is higher than 7 or so. I assume this is because solvated electrons react with protons to form their conjugate base: the hydrogen atom. The latter species is highly reactive as well, but it is not colored. Some people have reported a green color (from DMSO solutions) and I have also seen a few crystals turn red. I imagine a simple way to explain this is the classic "electron in a box" problem often given to first-year physics students. The bigger the "box" the electron is trapped in, the longer the wavelengths it will absorb (or scatter). The extreme case of this is metal: the outer-shell electrons in metals are essentially unbound and completely free to move, so the "box" is the whole substance of the metal. This is why metal is shiny. -James Holton MAD Scientist Todd Geders wrote: > Greetings, > > On a recent synchrotron trip, certain frozen samples were turning a > blue upon exposure to the beam. Attached is a representative image > from the crystal-centering camera. If you take snapshots down the > crystal, you can make blue dots. Note that it also colors the frozen > solution in addition to the crystal. > > Details on conditions: > > 20 micron beam, unattenuated beam, 12.000 keV, GM/CA at APS > Protein solution: 20mM HEPES-KOH pH 7.5, 100mM KCl, 0.1mM EDTA, 2mM DTT > Crystallization solution: 39% w/v PEG 6000, 0.1M HEPES-KOH pH 7.6, > 0.2M Ammonium sulfate > Cryo protection: Added 7% v/v glycerol as cryoprotectant > > Anyone have any ideas on what is causing the color change? > > Todd Geders > University of Minnesota > Dept. of Medicinal Chemistry > 308 Harvard St. SE, #8-101WDH > Minneapolis, MN 55455 > Office 2-163 WDH / Lab 2-160 WDH > Phone: 612-624-2448 > > > ------------------------------------------------------------------------ >