| It has always been a dream to film chemical reactions fast enough to see how molecules evolve from their initial structure, through intermediates to their final structure. Which atoms take part, are there intermediate steps and if so, what are they and for how long do they live? This is a great challenge since molecules change shape and composition extremely fast, from femtoseconds to milliseconds. With the advent of third generation synchrotrons facilities such as the ESRF in Grenoble, intense pulsed beams of hard X-rays are now available for probing atomic motions with down to 50 ps resolution. Beamline ID09B at ESRF is dedicated to time resolved diffraction from crystals and liquids. The molecules are typically initiated by a 1-2 ps long laser pulse and the structure is probed by a delayed X-ray pulse. The pump-probe sequence typically run at 1-1000 Hz and when the sample is exchange between shots, the diffracted signal is accumulated on a CCD camera. I will show a film of the dissociation of CO in myoglobin and hemoglobin crystals and compare that study with work in solution. The solution data is taken by SAXS/WAXS and that work shows that certain important intermediate structures are absent in crystals due the crystal packing. |
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