All chemical and biological reactions such as bond-making or bond-breaking, enzyme catalysis, or response to light, proceed via structural changes, in which atoms may move through substantial distances in the range of 0.01 - 1 nanometer. These structural changes can be very fast and span the time range from femtoseconds to seconds. Can they be observed directly, through the time-dependent changes in the scattering of intense, brief X-ray pulses derived from synchrotron sources such as the Advanced Photon Source at Argonne National Laboratory or the European Synchrotron Radiation Facility in Grenoble, France? We observe such processes in single crystals of biological macromolecules as they respond to a brief light pulse that initiates a light-driven structural reaction in the molecules in the crystal. Such experiments yield a time-dependent, time- and space-average structure over all the molecules in the crystal, and the results can be displayed as a "molecular movie". However, the time-dependence arises from the time-dependent changes in the populations of otherwise time-independent structures, each associated with an intermediate state in the overall reaction. Can the time-dependent data be analysed further to extract these metastable, short-lived structures, so that the "movie" reduces to a set of carefully-chosen "stills"? These considerations will be illustrated with experiments on the photolysis of the carbon monoxide complex of the heme protein, myoglobin, and on the light-driven response of a bacterial blue-light photoreceptor known as photoactive yellow protein.

ANL Physics Division Colloquium Schedule