Organized by the Institute of Experimental Physics, University of Bialystok and the Institute of Atomic Energy, Swierk under the auspices of the European Crystallographic Association; Committee of Crystallography, the Polish Academy of Sciences; and the Polish Neutron Scattering Society The School is financially supported in part by the International Union of Crystallography; the European Crystallographic Association; the European Office of Aerospace Research & Development; the Ministry of Science and Information Society Technologies; the Warsaw University of Technology; and the University of Bialystok.
		Structural transitions in proteins studied by X-ray diffraction Hans D. Bartunik Max Planck Unit for Structural Molecular Biology,MPG-ASMB c/o DESY, Hamburg, Germany Topics Dynamical changes in the conformation are essential for the biological functioning of many proteins and multi-protein assemblies. X-ray diffraction provides a unique tool for determining the three-dimensional structures of transient and intermediate states at high spatial resolution. Short-lived intermediates may be trapped by a number of techniques. Examples of applications in particular include investigations of productive catalytic reactions in enzymes. The crystalline environment implies a possible coexistence of protein molecules in different structural states along a reaction coordinate. The molecular packing may induce correlations between intra- and intermolecular motions. This is illustrated by the results of X-ray diffraction studies of heme proteins in trapped transient states at ultra-high resolution. Time-resolved crystallography provides a means for experimentally determining the time sequence in the population of intermediates. Stroboscopic Laue diffraction measurements recently have been used to obtain structural information on cyclicly repeated reactions in protein crystals on sub-ns time scales. In the case of high-symmetry space groups, complete structural information may be derived even from one single Laue diffraction exposure applying Bayesian deconvolution methods. Such applications presently are limited to relatively small proteins and highly ordered crystals. The hard X-ray FEL that is planned at DESY will extend the range of applications to systems of higher structural complexity and shorter time scales in the fs regime. Complementary techniques are needed for determining correlations between motions. Experimental observations are needed in addition to theoretical molecular dynamics simulations. Diffuse X-ray scattering from protein crystals potentially may be used for this purpose. A recent application to hemoglobin demonstrates the feasibility of quantitative evaluations of diffuse scattering at high resolution.