Protein dynamics on different timescales

Structure and dynamics determine the function of proteins. This contribution discusses two aspects of protein dynamics, the structural fluctuation and the structural relaxation connected with conformational changes. Myoglobin and haemoglobin were investigated. To cover a wide time range different experimental techniques had to be used. Moreover, measurements in a large temperature regime were used to separate contributions from different modes of motions. Phonon assisted Mössbauer effect using synchrotron radiation allowed the study of the harmonic vibrations which have characteristic times of 1 fs to 0.6 ps. They are present in the whole temperature range from cryogenic to room temperature. With a combination of neutron structure analysis and incoherent neutron scattering it was possible to distinguish three types of hydrogen mean square displacements which are present only above a characteristic temperature Tc: These are the backbone-like (slower than about 100 ps), methyl-like (partly slower partly faster than about 100 ps) and lysine-like (faster than about 100 ps) displacements. The exceptional high energy resolution of Mössbauer absorption on 57Fe allowed the measurement of quasi diffusive modes of molecular segments which have characteristic times slower than 1 ns and are present only above Tc. Conformational changes from the ligated to the unligated structure of myoglobin and haemoglobin were investigated by creating a metastable intermediate and observing the relaxation into the equilibrium conformation. A metastable state was obtained by X-ray irradiation. Structural relaxation was investigated as a function of time and temperature using the Mössbauer hyperfine interactions as indicator. Furthermore it was possible to measure intermediates created by photolysis of a ligand with temperature dependent X-ray structure analysis or time dependent X-ray structure analysis with the Laue technique. It was shown that the quasi diffusive structural fluctuations above Tc strongly facilitate structural relaxations.