Dynamics in protein crystals: inelastic light scattering and X-ray diffraction during microwave irradiation

Frequently, protein crystallographers see themselves confronted with the problem that although a protein forms crystals with high symmetry, its structure determination is not possible due to insufficient diffraction properties of the crystals. A possible cause for this phenomenon is that the proteins are assembled in several conformational substates in the crystal lattice, which causes an unsatisfactory order and poor diffraction properties of the crystal. The goal of this work was to answer the question whether the conformational disorder in poorly diffracting protein crystals can be decreased by microwave irradiation of the crystals. Normal mode analyses have shown that in many cases, where the structure of a protein is known in two different conformations, the associated movement, which transfers one conformation into the other can already be described by only one or two of the slowest vibrational normal modes of the respective protein. Hence it should be possible to induce transitions between conformational substates by a selective excitement of the slowest vibrational modes. For the selective excitement electromagnetic waves are applicable, which can couple to the dynamics of a protein via polar and charged sidechains of the amino acids and via dipoles between structural subunits.For the first time inelastic light scattering on protein crystals has been done from the GHz- to THz-frequency range with a tandem-Fabry-Pérot-interferometer.By the modification of a special microwave guide, XRD-measurements under simultaneous microwave irradiation became possible. Thus microwave effects on protein dynamics have been studied with atomic resolution for the first time