Anisotropic picosecond X-ray solution scattering from photoselectively aligned protein molecules

Anisotropic X-ray scattering patterns of transiently aligned protein molecules in solution are measured by using pump-probe X-ray solution scattering. When a linearly polarized laser pulse interacts with an ensemble of molecules, the population of excited molecules is created with their transition dipoles preferentially aligned along the laser polarization direction. We measured the X-ray scattering from the myoglobin protein molecules excited by a linearly polarized, short laser pulse and obtained anisotropic scattering patterns on a 100 ps time scale. An anisotropic scattering pattern contains higher structural information content than a typical isotropic pattern available from randomly oriented molecules. In addition, multiple independent diffraction patterns measured by using various laser polarization orientations will give a substantially increased amount of structural information compared with that from a single isotropic pattern. By monitoring the temporal change of the anisotropic scattering pattern from 100 ps to 1 μs, we observed the orientational dynamics of photogenerated myoglobin with the rotational diffusion time of ∼15 ns.