SPring-8, the large synchrotron radiation facility

Speaker : Dr. D.K. Saldin

Language : ENGLISH

Affiliate : Department of Physics, University of Wisconsin-Milwaukee

Title : Structure Determination by X-Ray Scattering

Abstract :
This talk will address two main topics, namely surface crystallography using conventional synchrotron X-ray sources, and structure determination of viruses and biomolecules using 4th generation X-ray sources such as the X-ray Free Electron Laser (XFEL).

Protein crystallography has been used to solve very complicated structures containing thousands of atoms from X-ray scattering data. It does this by a two-step process in which an approximate real-space electron density distribution is found by first estimating the phases of the measured X-ray amplitudes and taking an inverse Fourier transform. Then a detailed atomic model of the structure is postulated and varied to find the best fit to the measured diffraction data.

In general, the aim of surface crystallography is to find the relative positions of a much smaller number of atoms at the surface of a known bulk structure. Most work on surface crystallography dispenses with the first step of protein crystallography, and works only with the second step of refining a structural model. As with protein crystallography, much more complicated structures may be solved in surface crystallography if the second (refinement) step were preceded by the reconstruction of an approximate electron density model.

In this paper we describe how such an approximate electron density may be found in surface crystallography by exploiting the fact that the bulk structure is known. The method has some analogy to holography, and also to iterative phasing retrieval methods that have been facilitated by advances in computer technology. I will describe work with model systems and experimental data to demonstrate the effectiveness of such methods.

In the second part of the talk, I will consider the entirely new capabilities for structure work on individual uncrystallized particles afforded by 4thgeneration sources like the X-ray Free Electron Laser (XFEL) which produce X-rays some 10 orders of magnitude brighter than any existing X-ray source in pulses of a few tens of femotoseconds. However, since the particles have to be introduced into the beam in random unknown orientations, completely new methods of structure solution are needed. I will discuss one such method used to determine the structure of viruses.

Organizer : TAJIRI Hiroo
Mail : tajiri@spring8.or.jp
PHS : 3443