Structure of Single Particles from Randomly Oriented Ensembles Using an X-Ray Free Electron Laser

SS Kim, S Wibowo and DK Saldin*

Received: December 01, 2017;   Published: December 07, 2017

DOI: 10.26717/BJSTR.2017.01.000573

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Abstract

The method of time-resolved crystallography offers a method of studying fast changes in structure during a chemical reaction. However, being a crystallographic method it is restricted to crystal structures. The problem with this is that most chemical reactions under physiological conditions happen in solution and not on molecules which are part of a crystal. The main problem with trying to find chemical reactions in solution is that one needs a powerful source of X-rays which will give a measurable signal even from small numbers of randomly oriented molecules. The newly developed X-ray free electron laser allows this when combined with a novel theoretical technique. Of course we understand one of the strengths of working with crystals is that the signal comes not from one but from trillions of identically oriented unit cells. The fact that the unit cells are identically oriented means that the X-rays do not have to scatter off a single molecule (or unit cell) to give a sensible signal. Since all molecules are identically oriented information about an idealized average molecule may be obtained from scattering by a crystal even though the different photons scatter off different molecules. The problem is that most chemical reactions do not take place in crystals.

Two main developments allow us to overcome this limitation. One is the development of the X-ray free electron laser (XFEL) which is capable of producing X-rays many orders of magnitude brighter than any previous X-ray source. If it is possible to determine the structure of a single particle without the need for crystals we will have achieved our goal. Luckily there has been a corresponding increase in understanding of scattering by disordered arrays. One of the things that has become realized recently is that if one concentrates not on the bare intensities of scattering, but on what are known as their angular correlations these said correlations are characteristic, over most of their range, of the structure but on not of the orientation of the molecules, provided the scattering is from a dilute disordered ensemble. Consequently, the way has been opened for the study of molecules via scattering by ensembles that are not identically oriented. What is more, techniques have further been developed for following fast changes in the structure of such molecules in a pump-probe experiment.

Abbreviations: XFEL: X-ray Free Electron Laser; LCLS: Linac Coherent X-ray Source; PYP: Photoactive Yellow Protein; LCLS: Linac Coherent Light Source

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