Applied Crystallography

Biography

Biography: D. K. Saldin

Abstract

In general what is measured on x-ray scattering are the intensities of x-rays. What is more if the scattering units are all identical and identically oriented, a measurable intensity is found from the unit cell even with relatively weak x-rays. Ingenious methods have been devised in x-ray crystallography for deducing the phases of the scattered x-rays and consequently the amplitudes of scattering. A Fourier transform of the scattered amplitudes gives the electron density of the unit cell and this often allows the structure to be deduced. The advent of the new x-ray sources such as those from an x-ray free electron laser (XFEL) allows a paradigm shift for the measurement of molecular structure. The increase in intensity of ten billion-fold allows the possibity of structure determination even if proteins do not form crystals by scattering off individual molecules. A translation of the molecule gives rise to exactly the same intensities, but as rotation in general changes the intensities totally. However even in the case of rotation a quantity called the angular correlations, while dependent on the structure, do not depend of the state of rotation or translation and is thus ideally placed to allow structure determination of molecules typically injected into an XFEL in the form of particles of constant structure but unknown orientation or position. What is more, the absence of translational periodicity as in a crystal, allows the determination of the phases of the scattered amplitudes by an iterative phasing algorithm. We will discuss the use of angular correlations to determine the structures of proteins and viruses, with an XFEL and even raid time resolved changes in their structure