Biological microcrystals and pulsed synchrotron radiation

New strong and pulsed X-ray sources like the TT-XFEL open new ways for the investigation of structure and dynamics of biomolecules and biological systems. Completely new techniques are requested. The extreme intensity of the new X-ray sources will allow the use of micro- or nano-sized samples. However, it will cause strong radiation damage. Therefore, the diffraction pattern of several samples has to be combined for one structure determination. As consequence, several objects must be brought into the X-ray beam consecutively and fast to avoid delay times. Since the scattered intensity of very small objects is low, all parasitic scattering has to be minimized. This means that the incoming X-ray beam and the sample have to be in the vacuum.

Up to now, the particles are brought into the X–ray beam by an electro spray injector. X–ray pulses and particles interact randomly. In this project we develop an alternative technique. The central part of our work is the construction of a scattering chamber, which should fulfill the requests stated above. It has to be possible to bring the samples into the beam at will. For that purpose, many small objects are dispersed on a thin disc of “Diamond Like Carbon (DLC)” developed in another MAP project. They are marked with fluoresceine or quantum dots. Their positions are indicated by their fluorescence and  recorded by a CCD camera. A program finds the coordinates of the crystals on the disc and stores them in a computer. During the experiment, one particle is irradiated till radiation damage dominates. This may happen even after one X-ray pulse. Then, the next object is brought into the beam. The samples are in vacuum to minimize stray radiation. As a consequence, all movements of the sample as well as the adjustment of the beam stop are performed by precision motor drives remote controlled. The samples can be frozen with the help of Peltier elements. The diffraction picture is measured by a MAR image plate or alternatively by a CCD camera. The figure shows the principle of the experimental setup.

For a prove of principle, we start with conventional protein crystals and the X-ray beam of a rotating anode.  Step by step their size of the crystals should be decreased. Extremely small crystals can be obtained much easier than crystals, which are at present suitable for structure determination. An optimal working scattering chamber will mark a great step towards the final goal of structure determination of single protein molecules and aperiodic biological systems.