Single Crystal Diffractometry

Diffraction pattern from a microcrystal of an olygomeric organic compound

The advent of synchrotron radiation sources (SR) has provided sufficient flux and beam qualities for crystal structure determinations from micrometer-sized crystals (or “microcrystals”) [1]. With the development of third generation synchrotron radiation sources, very intense hard X-ray beams in the µm-range have become available with an acceptable divergence for single crystal experiments [2]. This allows for the probing of very small crystals of less than 250 µm³[3,4] and in exceptional cases sub-µm³ volumes[5]. One can therefore select crystals from powder grains (even from heterogeneous samples) for data collection and structure determination. This is particularly interesting for cases where structure determination from powder data was previously unsuccessful or where Rietveld analysis does not provide the required structural information.

Microgoniometer

The microgoniometer has been developed within the context of an EMBL/ESRF collaboration for protein crystallography. Its main features are:

The microgoniometer was initially designed for protein crystals of less than about 50 µm size. It is able to accept frozen samples or on-site sample freezing (sample cooling with a standard cold stream). The beam size can be defined by either 4, 10 or 30 µm diameter apertures. The crystal size requirements (e.g. needle) are :

A suite of software is provided for microgoniometer control (Microdiff GUI) and data collection (ProDC). The Microdiff GUI allows for perfect centering of the crystal either semi-automatically or manually through motor controlled PHI axis xyz translation stages.

Although the microgoniometer was designed initially for protein crystallography it is of course possible to use it for more general chemical crystallography. Typically, the resolution at the edge of the detector is 0.75 Å at l = 0.721 Å (when the detector is not tilted and PHI axis-detector distance is about 45 mm). Data collections are usually carried out at 100 K. Standard exposure times range from 0.5 s to 30 s per image, with a rotation from 1 to 6 degrees. The time taken for a full data set to be recorded is typically between 20 mn to 2 hours, following crystal quality, exposure time and rotation angle.

Sample Preparation

For protein crystals or fragile samples

Fig.1 Sample geometry for microgoniometer

As there is an integrated motorized sample centering system, it is not possible to use your own goniometer heads. All samples should be mounted on magnetic bases fitting onto a magnet diameter of 9.5 mm (HAMPTON magnet).

Cryo-loops for sensitive microcrystals (e.g. protein crystals):

HAMPTON loops with thin pins work fine. The thick (copper) pins are likely to cause problems because they provoke icing with the cryo-geometry (therefore please use thin pins!).

Geometry: