4 Channel ORTEC C-TRAIN-04
In a collaborative project between CCLRC Daresbury Laboratory and ORTEC a monolithic low profile Ge detector has been manufactured. Its design is based on the proven C-TRAIN technology already in use at Daresbury Laboratory for XAFS and XRF applications (Derbyshire et al., 1999). It provides the beamline with the capability of collecting fluorescence data for MAD experiments, and full range XAFS data on the same crystals as well as monitoring the redox state of a ‘metallic’ functional group during crystallographic data collection. In addition, it allows for a trace elements analysis prior to beginning anomalous/heavy atom derivative diffraction data collection.
The combination of compact detector snout, maximum ratio of active-to-total subtended solid angle, high count rate and high energy resolution etc., is extremely valuable for obtaining high-quality XRF and consequently XAFS data This detector, together with the XSPRESS (X-ray signal processing electronics for solid state detectors) digital electronics developed at Daresbury Laboratory, provides a high photon-count-rate capability in the region of several million photons/sec.
On-Line Metal Analysis Of Protein Crystals, Metallogenomics and Metalloproteomics
For a hypothetical protein, XRF data collection would allow the experimenter to establish the presence of a metal atom in a matter of minutes without scanning over a large number of absorption edges (Figure 9b), which by necessity is time consuming and exposes the crystal to unnecessary radiation dose from which the crystal may not even survive for any diffraction data collection. The second scenario could be establishing the presence of Se in the crystal without collecting diffraction data. This would allow much more efficient screening of crystals and is likely to prove very valuable in major structural genomics/proteomics programmes. The third example is the soaking of crystals, for example for heavy atom derivative, where again presence of a heavy atom can be identified from an XRF spectrum without scanning the monochromator or collecting diffraction data.
The above features are of particular importance to metalloproteins, which are expected to make up some 30% of a genome. In addition to the on-line screening for metals in hypothetical or known proteins, the ability to do XAFS/XANES on the same crystal opens a new capability of ensuring that the redox state of a crystal for which diffraction data are also collected and its structure determined. In addition, the single crystal XAFS capability allows one to exploit the polarisation properties of the synchrotron radiation X-rays and collect angular-resolved XAFS data capable of yielding 3D information around the metal atom at sub-atomic resolution (Bianconi, et al 1985; Hasnain & Hodgson). The combined use of X-ray Crystallography and X-ray Spectroscopy (XAFS) pioneered at Daresbury Laboratory (Hasnain, S.S., (2004); Strange & Hasnain (2003); Binsted and Hasnain, (1996); Cheung et. al., 2000) has already proved extremely powerful in a number of cases (Hasnain & Hodgson, 1999). Such an approach for studying metalloproteins from a genome/proteome would be facilitated by the availability of the new instrument.
Figure 1 metal tracing on a protein derivative crystal with exposure time of 15 sec. The first peak from the right is the scattering peak for due to the radiation used, the second and the third peak indicate the presence of Zn and Cu respectively.
