Page 189 - Macromolecular Crystallography

178 MACROMOLECULAR CRYS TALLOGRAPHY

Figure 12.3 SGX-CAT barcode reader station. The 2D barcodes on the base of each crystal mount are read on the
left. The scanner on the right reads the unique 1D barcodes on the vials used to ship the crystals. (NB: During operations, crystals
are maintained in liquid nitrogen throughout this process.)

bearing the 2D barcode. An alternative method Despite this obvious advantage, micromounts
has been developed recently, which uses microfab- have the drawback of requiring that the amount
ricated mounts created from polyimides such as of cryoprotectant be minimized to ensure reliable
Kapton (Thorne et al., 2003). These mounts have recognition of the fiducial marks. While this require-
greater rigidity than that of nylon loops, facilitating ment presents little problem when preparing a small
precise placement of the sample across a perfora- number of crystals, it can necessitate many extra
tion in the crystal mount. As with nylon loops, hours of labour when dealing with large numbers
different microfabricated mounts are available to of samples. Therefore, because of the high volume
match the hole in the mount with the size of the of crystals generated at SGX headquarters in San
crystal. Unlike nylon loops, the manufacturing pro- Diego, we continue to use nylon loops for mounting
cess for these new mounts guarantees uniformity of of crystals.
dimensions, which can be important during auto- Once a crystal has been mounted, it must be
matic positioning of protein crystals in the X-ray placed in the X-ray beam. Most modern goniostats
beam (see below). These polyimide mounts include are motorized, permitting this process to be exe-
registration or fiducial ‘cross hairs’, which visual cuted remotely or, for high-throughput data collec-
recognition systems can use to automatically place tion, automatically. At third-generation synchrotron
crystals precisely in the X-ray beam. sources, highly focused X-ray beams and small