An overview of heavy-atom derivatization of protein crystals

Abstract

Heavy-atom derivatization is one of the oldest techniques for obtaining phase information for protein crystals and, although it is no longer the first choice, it remains a useful technique for obtaining phases for unknown structures and for low-resolution data sets. It is also valuable for confirming the chain trace in low-resolution electron-density maps. This overview provides a summary of the technique and is aimed at first-time users of the method. It includes guidelines on when to use it, which heavy atoms are most likely to work, how to prepare heavy-atom solutions, how to derivatize crystals and how to determine whether a crystal is in fact a derivative.

Keywords: crystallography; derivatization of crystals; heavy atoms; phasing techniques; protein crystals.

Figure 1

Techniques for the phasing of macromolecular structures.

Figure 2

The periodic table with class A and class B heavy atoms shown in yellow and blue and intermediate elements in purple (adapted from Fig. 8.4 in Blundell & Johnson, 1976 ▸). The absorption edges (energies and wavelengths) and f′′ at the edge are shown (data from the http://skuld.bmsc.washington.edu/scatter/AS_periodic.html website). Some elements (marked with an asterisk) have a range of absorption edges from which to choose and thus for each element an edge at a suitable energy for data collection at a synchrotron has been selected.