The absorption edge of protein-bound mercury and a double-edge strategy for HgMAD data acquisition

The L-III absorption edge of protein-bound mercury (Hg) has been experiment ally determined using X-ray data collection from a crystal. This absorption edge is 12 291 eV, 4 eV higher than the theoretical value of elemental Hg. Considering the possible shift of the Hg absorption edge with the chemical environment in different protein crystals, a double-edge strategy for mult iwavelength anomalous diffraction (MAD) data collection has been developed. The approach provides a convenient way to optimize the dispersive signal b etween a remote wavelength and two edge wavelengths separated from each oth er by 4 eV. The dispersive signals derived from both edges are used, along with anomalous signals, in MAD phasing and phase refinement. This approach has been used in the crystal structure determination of three proteins cont aining one Hg atom per 186-196 amino-acid residues at 2.0, 2.6 and 2.7 Angs trom resolution. A set of four wavelengths is recommended for HgMAD data ac quisition: 1.0087 Angstrom (12 291 eV, edge1), 1.0084 Angstrom (12 295 eV, edge2), 1.0064 Angstrom (12 320 eV, peak) and 0.9918 Angstrom (12 500 eV, r emote). Although it is no longer necessary to determine the L-III absorptio n edge of protein-bound Hg experimentally, an initial fluorescence scan on the crystal for data collection is still necessary to verify the existence of Hg in the crystal.