Isomorphous replacement with optimized anomalous scattering applied to protein crystallography

Isomorphous replacement is an essential technique for the de novo solution of macromolecular crystal structures. The use of the anomalous-dispersion effect of the heavy atom in the derivative leads to the acronyms SIRAS or MIRAS (single or multiple isomorphous replacement with anomalous scattering) as the term for the phase determination method. Synchrotron radiation is tuneable over a wavelength range encompassing the absorption edges of the typically used derivative atoms such as Hg, Au and Pt. Hence, it is possible to optimize the anomalous-scattering signal of such atoms by appropriate choice of a wavelength between ~0.8 and 1.1 Å. This paper reports a comparison of this method (which we call SIROAS; O for optimized) and SIRAS on related mercury derivatives of glutamate dehydrogenase. The anomalous-scattering signal is enhanced in the SIROAS experiment and this results in an increase of the phasing power of the derivative data.