Away from the edge: SAD phasing from the sulfur anomalous signal measured in-house with chromium radiation

Acta Cryst. (2003). D59, 1943–1957

Experimental electron-density map of thaumatin for the region including Trp51 generated from data collected with Cr Kα data to 3.0 Å resolution and sulfur SAD phases.

The solution of protein structures using the anomalous scattering from sulfur in native proteins (S-SAD) is becoming more widespread. Most often Cu Kα (λ = 1.5418 Å) radiation is used, for which Δf for sulfur is 0.56 electrons. The authors have developed a system for the home laboratory employing Cr Kα radiation (λ = 2.2909 Å) which doubles the anomalous signal from sulfur to 1.14 electrons. The authors describe the hardware and software tools necessary to take advantage of this enhanced anomalous signal in the test proteins thaumatin and trypsin. The results show that interpretable maps can be generated in the resolution range 3.5–2.8 Å using much less data than required for Cu S-SAD. Caveats associated with the experiment, radiation damage and absorption, are described; solutions to these caveats are also provided.