Cheng Yang
Rigaku Americas Corporation, The Woodlands, Texas
Abstract:
The phase problem in macromolecular crystallography has been mitigated
dramatically in recent years by advances in methodology and instrumentation. SAD phasing
has now become the primary de novo phasing method. A search of
the PDB of structures released in 2006 reveals the number of structures
solved by SAD phasing exceeds those solved by MAD for the first time. A
number of these examples of successful S-SAD and Se-SAD phasing used Cr
radiation (λ = 2.29 Å), which can double the anomalous signal of sulfur
and selenium compared to Cu radiation. This report reviews recent
results from phasing with the enhanced anomalous signal provided by Cr
radiation to demonstrate this longer wavelength can be used to solve
de novo structures. Selenium, as the heavy atom, with Cr radiation
can provide sufficient anomalous scattering for routine phasing. Cr
radiation opens a new path to extracting the weak anomalous signal from
sulfur to phase native protein data. With the addition of Cr radiation
to the crystallographer’s toolkit, in-house X-ray sources can routinely
provide at least two wavelength options. The combination of diffraction
data collected using both Cu (λ = 1.54 Å) and Cr radiation can improve
the electron density tremendously. Anomalous scattering from sulfur can
also assist in molecular replacement solutions. Finally, the data
collected with Cr radiation can be used to refine a structure.
Ultimately, this makes it possible to solve a protein structure with a
single data set.
This in-house phasing approach we describe has been given the label “know before you go” by John Rose and B.C. Wang at the University of Georgia. This method improves the efficiency of the solution of macromolecular crystal structures and usage of the synchrotron beam time.
abstract (pdf)
2008 Run
Nov 19th - Dec 22nd
