Experimental Phasing with Xo4

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2 Powerful Phasing Agents For Native Crystals

Xo4/Crystallophores are straightforward Experimental Phasing Agents facilitating crystal structure determination using SAD or MAD methods, due to the exceptional anomalous properties of f-block elements. They overcome the tedious and time-consuming work of seleno-methionine labelling or of heavy-atom incorporation.
Xo4 are fully compatible with conventional phasing methods. Prior to data collection, carry out a rapid (from 30 sec to 5 min) soak of crystals in a concentrated Xo4 solution (100mM) in mother liquor supplemented with a cryo-protectant. AVOID BACK SOAKING


To fully exploit the anomalous contribution of Terbium, an X-ray fluorescence scan can be performed to precisely define the peak and the inflection wavelengths of the Tb LIII absorption peak and to set the energy of the X-ray beam.

• Tb3+ has 62 electrons making it ideal for heavy atom phasing.
• Powerful tool for SAD and SIRAS phasing: Terbium’s LIII absorption edge is around 1.6 Å (7.5 KeV, f” ~ 30 electrons), with significant anomalous signal present far from the edge.


LuXo4 has been designed to improve phasing properties : its LIII absorption edge, at 1.34Å, is easily accessed with most synchrotron-radiation sources leading to an anomalous contribution of about 30e-. This contribution is also important (10e-) on a fixed-wavelength (1Å) beamline allowing non-optimized SAD phasing.

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