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Notes on phasing with Ca

Hoang et al., Nature 2003
They solved the structure of Osteocalcin at 2Å by single anomalous scattering method (SIRAS?) detecting 3 Ca2+ ions and two S atoms (involved in a disulphide bridge). However, they solved a fragment of residues 13-49 (rather small!). Construct was 1-50, "relative molecular mass 5,737". INTERESTINGLY, they used λ=1.7 Å; SOLVE was used to located 3Ca2+ and 2S atoms, then RESOLVE to improve phases by solvent flattening.

Mueller-Diechmann et al., ActaD 2005
The anomalous scattering of S (or P) is small but can be measured reliably also using Cu Kα radiation (λ=1.54 Å). Data collection in the λ range of 1.5-3.0Å can be handled at synchrotron or at home sources using for example Cr Kα radiation (λ=2.29 Å). WHAT IS THE OPTIMAL λ DEFINED AS THE ONE YIELDING THE HIGHEST ANOMALOUS SIGNAL-TO-NOISE RATIO? SEE Weiss, Sicker & Hilgenfeld 2001 (REF in Mueller-Diechmann et al.) for "optimal lambda of 1.9Å for thermolysin where the largest part of the phasing signal originates from Ca ions".

Example of phasing by Ca given here are:
- PPE (porcine pancreatic elastase), crystallized in the presence of 0.04M CaCl2, and
- Thermolysin, crystallized in the presence of 1.4M calcium acetate.

Maximal R_anom/R_r.i.m was obtained at λ=2.08 and λ=2.12, respectively. 2.1Å is shown to be the optimal λ to obtain the highest anomalous signal-to-noise ratio, and "this is independent of the anomalously scattering substructure, provided that no dominating absorption edge is present in the λ range considered". When for some reason it is not possible to collect around 2.1, it is suggested to collect data at a slightly shorter λ, which will also make data processing easier. Then, "the strategy should be to set the detector at the minimum distance to the crystal and choose the λ according to the max resolution one wants to collect."

Dauter et al, ActaD 2001/Jolly SAD
Example of phasing subtilisin on three calcium ions is given. Crystals were grown with 1M CaCl2! INTERESTINGLY, data were collected at λ=1.54 Å using synchrotron radiation.