No subject

Mon Jun 7 08:57:32 PDT 2010

.H. & Jensen, L.H. citing Wilson A.J.C. [1950], Acta Cryst. 3, 397), th=
e expected R-factors for the atoms of the structure randomly distributed in=
 the unit cell (or asymmetric unit) are 0.83 for centric reflections and 0.=
59 for acentric reflections.<br>

The values you indicate are well below the figures mentioned above, so they=
 indicate that what you have is not random. But this is not sufficient to t=
ell you if what you have is a proper MR solution or not:<br>
What about the packing in the unit cell? Do you have crystal-forming contac=
ts in the 3 directions of space that explain the formation of the crystal? =
If there are voids, there might be molecule(s) missing. What is the value o=
f the Z-score provided by Phaser (if you have used Phaser)? In the resultin=
g electron density map, do you see electron density that correspond to the =
&quot;mutations&quot; you have to carry out in order to go from model to ta=
rget structure? If you see such electron density, then it is very likely th=
at you indeed have a molecular replacement solution.<br>

The value of the Rmerge (you probably mean Rsym) you give (0.5) will probab=
ly not satisfy referees for a publication. What can always be done is to us=
e several crystals for data collection, starting data collection at differe=
nt (unique) positions, and merge the resulting data (restricting the data p=
rocessing to the frames where the value of Rsym is acceptable). You should =
then end up with a merged data set that has a lower Rmerge value. We used t=
o do this all the time in the days prior to cryo-cooling of crystals, when =
crystals were mounted in capillaries. Rmerge values of 10 to 15 % were typi=
cal then. That is if the value of the Rsym is indeed due to decay. High Rsy=
m values are also obtained in cases of crystal twinning - without proper tr=
eatment- , improper space group assignment (although in that case, much muc=
h higher values are usually observed).<br>

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