Dear All,
I would have a question regarding the effect of non-crystallographic
symmetry (NCS) on the data:parameter ratio in refinement.
I am working with X-ray data to a maximum resolution of 4.1-4.4
Angstroem, 79 % solvent content, in P6222 space group; with 22 300
unique reflections and expected 1132 amino acid residues in the
asymmetric unit, proper 2-fold rotational NCS (SAD phased and no high-
resolution molecular replacement or homology model available).
Assuming refinement of x,y,z, B and a polyalanine model (i.e. ca. 5700
atoms), this would equal an observation:parameter ratio of roughly
1:1. This I think would be equivalent to a "normal" protein with 50 %
solvent content, diffracting to better than 3 Angstroem resolution
(from the statistics I could find, at that resolution a mean
data:parameter ratio of ca. 0.9:1 can be expected for refinement of
x,y,z, and individual isotropic B; ignoring bond angle/length
geometrical restraints at the moment).
My question is how I could factor in the 2-fold rotational NCS for the
estimate of the observations, assuming tight NCS restraints (or even
constraint). It is normally assumed NCS reduces the noise by a factor
of the square root of the NCS order, but I would be more interested
how much it adds on the observation side (used as a restraint) or
reduction of the parameters (used as a constraint). I don't suppose it
would be correct to assume that the 2-fold NCS would half the number
of parameters to refine (assuming an NCS constraint)?
Regards,
Florian
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Florian Schmitzberger
Biological Chemistry and Molecular Pharmacology
Harvard Medical School
250 Longwood Avenue, SGM 130
Boston, MA 02115, US
Tel: 001 617 432 5602
