I think that two issues are being confused here. When the anomalous signal
is very weak, e.g. for sulfur-SAD phasing, the anomalous differences are
comparable with the intensity esds. By averaging over many measurements we
can reduce these esds and so improve the signal to noise ratio and the
chances of solving the structure. In your case you have a whopping
anomalous signal so there is no need for such averaging. On the other hand
you have only measured a fraction of the data. However the data to
parameter ratio (assuming three parameters for one barium site) is still
more than adequate. A better comparison would be with small molecule direct
methods, where structures are frequently solved (e.g. with shelxd) with
less than half the data by chemists impatient to see if they have
synthesized the right compound.
George
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-22582
On Wed, 4 May 2011, Dhanasekaran Varudharasu wrote:
> Dear all,
>
> We have solved the sturcture of hen egg white lysozyme
> with a barium ion at 2.7 fold data redundancy. Data collected at in-house
> copper K-alpha source to 2.22 A resolution with 1 degree oscillation step
> per frame. The substructure was correctly identified with just 8 frames ( up
> to 7 frams the SHELXC did not produced the hkl files) when using SHELXD and
> phasing was successful when using SHELXE-Beta trail version with 35 frames.
> We can not belive it because the data used for substructure solution has
> only 37% completeness. The crystallographers frequently says that in SAD
> phasing to find the correct anomalous substructe the data reduendancy should
> be high enough. But in our case the SHELXD was identified the heavy atom
> site correctly at this very low completeness.
>
> I welcome your suggestions and comments.
>
> Thanks
> with regards
> Dhana
>
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