It is mostly because in the higher angles intensity of the reflection is lower, precision is lower and anomalous signal is washed out by counting statistics.
For very well diffracting test crystals anomalous signal is MEASURABLE to very high resolution providing good enough I/sigma(I) is generated.
FF
Dr Felix Frolow
Professor of Structural Biology and Biotechnology
Department of Molecular Microbiology
and Biotechnology
Tel Aviv University 69978, Israel
Acta Crystallographica F, co-editor
e-mail: [log in to unmask]
Tel: ++972-3640-8723
Fax: ++972-3640-9407
Cellular: 0547 459 608
On Oct 14, 2010, at 23:42 , Tim Gruene wrote:
> On Thu, Oct 14, 2010 at 04:28:26PM -0500, Jacob Keller wrote:
>> I have always found this angle independence difficult. Why, if the anomalous scattering is truly angle-independent, don't we just put the detector at 90 or 180deg and solve the HA substructure by Patterson or direct methods using the pure anomalous scattering intensities? Or why don't we see pure "anomalous spots" at really high resolution? I think Bart Hazes' B-factor idea is right, perhaps, but I think the lack of pure anomalous intensities needs to be explained before understanding the angle-independence argument.
>>
> We don't do this because your crystal is angle dependent - it usually does not
> have the required degree of order to scatter thus far, so the anomalous signal
> drowns in the noise.
>
>> JPK
> --
> --
> Tim Gruene
> Institut fuer anorganische Chemie
> Tammannstr. 4
> D-37077 Goettingen
>
> phone: +49 (0)551 39 22149
>
> GPG Key ID = A46BEE1A
>
|