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CCP4BB  August 2007

CCP4BB August 2007

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Subject:

Re: Strange diffraction images

From:

Raji Edayathumangalam <[log in to unmask]>

Reply-To:

[log in to unmask]

Date:

Mon, 27 Aug 2007 12:36:08 -0400

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Very dumb question perhaps:

If there were two interpenetrating lattices of slightly different cell dimensions, would we not
expect that the indexing program would leave out a lot of the spots as "unpredicted" or "uncovered"?

Could someone clarify with respect to the diffraction pattern that has just been posted (diff45..png)?

Raji



---------Included Message----------
>Some small molecule crystallographers have specialized in solving and 
>refining structures that, exactly as you describe it, consist of two (or 
>more) interpenetrating, non-commensurable lattices. The usual approach is 
>to decribe the crystal in up to six dimensional space. The programs SAINT 
>and EVALCCD are able to integrate such diffraction patterns and
>SADABS is able to scale them. However the case in point is probably 
>commensurate.
>
>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. +49-551-39-2582
>
>
>On Mon, 27 Aug 2007, Jacob Keller wrote:
>
>> What a beautiful and interesting diffraction pattern!
>> 
>> To me, it seems that there is a blurred set of spots with different cell dimensions, although
>> nearly the same, underlying the ordered diffraction pattern. A possible interpretation occurred to
>> me, that the ordered part of the crystal is supported by a less-ordered lattice of slightly
>> different dimensions, which, because the crystal is a like a layer-cake of 2-d crystals, need not
>> be commensurable in the short range with the ordered lattice. The nicely-ordered "cake" part of the 
>> crystal you solved, but the "frosting" between is of a different, less ordered nature, giving rise
>> to the diffuse pattern which has slightly different lattice spacing. I would have to see more
>> images to know whether this apparent lattice-spacing phenomenon is consistent, but it at least
>> seems that way to me from the images you put on the web. I would shudder to think of indexing it,
>> however.
>> 
>> All the best,
>> 
>> Jacob Keller
>> 
>> ps I wonder whether a crystal was ever solved which had two interpenetrating, non-commensurable
>> lattices in it. That would be pretty fantastic.
>
>
>Jacob,
>
>Some small molecule crystallographers have specialized in solving and
>refining structures that, exactly as you describe it, consist of two
>interpenetrating, non-commensurate lattices. The usual approach is
>to index the diffraction pattern in multiple dimensional space 
>('superspace'). The programs SAINT and EVALCCD are able to integrate 
>diffraction patterns in up to six dimensions, SADABS is able to scale 
>them and the refinement is almost always performed with Petricek's 
>program JANA2000: 
>
>http://www-xray.fzu.cz/jana/Jana2000/jana.html 
>
>However the case in point is probably commensurate.
>
>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. +49-551-39-2582
>
>
---------End of Included Message----------

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