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More about unit cells

Harry Powell hrp1000 at cam.ac.uk
Thu Jul 11 09:28:30 EST 2002


> Well, having tried he indexing with Denzo/Mosflm of both data set (in fact, 
> several), both programs behave consistently giving the same results: 
>         1Mar ImagePlate: 60 60 130
>         2 Hamburg : 60 60 270

Are the "odd l" reflections all weak compared with the "even l"
reflections? Can you see them all clearly in the images collected both at
the synchrotron and at your home source? The usual source of this
phenomenon is when the spot finding routines have missed the "odd l"
reflections because their intensities/background ratios fall below some

Spots on images collected at synchrotrons tend to be "tighter" and stand
out better from background, mainly due to the lower divergence of the
incident beam.

> it). Can it be some kind of twinning? Teorethical all cruystal came form 
> teh same drop and are smiliar. Would it be possible to get so diffrent but 
> similar crystals in the same drop/ condition? Can it be a device 
> (diffractometer) artifact?

I have collected data on three DNA crystals from the same drop which have
had unit cell volumes in the ratio 1:2:4, which were not related by simple
doubling of the cell edges, so it's possible!

Dr Harry Powell, MRC Laboratory of Molecular Biology,
MRC Centre, Hills Road, Cambridge, CB2 2QH

See http://www.mrc-lmb.cam.ac.uk/harry  for Mosflm, BCA and
crystal growing details

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