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Re: [ccp4bb]: Data processing

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On Fri, 30 Mar 2001, Carlo Ciatto wrote:

> Well, it must be triclinic...but the refined cell is the following:
> 89.211  89.214  150.819  90.000  89.989  87.153 ----> a=b, alpha=beta=90.

Well, it could genuinely be triclinic with two 90.0 degree angles - the
crystal system only imposes the metric values on the cell parameters -
they are "necessary but not sufficient"; it's the internal symmetry that
defines the crystal system (an ultimately space group). Many
crystallographers who have collected data on more than a few crystals have
an example of beta = 90.000 for a monoclinic cell, and I'm sure there are
some out there who have triclinic cells with three 90.00 degree angles.

> Any ideas or suggestions? Where should one look for possible problems or
> mistakes (before scaling)? I must confess my unease with symmetry, maybe
> the Denzo table is showing me something that I cannot see.

Something to look for would be incorrect main beam position. Have a look
at the images and see if there is apparent symmetry of intensities with
the indices shifted by + or - 1. 

Other things could be high absorption coupled with a badly centred

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