Dear Francisco,
With a c axis of 650 Angs your phi-overlap problem is severe. When this
axis is close to being parallel to the beam, the angular distance in radians
between h,k,l and h,k,l+1 for a reflection hkl close to the top or bottom of
an image is the angle spanned by c* viewed at a distance d* (=d*[h,k,l]),
i.e. roughly c*/d*. This gives an angular distance of 1 degree for a c axis
of 180 Angs at a resolution of pi (=3.14159...) Angs. In your case, at a
resolution of 3.25 Angs this angular distance is only 1/200 radian, i.e.
0.286 degree; at higher resolution, it is proportionally smaller. Therefore,
even a small mosaicity (0.2?) will give you severe phi overlap at the top
and bottom of your images (assuming that the spindle axis is horizontal).
The best you can hope for is that the estimation of that mosaicity by
matching predicted spots to observed spots will give a correct rejection for
affected reflections - until refinement against measurements that are linear
combinations of intensities of several reflections, rather than intensities
of individual reflections, becomes possible.
With best wishes,
Gerard.
--
On Thu, May 22, 2008 at 03:47:24PM +0100, [log in to unmask] wrote:
> Dear All
>
> I am processing data from a crystal for a large macromolecular complex
> with mosflm. Cell dimensions are around 120 x 150 x 650, with a p222
> spacegroup. To avoid overlaps, we have collected data with a oscillation
> of 0.1 degrees.
>
> When I try to process the data with mosflm, mosaicity decreases along the
> processing to a very low values (0.05 to 0.1 depending on the images). The
> result is that I miss a lot of spots from the images.
>
> I would appreciate any help regarding :
>
> 1.- What's is the best strategy to process such a dataset ?
> 2.- Which are the critical parameters in mosflm to avoid these problems,
> and how to modify them ?
> 3.- and finally, can I use this data or your advice is to try to get
> crystals in a different spacegroup ?
>
> Many thanks
>
> Best regards
>
> Francisco
>
>
>
> -----------------------------------------
> Francisco J. Enguita, Ph.D.
> Macromolecular Crystallography Laboratory
> ITQB
> EAN, Av. da República
> 2781-901 Oeiras
> Portugal
> Phone : +351-21-4469663
> Fax : +351-21-4433644
> E-mail : [log in to unmask]
> -----------------------------------------
--
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* Gerard Bricogne [log in to unmask] *
* *
* Global Phasing Ltd. *
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