Hi,
It certainly can vary along a crystal – I don't really remember any more whether I specifically mentioned it in (shameless plug follows) my paper in jmb in 1987 but when I solved the structure of MLE, by mir, it was twinned by merohedry. This was the first de novo protein structure that was so twinned.
The crystals were long fat needles, and I noticed that the twin fraction varied along the crystal. (Indeed,that was how Ifirst noticed that they were twinned, because the space group varied from I422 to i4 along the crystal). Clearly, if as you collect the dataset, the irradiated volume changes the twin fraction can change too.
My paper on the subject is almost unknown – evidence of the importance of making a noise about yourself. The other two early discussions of merohedral twinning in macromolecules are in papers from Bob Sweet on monellin , and Doug Rees.
Adrian
Sent from my iPhone
> On 23 Oct 2015, at 21:49, Keller, Jacob <[log in to unmask]> wrote:
>
> Dear Crystallographers,
>
> I am looking for a reference or precedent for variation of twin fraction within a single dataset--does anyone know of such? Could also be in the small molecule literature, but I can't think of a great way to search for this particular phenomenon. Any pointers in the right direction much appreciated.
>
> All the best,
>
> Jacob Keller
>
> *******************************************
> Jacob Pearson Keller, PhD
> Looger Lab/HHMI Janelia Research Campus
> 19700 Helix Dr, Ashburn, VA 20147
> email: [log in to unmask]
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