Re: twinning

[Chris Colbert <colbc@wladziu.bio.purdue.edu>]

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Titia Sixma wrote:
>
>I'ld love some answers to the following totally non-ccp4 related, but
>certainly crystallographic question (and yes, I'll make an overview
>afterwards):
>
>a) What conditions have people succesfully used to overcome twinning in
>crystallization?
>
>b) What MIR structures have been solved using (de)twinned data
>(besides cephalosporin synthase)? 
>	
>c) Have any structures been solved by MAD using detwinned data?
>
>d) Has anybody experience in detwinning pseudomerohedral twinned data?
>
>Many thanks!
>

Hi Titia,

Well, I have run into two twinning problems in my career as a graduate 
student so I will relate my experiences to your questions.

a) The obvious answer is to crystallize the protein in a space group that
does not allow twinning.  Of course, that is not a helpful answer.  In
my case I found untwinned or very low twinned crystals within the same
drops as near perfectly twinned ones.  My solution was a brute force one,
collect data, test for twinning, move to the next crystal.  I was lucky
and found some untwinned crystals.  Sometimes if the twinning is caused
by crystals becoming intergrown, you can try to grow fewer crystals per
drop.  Or try growing the crystals slower, ie. lower the temperature, 
add glycerol(never tried this), etc.  These are not guaranteed, but 
worth the effort if you get an untwinned crystal.

b) Everyone else has covered this pretty well.  I think currently the
best way to solve a twinned crystal is through MR.

c) I have tried, but alas no success as yet.  Theoretically this can
be done.  However, when the data is detwinned you must take precautions
that the twin operator only relates I+ to I+ and I- to I-.  Otherwise,
the detwinning loses your anomalous signal.  As I understand the CCP4
detwin program does not support detwinning of anomalous data.  

That brings up the next point.  For the most part a SeMet MAD experiment
is a low signal experiment.  (I am assuming you are talking about a
SeMet MAD experiment)  Detwinning while helpful is not perfect and will
remove some if not all of the signal.  Also, with the presence of a twin
operator,  you will have to solutions related to each other by the twin
operator.  So, finding the consistent solution with its cross peaks becomes
a major under taking.

In my opinion, to solve a MAD twin problem you will need to have a very
high signal MAD experiment with few sites.

d) I must point you to Todd Yeates crystal twinning server.  This web site

http://www.doe-mbi.ucla.edu/Services/Twinning

is very helpful in identifying twinning and determining initial estimates for
the twin fraction and the twin operator.  It also has some great general
information about twinning to read.

 
Well, I hope this is a little helpful.  Good Luck!


Chris

***********************************************************
Christopher L. Colbert
Purdue University Macromolecular Crystallography         
Lilly Hall of Life Sciences                         
West Lafayette, IN 47907-1392			    
***********************************************************