Re: [ccp4bb]: p61 vs p65 flipped

["Eleanor J. Dodson" <ccp4@ysbl.york.ac.uk>]

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Ed wrote:
> 
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> 
> Apologies for the not strictly CCP4 question...
> 
> I'm trying to find Se sites in MAD data from crystals of space group p61 or
> p65. I was under the impression that the Patterson maps should be the same
> for both and that I therefore only needed to solve the Pattersons once.
> Experimental Patterson maps from peak data look the same for both. However,
> Solve is giving me different solutions for p61 compared with p65.


 Solve uses the Patterson for the first solution, then tests both
possibilities
 (xyz in P61 or -x,-y,-z in P65 - there are only two - see below..)
using Fourier analysis with the generated phases.
One solution on the correct hand gives sensible phases - the othe does
not, so by using the maps to check the sites in turn SOLVE can select a
most probable hand..

> 
> Also, can anybody out there tell me whether there are 4 different phasing
> possibilities: p61 with sites flipped or unflipped, and p65 flipped,
> unflipped. 

 There are only two possiblities. As you say, the Pattersons are
identical, but the vectors you search for are different for the two
space groups. So you get only two interpretations..
For this pair of spacegroups the two solutions ar either xyz or
-x,-y,-z. 

Another way of looking at it is to think that if you have a screw axis
rotating up in steps of 1/6 ever 60 gegrees, and you chang hand the
screw axis will now rotate -1/6th (or 5/6ths) every 60 degrees ..

eleanor