P4(1) and P4(3) are enantiomorphic space groups. The only difference is the helix (one way, or another). No difference in the diffraction pattern. Hence a program (or a crystallographer) cannot distinguish the 2 based on the diffraction pattern.
Once you start phasing, e.g. by molecular replacement, there is one solution for one enantiomer and far worse statistics for the other enantiomer. This solves it. And I'd like to point out that it is always a good idea to keep in mind the alternative space groups even if you think you are solving the structure but haven't solved it completely yet. Fred. > Message du 11/10/11 21:02 > De : "Young-Jin Cho" > A : CCP4BB@JISCMAIL.AC.UK > Copie à : > Objet : [ccp4bb] Pointless (P41) vs. Phaser (P43) > > Hi all, > I recently got diffraction data of 214 AA protein. When I processed > the data, pointless suggested me a space group as P41. However, when I > ran Phaser with 'all choices of alternate space group', it gave me a > pdb file with P43. Additionally, phenix.xtriage suggested me P422 with > twin laws (h,-k,-l). > Anyway, it seems like P43 is right space group (I tried to process and > refine all possible space groups, and based on R values P43 is most > reasonable). > At this point, I'd like to know more first why different programs > suggested different space group, and which is the most reliable way to > determine the space group. Also, what is the biggest difference > between these space groups. In practice, which way is the best method > to get a right space group other than trying all possible space groups > as I just did. > > Thanks in advance, > > Young-Jin >
