Dear Matthew, If it is P43 and merohedrally twinned, you should not see systematic absences for the 21 axis. In fact there is no way of getting the P43212 systematic absences from a twinned crystal of lower symmetry. I presume that you also tried MR in P41212.
Best wishes, George Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-22582 On Wed, 22 Jul 2009, Matthew Franklin wrote: > Hi all - > > I'm trying to solve the structure of an antibody-receptor complex, and I've > hit a wall which may be due to a twinned crystal form. This crystal has the > apparent space group P43212, with cell constants a=64.02 c=274.83. Solvent > content analysis using this space group suggests 1 mol/asu, with 47% solvent, > which would be fairly consistent with the diffraction limit of about 2.8 A. > I've been able to place the antibody using molecular replacement and refine > it to R=0.289, Rf=0.338. There is some density for the receptor, which > represents about 20% of the mass of the complex, but not clear enough to > build into, and all efforts to improve the density or place the receptor by > molecular replacement have failed. > > Well, tough luck, you might say, but I noticed at the very beginning of the > process that this crystal form may be a perfect twin. The "4th moment of E" > graph from Truncate shows nearly all resolution bins with values of 1.4 - > 1.6, except in the very topmost few bins where the values rise up to 2. The > other moment graphs are likewise at their "perfect twin" values across the > resolution range. The cumulative intensity distribution graph shows the > observed values are about 30% lower than the expected values for both > acentric and centric reflections. As I understand it, both of these are > strong indicators of twinning, and the twin fraction analysis in DETWIN > suggests a twin fraction of ~0.5. > > So what do I do now? I think I'm supposed to reprocess the data in the space > group without the twin transformation (which would be P43), then run > molecular replacement which should show me solutions for both halves of the > twin. However, all I'm seeing is two copies of the antibody in the (P43) > asymmetric unit, which don't overlap, and both of which need to be present in > the same unit cell in order to form a 3-dimensional lattice. This is exactly > what I would expect to see if there were no twinning present. > > So my question is, is this crystal form twinned or not? Is there some way > that the intensity statistics could be misleading me? On the other side, am > I doing something wrong with the structure determination if the crystal is > twinned? How should I proceed? > > Thanks for any help anyone can provide. > > - Matt > > > -- > Matthew Franklin , Ph.D. > Senior Scientist, ImClone Systems, > a wholly owned subsidiary of Eli Lilly & Company > 180 Varick Street, 6th floor > New York, NY 10014 > phone:(917)606-4116 fax:(212)645-2054 > > > > Confidentiality Note: > This e-mail, and any attachment to it, contains privileged and confidential > information intended only for the use of the individual(s) or entity named on > the e-mail. If the reader of this e-mail is not the intended recipient, or > the employee or agent responsible for delivering it to the intended > recipient, you are hereby notified that reading it is strictly prohibited. If > you have received this e-mail in error, please immediately return it to the > sender and delete it from your system. > > Thank you. >
