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.
