Hi Jeffrey Re-indexing this in the conventional setting (i.e. beta nearest to 90 - for reasons no-one has yet been able to explain to me the standard d.p. programs do not make this choice automatically!) gives you space group I2 with cell:
a=58.1 b=95.8 c=76.67 beta=90.24 With beta now so close to 90, twinning is quite likely. Also it's quite likely that this cell shows the relationship to I222 much more clearly than the C2 one, and if it is twinned the 2-fold axes of the I-centred orthorhombic cell will almost certainly be the twinning axes in the I2 cell. Finally I would not base any determination on the *centric* moments, simply because there are so few centric reflections in monoclinic! You must look at the *acentric* moments. Also look at the stats from the detwin program using the re-indexed data with the h,-k,-l or -h,-k,l twin operators; this gives you an estimate of the twin fraction: actual detwinning of the data may or may not prove useful! HTH -- Ian > -----Original Message----- > From: [email protected] [mailto:[email protected]] On > Behalf Of Jeffrey D Brodin > Sent: 16 June 2009 17:24 > To: [email protected] > Subject: Possible C2 Twinning > > Dear ccp4bb, > > I've recently run into a problem refining a crystal structure that I > indexed and performed a molecular replacement on as C2. The unit cell > dimensions are a=96, b=95.8, c=58.1 and beta=127; Rmrg=.195. The data go > down to a resolution of 2.8 angstroms. The data also index in I222, but > scaling statistics are worse (Rmrg = .598) and Molrep does not find an > acceptable solution. Using the C2 space group Molrep finds a solution > with the expected 4 monomers (protein size is ~12.5kd) in the asymmetric > unit, but I have only been able to refine it to an R/Rfree of 33/37. The > asymmetric unit consists of two dimers, and performing a two-fold rotation > around an axis perpendicular to the helices of the dimers generates the > tetramer that we would expect from other experimental data. Also, the > backbone density for two of the monomers is much worse than normal. > Truncate shows possible twinning in that the 4th centric moment of E has > an observed value of 2, which is what would be expected for a perfect twin > (actual expected value is 3) and the first and third centric moments are > .87 and 1.45 respectively, with expected values of .8 and 1.6 and .89 and > 1.33 for perfect twins. I am new to resolving twinning issues and was > wondering if anyone has any ideas as to whether I may have the space group > wrong or if there is a twinning operator that I could use to resolve this > issue. Thanks in advance for your help, > > Jeff Disclaimer This communication is confidential and may contain privileged information intended solely for the named addressee(s). It may not be used or disclosed except for the purpose for which it has been sent. If you are not the intended recipient you must not review, use, disclose, copy, distribute or take any action in reliance upon it. If you have received this communication in error, please notify Astex Therapeutics Ltd by emailing [email protected] and destroy all copies of the message and any attached documents. Astex Therapeutics Ltd monitors, controls and protects all its messaging traffic in compliance with its corporate email policy. The Company accepts no liability or responsibility for any onward transmission or use of emails and attachments having left the Astex Therapeutics domain. Unless expressly stated, opinions in this message are those of the individual sender and not of Astex Therapeutics Ltd. The recipient should check this email and any attachments for the presence of computer viruses. Astex Therapeutics Ltd accepts no liability for damage caused by any virus transmitted by this email. E-mail is susceptible to data corruption, interception, unauthorized amendment, and tampering, Astex Therapeutics Ltd only send and receive e-mails on the basis that the Company is not liable for any such alteration or any consequences thereof. Astex Therapeutics Ltd., Registered in England at 436 Cambridge Science Park, Cambridge CB4 0QA under number 3751674
