Here's an interesting paper on "The Genesis of Twinned Crystals" (from 1945! - it's even printed on simulated aged paper and if you look closely the "aging" has an unrealistic plane-group pattern!):
http://www.minsocam.org/msa/collectors_corner/arc/twinorig.htm It's by the eminent crystallographer Martin J. Buerger who was Prof. of Mineralogy at MIT (http://en.wikipedia.org/wiki/Buergerite) (those of us old enough will remember his classic textbooks "Elementary Crystallography" and "the Precession Method in Crystallography": http://www.amazon.com/s/ref=nb_ss_b/104-3443522-7876740?url=search-alias %3Dstripbooks&field-keywords=%22martin+j.+buerger%22&Go.x=9&Go.y=10&Go=G o). Anyway, the point is that crystals are never "grown under identical conditions" even in the same pot - crystallisation by definition is a dynamic process so that the precise conditions will vary over time and even from place to place in the pot, because as a crystal grows the protein or whatever is depleted from solution and so the conditions in the solution phase inevitably change over time. Crystallisation is necessarily a non-equilibrium process from a supersaturated solution (if it were truly in equilibrium it would take an infinite time for a crystal to grow!) - and the degree of supersaturation is known to be correlated with the frequency of occurrence of growth defects such as twinning. As the material is depleted from solution the degree of supersaturation is reduced, so the rate of growth slows down and defects including twinning become less likely. Hence one would hypothesise that the more highly twinned crystals in a pot would be the ones that had formed first (or at least formed fastest), and in the case of the variably twinned needle crystal that I mentioned earlier one would expect that the crystal had grown from the twinned end towards the non-twinned end - I don't know if anyone has actually looked at this in detail! -- Ian > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Mark Mayer > Sent: 17 April 2007 20:39 > To: CCP4BB@JISCMAIL.AC.UK > Subject: twin fraction varies between crystals? > > For cases where people have had merohedral twinning, did the > twin fraction vary substantially > between individual crystals grown under indentical > conditions? I have no prior experience with > merohedral twinning, and was surprised to see that the twin > fraction varied substantially as detailed > below, and that by screening we were able to get untwinned xtals. > > The project started with a weak home data set for which the > twin fraction was 0.478, and which > scaled in both H3 and H32. We just came back from APS with > data sets from another three crystals, > for which the ML twin fraction, estimated using > phenix.xtriage with scalepack merged intensities as > input, varied from 0.335, 0.219 and 0.02. The latter is > refining very nicely, in H3 and will not scale in > H32. > > Thanks - Mark > > 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
