I just wanted to disagree with Roger's word choice, but not his argument (this is a "flame"-free response). Forget about "packing" and "packable" as there is no outside force doing the work. The molecules are just falling into a local energy minimum where favorable intra- and intermolecular interactions predominate. It is difference in the behavior of the ensemble versus of a solubilized, dispersed species (be it monomer or dimer). It is a phase behavior issue. Concerning Sebastian's case, while it is uncommon, the idea that a monomer has a crystalline phase state while the dimer does not is perfectly reasonable, and the crystals of the monomer grow due to mass action. I am sure the number of verified examples of this are limited. However, there are many cases where dimeric and tetrameric enzymes can be shown to be fully saturated with one or another bound substrate in solution, but show one or more empty active sites in the crystal. I know of several cases where this occurs, showing that selection of the species with the best set of favorable intra- and intermolecular interactions occurs.
Regards, Michael **************************************************************** R. Michael Garavito, Ph.D. Professor of Biochemistry & Molecular Biology 603 Wilson Rd., Rm. 513 Michigan State University East Lansing, MI 48824-1319 Office: (517) 355-9724 Lab: (517) 353-9125 FAX: (517) 353-9334 Email: rmgarav...@gmail.com **************************************************************** On Apr 8, 2015, at 9:28 AM, Roger Rowlett <rrowl...@colgate.edu> wrote: > The problem with crystallization is that is selects for the least soluble, > most packable species. Sometimes that works against what you would like to > know. That could include oligomerization state as well as conformational > state. For example, some of the allosteric carbonic anhydrases stubbornly > crystallize only in the T-state, despite crystallization conditions that are > known to preferentially stabilize the R-state, and for which the predominant > R-state population can be confirmed by other methods. > > Cheers, > > _______________________________________ > Roger S. Rowlett > Gordon & Dorothy Kline Professor > Department of Chemistry > Colgate University > 13 Oak Drive > Hamilton, NY 13346 > > tel: (315)-228-7245 > ofc: (315)-228-7395 > fax: (315)-228-7935 > email: rrowl...@colgate.edu > > On 4/8/2015 9:07 AM, Sebastiaan Werten wrote: >> Dear all, >> >> we are currently working on a protein that is known to exist in a >> monomer-dimer equilibrium. At the high concentrations used for >> crystallisation assays, the dimer is predominant and the monomer practically >> undetectable. >> >> Nevertheless, one of the crystal forms that we have obtained contains the >> monomeric species, not the dimer. >> >> I was wondering if anyone is aware of similar (published) cases, and if the >> phenomenon as such has been discussed in detail anywhere? >> >> I did literature searches but so far couldn't find anything. >> >> Any pointers would be much appreciated! >> >> Best wishes, >> >> Sebastiaan Werten. >> >
