[ccp4bb] 18A rule reference
Hi all, I've had an interesting question from an undergradute student, asking if there's any specific reference for the 18Å rule of thumb in small molecule crystallography for dividing the volume of a unit cell by 18 for the approximate number of non-hydrogen atoms. I'm afraid my google-fu has failed me, but I was curious as to whether there is a reference associated with the practice of if it's just one of those handy little pieces of advice for crystal screening. Thanks, Natalie Natalie J. Tatum http://about.me/tatum.nj PhD Researcher Pohl Group Durham University
Re: [ccp4bb] 18A rule reference
Divide by 11 if you count all atoms, including H-atoms; divide by 20-22 if you only count non-H-atoms, to get the approximately number of atoms in the unit cell. On Mon, May 5, 2014 6:07 am, Natalie Tatum wrote: Hi all, I've had an interesting question from an undergradute student, asking if there's any specific reference for the 18Ã rule of thumb in small molecule crystallography for dividing the volume of a unit cell by 18 for the approximate number of non-hydrogen atoms. I'm afraid my google-fu has failed me, but I was curious as to whether there is a reference associated with the practice of if it's just one of those handy little pieces of advice for crystal screening. Thanks, Natalie
Re: [ccp4bb] 18A rule reference
Hi Natalie Try - P. Román, C. Guzmán-Miralles and A. Luque, Acta Cryst. (1993). B49, 383-386[ doi:10.1107/S0108768192007201 ] Estimate of a relationship between the number of atoms and the volumes of the unit cells of organic compounds references therein On 5 May 2014, at 12:07, Natalie Tatum wrote: Hi all, I've had an interesting question from an undergradute student, asking if there's any specific reference for the 18Å rule of thumb in small molecule crystallography for dividing the volume of a unit cell by 18 for the approximate number of non-hydrogen atoms. I'm afraid my google-fu has failed me, but I was curious as to whether there is a reference associated with the practice of if it's just one of those handy little pieces of advice for crystal screening. Thanks, Natalie Natalie J. Tatum http://about.me/tatum.nj PhD Researcher Pohl Group Durham University Harry -- ** note change of address ** Dr Harry Powell, MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge, CB2 0QH
[ccp4bb] Teleology of Inverted Repeat Transporters (and oligomers)
Dear Crystallographers (teleology-haters exempt here), Does anyone know of any references discussing teleology of inverted repeats in transporters, i.e., what design sense does it make to use this architecture, why is it so common even in the absence of sequence similarity? Is there some underlying feature of this general design principle that helps transporters work well? I suspect someone has discussed it somewhere? Similarly, generally, with regard to oligomers, which are so common: what's the advantage? Isn't it just as likely to generate a protein-protein interface of proteins AB as making an AA interface? Or perhaps it's not? It just seems to me that oligomerization is way too over-represented in the structural world to be by chance. All the best, Jacob *** Jacob Pearson Keller, PhD Looger Lab/HHMI Janelia Farms Research Campus 19700 Helix Dr, Ashburn, VA 20147 email: kell...@janelia.hhmi.org ***
[ccp4bb] 18A rule reference
Thanks all for the links, everyone. A couple of interesting reads. Hi Natalie Try - P. Román, C. Guzmán-Miralles and A. Luque, Acta Cryst. (1993). B49, 383-386[ doi:10.1107/S0108768192007201 ] Estimate of a relationship between the number of atoms and the volumes of the unit cells of organic compounds references therein On 5 May 2014, at 12:07, Natalie Tatum wrote: Hi all, I've had an interesting question from an undergradute student, asking if there's any specific reference for the 18Å rule of thumb in small molecule crystallography for dividing the volume of a unit cell by 18 for the approximate number of non-hydrogen atoms. I'm afraid my google-fu has failed me, but I was curious as to whether there is a reference associated with the practice of if it's just one of those handy little pieces of advice for crystal screening. Thanks, Natalie Natalie J. Tatum http://about.me/tatum.nj PhD Researcher Pohl Group Durham University Harry -- ** note change of address ** Dr Harry Powell, MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge, CB2 0QH
