Dear Paul, could you check if the crystals are optically active and find out which way they rotate the polarisation of light? I am not sure how to deduce the screw direction from this, I would first try to determine an angle...
Best, Tim On Mon, Jun 01, 2015 at 04:41:41PM -0400, Paul Paukstelis wrote: > All the crystals are unipyramidal, regardless of how they are grown. > The crystal habit seems to be dictated by the arrangement of the > molecules in the unit cell. The structure is solved, so this is > really an exercise in trying to figure out if what we see in the > crystal packing can explain the morphology. > > --paul > > On 06/01/2015 04:25 PM, Edward A. Berry wrote: > >Is it possible to distinguish P64 from P62 without having > >basically solved the structure? > > > >Given that it is P64, is the +ive direction of the c axis > >arbitrary? A left-hand helix is left hand either way you point it, > >and the molecules in the asymmetric units could be pointing the > >opposite way. > >Clearly the lattice is symmetric, so you could start out with > >either orientation and end up solving the structure. Phaser offers > >to try both enantiomers, but not to try with rotating the lattice > >to reverse the c axis in case you got that wrong. > > > >I too would first guess that the truncated base is where it ran > >into the glass, and the flat tip > >may be where it hit the air-water interface; the basic morphology > >being hexagonal bipyramid. > >Unless there are dozens of crystals with the same habit. > > > >eab > > > >On 06/01/2015 03:45 PM, Roberts, Sue A - (suer) wrote: > >>Can't you break the ambiguity if there is significant anomalous > >>signal? So, you'd need to have collected data from a protein > >>crystal with SeMet, or a heavy atom, or near the S edge, and > >>know the orientation of the crystal w.r.t. the direction of the > >>unique crystal axis (for instance from face indexing.) > >> > >>Sue > >> > >>Dr. Sue A. Roberts > >>Dept. of Chemistry and Biochemistry > >>University of Arizona > >>1306 E. University Blvd, Tucson, AZ 85721 > >>Phone: 520 621 4168 > >>[email protected] > >> > >> > >>-----Original Message----- > >>From: CCP4 bulletin board [mailto:[email protected]] On > >>Behalf Of Phil Jeffrey > >>Sent: Monday, June 01, 2015 12:24 PM > >>To: [email protected] > >>Subject: Re: [ccp4bb] crystal habit/morphology and the > >>relationship to unit cell contents > >> > >>I would have thought that what the indexing routine defined as > >>[001] vs [00-1] would be essentially random as one would obtain > >>the equivalent indexing in 622 in both "up" and "down" alignment > >>of the crystallographic a/b/c axes with respect to crystal > >>morphology. > >> > >>Phil Jeffrey > >>Princeton > >> > >> > >> > >>On 6/1/15 1:44 PM, Scott Lovell wrote: > >>>Hi Paul, > >>> > >>>If you have access to diffractometer equipped with a 4-circle > >>>goniometer, you should be able to index the faces of the > >>>crystals. All you need to do is collect some images to index > >>>the lattice and determine the orientation matrix. Most > >>>instruments have software that allows one to then orient > >>>specific faces or crystallographic directions relative to > >>>various directions of the instrument (eg. camera, phi axis, > >>>direct beam, etc). So after indexing, you could then orient > >>>the [001] direction of the crystal towards the camera to > >>>determine if this is the "top" or the "base". You can also > >>>determine the direction of the a/b axes [100] and [010] > >>>relative to the crystal and index the other faces. If you can > >>>also measure the interfacial angles, this may help you to > >>>confirm the indices. > >>> > >>>If you do this for a number of samples, is the "top" face > >>>always the [001] direction or is it the [00-1] direction for > >>>other crystals? Assuming that you are growing these crystals > >>>by hanging drop, my guess is that the "base" is in contact > >>>with the coverslip during growth and you observe this half > >>>pyramid habit. If you were to grow the crystals using the > >>>floating drop method, to prevent contact with the plate > >>>materials, would the crystals form a bipyramidal habit? Or do > >>>you see crystals in the current drop that have the same habit > >>>but are not in contact with the plate materials? > >>> > >>>Scott > >>> > >>> > >>>-----Original Message----- > >>>From: CCP4 bulletin board [mailto:[email protected]] On Behalf Of > >>>Paul Paukstelis > >>>Sent: Monday, June 01, 2015 11:21 AM > >>>To: [email protected] > >>>Subject: [ccp4bb] crystal habit/morphology and the relationship to > >>>unit cell contents > >>> > >>>I'm interested in knowing how to figure out the relationship > >>>between the unit cell contents and the crystal habit in these > >>>crystals (small attachment, two roughly orthogonal views). > >>> > >>>Space group is P64 (enantiomeric) , and you can clearly see > >>>the six-fold. The question becomes how to determine which > >>>direction the screw axis is going with respect to "top" and > >>>the "base" of the pyramidal crystals (right image) so I can > >>>gauge how/why the crystals grow this way based on the cell > >>>contents. > >>> > >>>Thanks in advance. > >>> > >>>--paul > >>> > -- -- Dr Tim Gruene Institut fuer anorganische Chemie Tammannstr. 4 D-37077 Goettingen phone: +49 (0)551 39 22149 GPG Key ID = A46BEE1A
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