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
> >>[log in to unmask]
> >>
> >>
> >>-----Original Message-----
> >>From: CCP4 bulletin board [mailto:[log in to unmask]] On
> >>Behalf Of Phil Jeffrey
> >>Sent: Monday, June 01, 2015 12:24 PM
> >>To: [log in to unmask]
> >>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:[log in to unmask]] On Behalf Of
> >>>Paul Paukstelis
> >>>Sent: Monday, June 01, 2015 11:21 AM
> >>>To: [log in to unmask]
> >>>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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