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
