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
s...@email.arizona.edu
-----Original Message-----
From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Phil
Jeffrey
Sent: Monday, June 01, 2015 12:24 PM
To: CCP4BB@JISCMAIL.AC.UK
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:CCP4BB@JISCMAIL.AC.UK] On Behalf Of
Paul Paukstelis
Sent: Monday, June 01, 2015 11:21 AM
To: CCP4BB@JISCMAIL.AC.UK
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
