>shouldn't the photons interfere in a phase-dependent way, as they do in a
non-twinned crystal
How are the photons supposed to be interacting in a normal crystal -
scattering, again, many threads - is a single photon process.
In 'standard' X-ray scattering - absent of more complicated scenarios not
relevant here - there is no coherence
between photons nor is inter-photon coherence necessary, eliminating any
options of 'phase -related interaction'
between them.
Again, the curse of the dual-beam Bragg picture hits home......
Best, BR
-----Original Message-----
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of
Keller, Jacob
Sent: Monday, August 17, 2015 4:52 PM
To: [log in to unmask]
Subject: [ccp4bb] Twinning Question
Dear Crystallographers,
I am trying to understand twinning as fully as possible, and the following
question has arisen. Imagine a pretty normal twinning case in which there is
one twin operator which allows for two orientations, perhaps of many
crystalline fibers constituting a crystal. Despite the presence of this
randomized orientational variability, however, the lattice is completely
homogeneous, which leads to uniformity of the diffraction pattern, and no
spot-splitting. In what manner, then, do the diffracting photons interfere
with each other? (or with themselves, as some express it)?
I think the prevailing way to think of this is that the intensities simply
add, which tends to make the spots more uniform in intensity, since every
spot is now the twin-fraction-weighted average of two spots. But I wonder:
if these fibers are randomly interspersed within the crystal, shouldn't the
photons interfere in a phase-dependent way, as they do in a non-twinned
crystal, such that the diffraction spots should not be simply the sum of
intensities of two mutually-flipped crystals?
Another way to think of it: shouldn't the opposite-polarity fibers
contribute a second, flipped "conformation" of the protein, with occupancy
equal to the twin fraction? This would be roughly analogous to having an NCS
of 0.5. Is this perhaps what is happening under the hood/bonnet of twin
refinement? If not, could it be?
The phase-dependent way of combining the diffraction should also generally
result in more uniformity of intensities, since the underlying diffracting
object has been averaged to something closer to a sphere, but I think it
would result in a different distribution than just adding intensities. And,
if this way is truer to what is actually happening in a crystal, perhaps it
will be helpful in solving notoriously-difficult twinned structures?
Doubtless in real crystals there is probably some of each type of
diffraction-combination, and different crystals might have variable amounts
of each type.
I would guess that these possibilities would have been addressed, but I have
not come across the sources yet, or the appropriate terminology, being
relatively new to the world of twinning.
All the best, and any pointers in the right direction, through literature or
otherwise, would be appreciated,
Jacob Keller
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Jacob Pearson Keller, PhD
Looger Lab/HHMI Janelia Research Campus
19700 Helix Dr, Ashburn, VA 20147
email: [log in to unmask]
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