> A protein would only scatter but not diffract
or Diffract but not scatter? isn't diffraction a kind of scattering?
But yes, the atoms in the unit cell may seem random in that
distance range (in fact this is assumed in wilson scattering)
but in a perfect crystal they will be the same in each unit cell.
But the scattering rom each unit cell will be systematically out
of phase, so the resultant vector will go round and round the
argand diagram without adding up to anything measureable
(Fig 4.15 in Drenth 1st edition), unless the Laue conditions
are met, and this leads to diffraction spots.
Now if the crystal is not perfect, each unit cell won't be the
same in hi-res details, cancelation will be imperfect, and this may
be related to the "diffuse scatter" which can result from certain
types of disorder.
Still I would expect to see peaks in a wilson plot around bond-length
resolution, similar to the peaks due to secondary structure at lower
resolution.
ed
Tim Gruene wrote:
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Dear Jacob,
A protein would only scatter but not diffract, the latter - in my
understanding - being the result of constructive interference from a
regular array of unit cells .
A powder pattern is the superposition of many small crystals amongst
which you don't observe interference.
Tim
On 05/09/12 16:16, Jacob Keller wrote:
Dear Crystallographers,
the "saxs on crystals" thread reminded me of a question I have had
for a while, and never having collected data better than ~1.6 Ang
or so, cannot answer myself from experience: I would think that
there might be powder-like diffraction rings at distances
corresponding to the various covalent bond lengths in proteins
(1.2-1.5 Ang), but have never heard of such. My thinking is that
the protein itself is essentially a powder sample within the unit
cell consisting of many small, randomly-oriented molecules (amino
acids) with their covalent bonds. Do the rings in fact exist, and
if not, why not? Maybe the electron density is not as "atomic," or
discrete, as the nuclei are? I wonder whether generally data
collected to beyond ~1 Ang have an intensity "bump" at those
covalent bond lengths, as I believe is seen in nucleic
acid-containing structures at the base-stacking distance (at the
right orientation)?
Jacob
- --
- --
Dr Tim Gruene
Institut fuer anorganische Chemie
Tammannstr. 4
D-37077 Goettingen
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