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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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