> cases out there (and so far I have heard of a disulfide bond on a
> 2-fold connecting two homodimers).
I'm slightly puzzled by this example. If the S-S bond is on the
special position, then the rest of the molecule can't have 2-fold
symmetry, so would have to be rotationally disordered with occupancy =
0.5 to avoid clashing with its symmetry mate:
*
X -- C *
\ *
S
|
S
* \
* C -- X
*
where the *'s indicate the 2-fold axis (i.e. vertically in the plane
of the page). In this case, for the reasons I gave in my previous
post there's no reason for the disordered S atoms to be exactly on the
2-fold; it would be pure coincidence if they were. If you mean
instead that the 2-fold is _perpendicular_ to the S-S bond (i.e.
coming straight out of the page in the diagram), the molecule does
indeed have 2-fold symmetry and can be ordered with occupancy = 1, but
then the S atoms are not on special positions, so this would not be an
example of protein atoms _on_ a special position.
One could imagine an example, say where the same side-chain on each
monomer is cross-linked (e.g. LYS with glutaraldehyde), forming the
homodimer:
X -- C -- N = C -- C -- C -- C -- C = N -- C -- X
Here the central C atom could be on a 2-fold (i.e. axis perpendicular
to the page) special position without rotational disorder. I've no
idea whether such a structure actually exists!
Cheers
-- Ian
