I have a semantics question, and I know how much this forum loves
discussing semantics.
We've all experienced non-isomorphism, where two crystals, perhaps even
grown from the same drop, yield different data. Different enough so that
merging them makes your overall data quality worse. I'd say that is a
fairly reasonable definition of non-isomorphism? Most of the time unit
cell changes are telling, but in the end it is the I/sigma and
resolution limit that we care about the most.
Now, of course, even for non-isomorphous data sets you can usually
"solve" the non-isomorphous data without actually doing molecular
replacement. All you usually need to do is run pointless using the PDB
file from the first crystal as a reference, and it will re-index the
data to match the model. Then you just do a few cycles of rigid body
and you're off and running. A nice side-effect of this is that all your
PDB files will line up when you load them into coot. No worries about
indexing ambiguities, space group assignment, or origin choice. Phaser
is a great program, but you don't have to run it on everything.
My question is: what about when you DO have to run Phaser to solve that
other crystal from the same drop? What if the space group is the same,
the unit cell is kinda-sorta the same, but the coordinates have moved
enough so as to be outside the radius of convergence of rigid-body
refinement? Does that qualify as a different "crystal form" or
different "crystal habit"? Or is it the same form, and just really
non-isomorphous?
Opinions?
-James Holton
MAD Scientist
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