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You did not indicate the four possible space groups. I am assuming that
the systematic absences in all data sets were inconclusive. Given that
you are using MIR - often one can limit the possibilities by looking at
the Harker sections of difference Patterson (Fph-Fp)^2 maps. For
instance, if you are working in a primitive orthorhombic space group
(P2x2x2x) - there are eight possibilities - but the Harker sections are
different for each one.
If memory serves, there are a few space groups that you might be left with
two choices - P31 vs P32 I think - but you could limit the number of
choices.
Then, if you still are not sure, you could look up the frequency that each
of your space groups show up in the PDB. This does not mean that you will
always get the correct answer - but if I had to guess - but if one of
your space group choices have neve been seen with a protein, then it is
highly unlikely that you have it. (of course if you do - that would be
nifty).
Good luck,
Ezra
