In my experience, the success of molecular replacement depends primarily
on the quality of the model. Thus if your model is good, even P1 will
work. Two extreme cases that I encountered were searching with a
monomer for what turned out to be 4 tetramers (thus first search only
accounted for 1/16 of the asu) and getting a solid solution from 40%
complete dataset at 3A-ish resolution with the search model being 25% of
the asu. On the other end of the spectrum, I've seen molecular
replacement fail to find the second copy using the first copy from the
final refined structure. Bottomline: the limit to which MR can be
pushed is inversely proportional to the quality of your model.
Your question is that kind which is easier to answer if you specify what
is that you are trying to accomplish.
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Hurry up, before we all come back to our senses!
Julian, King of Lemurs
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