[ccp4bb]: Crystal packing

[Lan Xu <xulan@wistar.upenn.edu>]

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Hi, all. I have a question regarding crystal packing and protein contact.
I have solved the structure of a receptor-binding protein in two 
different space groups. Form I is P2221, Form II is C2221, both have one 
molecule in an asymmetric unit. Recently, we used molecular replacement 
to solve the structure for the third crystal form (Form III). It belongs 
to C2221, but has two molecules in one asymmetric unit.
When we examine the crystal packing of these three crystal forms, the 
same dimer is present in all three crystal forms. However, all the 
biochemical and biophysical evidence suggest that the protein is a 
monomer in solution.
A buried surface per subunit in the dimer is ~1206 Å2, which represents 
~5.5% of the surface area of the monomer.

My questions are how do we distinguish between a biological dimer or a 
dimer because of crystal packing? Whether a buried surface area of only 
5.5% of the total surface area is stable for a dimeric interaction? Is 
there any reference that I can refer to?

Thanks.

Lan Xu