Interferon is a protein.  It is dried into a powder which should be able 
to be frozen with
no problem.  But when proteins are dissolved into a liquid solution, 
freezing can be tricky.
As a scientist working with a variety of proteins, I can tell you that 
many proteins will start
to break down if frozen more than once.  And rapid freezing is preferable 
to slow freezing.
At home, the closest thing to rapid freezing could be done by putting the 
protein tube into ice in the back of
the freezer lowering
the temp more rapidly than if you just stick it on the shelf in the 
freezer.  Proteins also generally
survive freezing better if they are more dilute and freezing in smaller 
portions will allow the
freezing to go faster than freezing in big portions.

If a protein is degraded by freezing, it's structure starts to unwind. 
Some molecules of the
protein in the solution may degrade and others won't so you may still see 
that the protein has
some activity or it may lose all activity.  You might see some precipitate 
in the solution (always
a bad sign) or you might not.  Without some kind of activity readout, it 
seems impossible
to me to know whether freezing is causing a problem.

In summary, I would make up the solution to the appropriate dilution and 
freeze it in the smallest
portions that will work (need appropriate sized containers) in the coldest 
part of the freezer in ice.
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