3. Precipitated protein.
In the initial screen:
This is not a good starting point. Protein possibly denatured, judged upon the dark/brown appearence of the precipitate.
Tip: Dilute the drop with water, if the protein does not dissolve, the protein is probably denatured.
In older drops a skin of denatured protein can appear on the surface of the drop.If the surface has a film that is not wrinkled it could be phase separation.