I'm trying to crystallize a small, soluble part of a protein (~15kDa, 152AA). I did some standard screens (Crystal Screen I & II + Index screen) with a protein concentration of 25 or 45mg/mL in an 1:1 (0.75µL)96 well set up. In most of the conditions I got phase separation (mostly PEG conditions)! Precipitation was formed in conditions with salt. I did not have phase separation with the control (buffer only, see below). For so far I know my protein was soluble up to a concentration of 60mg/mL (I didn't went higher). Its predicted to have a lot of beta-strands (according to CD-spectra and secondary structure predictions). 

- What is the molecular basis of phase separation? I mean what is going on at molecular level? I would suspect that my protein is not soluble in a PEG environment, is this correct?

- What can I do to prevent my protein or buffer (?) going into phases? Is it temperature dependent? Are there additives I can add? Do I need to lower the salt concentration? 

- Are there examples (some of your personal experience) where phase separation was a good thing? 

For your record: the protein is in a 150mM NaCl, 20mM HEPES pH7.5 buffer and the pI is 5-6. It is cloned with a his-tag (but cleaving the his-tag didn't change much). 

Best Regards, 

Ruben