I'm trying to understand more about nucleation work, since I have no background about physic chemistry or even not much chemistry. But this concept came out many times during my way to know more information, and I can not find directly details from internet. Best regards.
Polyethylene glycol (PEG) can promote the precipitation of some proteins by excluding their molecules from accessing to a significant volume of water, since a large number of water molecules interact strongly with each hydrophilic macromolecule of PEG. Eventually that may bring the protein to precipitate, as its solubility limit is attained or exceeded. Often, depending on the solution composition and temperature, aqueous two-phase separation may occur concomitantly with protein precipitation from a protein-rich aqueous phase, from which a PEG-rich aqueous phase uptakes water.
You may find useful refer to: D.H. Atha, K.C. Ingham, "Mechanism of precipitation of proteins by polyethylene glycols", J. Biol. Chem. 256(23) 1981, 12108–12117; http://www.jbc.org/content/256/23/12108.long
Carlos Araújo Queiroz. Thanks for your information. For me, I just know the different mechanism that from both ammonium sulfate and PEG for protein precipitation. Seems like not long time that people try to apply the "depletion mechanism" to explain protein nucleation. I read one paper that elucidate the molecular level nucleation mechanism, it's necessary to know both ammonium sulfate and PEG mechanism first. Here is the paper.
Van Driessche, Alexander ES, et al. "Molecular nucleation mechanisms and control strategies for crystal polymorph selection." Nature 556.7699 (2018): 89.
https://www.nature.com/articles/nature25971
Meng, you are right. Depletion has been used to explain protein crystallization by PEG addition. The paper that Carlos cited is rather old. It was written before depletion was well understood.
In the end, optimizing protein crystallization is about optimizing the second virial coefficient of the protein molecules, whether by salt or polymer. Here's one relevant open access paper that I read recently: A. Saluja et al., "Diffusion and Sedimentation Interaction Parameters for Measuring the Second Virial Coefficient and Their Utility as Predictors of Protein Aggregation", Biophysical Journal Volume 99 October 2010 2657–2665. I would also recommend "Depletion interactions and protein crystallization" by Chuck Zucoski in J. Crystal Growth. You can find more references by Googling "Protein crystallization depletion".
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