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Some non-ionic polymers (e.g. chitosan and polyethylene glycol) are also commonly used as protein precipitating agents. What is the principle of protein precipitation resulting from the addition of these polymers? Is this type of precipitation reaction a reversible or non-reversible precipitation?

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  • $\begingroup$ Perhaps a bit too technical, but would this answer your question? At least in part? $\endgroup$
    – S Pr
    Oct 13, 2021 at 14:25
  • $\begingroup$ Two interpretations of such precipitation events have been proposeda while ago according to the paper above: attractive depletion (Asakura & Oosawa, 1958) and excluded volume (Iverius and Laurent, 1967, Polson, 1977). These are discussed there too! $\endgroup$
    – S Pr
    Oct 13, 2021 at 14:28

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Protein precipitation by these non-ionic polymer is based on the principle of steric hindrance. These polymers compete with proteins (Mahadevan, 1992) for space in the solvent. Congestion caused by the physical presence of surrounding polymers force the proteins to exclude from region occupied by the polymers and concentrate together until their solubility is exceeded (Atha & Ingham, 1981). In this case, protein precipitation occurs.

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    $\begingroup$ "Steric hindrance" is usually invoked in the context of molecular shape as it relates to chemical or enzymatic reactions, i.e. how the relative organization of atoms in a molecule constrains the shape space of that molecule. I'm not sure if it's the right term to use in discussion of this system, but I'm open to differing opinions. $\endgroup$
    – acvill
    Oct 14, 2021 at 15:20
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    $\begingroup$ Moreover, your answer could be improved by replacing some of your terms with more concrete explanations of the physical interactions. Re: what electrostatic phenomena underlie polymers competing with proteins and congesting the solvent space? There is also a minor logical jump from exceeding solubility to protein precipitation that could be addressed. $\endgroup$
    – acvill
    Oct 14, 2021 at 16:11
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    $\begingroup$ @acvill - yeah, this is mechanism is molecular crowding, not steric hindrance $\endgroup$
    – bob1
    Dec 5, 2021 at 22:49

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