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Re: salting out, deionized H2O,.. (fwd)
Sender: bruce_bush@merck.com (Bruce Bush)
Subject: Re: WSN: salting out, deionized H2O,..
I would welcome discussion of "salting-out" as mediated by HYDROPHOBIC
interactions. Joe Wolfe's comment below applies to colloids or proteins
that have CHARGED surfaces (including distributions of charges of both signs,
even when the total charge is zero). When biochemists observe that protein-
ligand or protein-protein association gets stronger at high salt,
they often ascribe this to stronger interaction among NONPOLAR regions.
In some cases, the binding region is known crystallographically to be nonpolar.
Can anyone say how general this phenomenon is? The explanation offered to me
by biochemists is that salt stabilizes bulk water (versus water at the
solute surface) and thus increases surface tension (the energy penalty
per area of water:solute surface). Association or aggregation tends to
decrease total surface area; hence high salt / high surface tension drives
equilibria toward association. Is this the essential reason? Do real
(macroscopic) surface tensions increase with salt concentration?
Has anyone captured this effect in a computational model? The effect seems
not to be a mean-field effect; fluctuations in water structure or in local
charge concentration may be involved. (Individual ions of the salt are
repelled from the nonpolar, low-dielectric protein region, and vice versa).
-- Bruce Bush
bruce_bush@merck.com
Joe Wolfe replies: To Soaring Bear at Univ Arizona,
regarding your "nice practical question for a theoretical group.
> you might want to consider the area of 'salting out'
> where raising ion level causes proteins to precipitate out
> ... question sounds very much like a standard effect in colloid science.
> The usual situation is this: identical colloidal particles
> have similar charge of several or more units which
> provides a repulsive interaction....
>.. Adding ions to the solution reduces the Debye length
> and allows the particles to approach more closely to
> a range at which thermal fluctuations may bring them
> within the range of the attractive forces. ...
> Joe Wolfe, Physics, University of New South Wales > Sydney 2052 Australia