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Experimental System II: Concentrated Aqueous Solutions
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The Hofmeister Series 
In addition
to studies that examine bulk silica materials, the Eggers laboratory strives to understand how small solutes mediate
their effects on biomolecules. Of special interest are natural osmolytes,
soluble crowding agents, and salt
combinations from the well-known Hofmeister series of ions. The two series below
are ranked such that the ions having the most favorable effects on protein
stability are on the left and those having the most destabilizing effects are on
the right. The underlying basis for
the Hofmeister series has been debated often,
but it seems possible that the order is dictated by the effects of each ion on
the properties of water, though no conclusive evidence exists to support this
idea. One paradox should be mentioned (which is often botched in the
literature): anions of high charge density, i.e. kosmotropes, are
generally favorable for protein structure, whereas cations of high charge
density are unfavorable. In other words, many investigators fail to recognize
that cations of low charge density (chaotropes by definition) are favorable for
protein structure and stability. The term "chaotrope" should not be used
interchangeably with "denaturant," as commonly practiced by biochemists, because
chaotropic cations are generally good!
The Eggers laboratory is currently pursuing a
combination of thermodynamic techniques, including calorimetry, to try and back
out the free energy of bulk water as a function of solute concentration and
identity. We believe this information can be used to estimate the total free
energy contribution of water in many reactions of biological interest, including
protein folding. Historically, most of the thermodynamic
framework for
treating aqueous solutions has been developed by physical chemists who tend
to define their key parameters at infinite dilution. Because the cytoplasm of a
living cell is highly concentrated, we believe that studies aimed at
characterizing concentrated (non-ideal) solutions are a necessary step toward
understanding the underpinnings of cell biology.
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