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Abstract
Small ions of high charge density (kosmotropes) bind water molecules strongly, whereas
large monovalent ions of low charge density (chaotropes) bind water molecules weakly
relative to the strength of water-water interactions in bulk solution. The standard
heat of solution of a crystalline alkali halide is shown here to be negative (exothermic)
only when one ion is a kosmotrope and the ion of opposite charge is a chaotrope; this
standard heat of solution is known to become proportionally more positive as the difference
between the absolute heats of hydration of the corresponding gaseous anion and cation
decreases. This suggests that inner sphere ion pairs are preferentially formed between
oppositely charged ions with matching absolute enthalpies of hydration, and that biological
organization arises from the noncovalent association of moieties with matching absolute
free energies of solution, except where free energy is expended to keep them apart.
The major intracellular anions (phosphates and carboxylates) are kosmotropes, whereas
the major intracellular monovalent cations (K+; arg, his, and lys side chains) are
chaotropes; together they form highly soluble, solvent-separated ion pairs that keep
the contents of the cell in solution.