Charge density-dependent strength of hydration and biological structure

Area of Science:

• Biophysical chemistry
• Biochemistry
• Ion-solvation dynamics

Background:

• Ions are classified as kosmotropes (strong water binding) or chaotropes (weak water binding) based on charge density.
• The heat of solution for alkali halides is exothermic only when kosmotropes and chaotropes form ion pairs.

Purpose of the Study:

• To investigate the relationship between ion hydration enthalpies and the formation of inner sphere ion pairs.
• To propose a model for biological organization based on the noncovalent association of moieties with matching free energies of solution.

Main Methods:

• Analysis of standard heat of solution data for crystalline alkali halides.
• Correlation of ion properties (charge density, hydration enthalpy) with ion pair formation.
• Examination of intracellular ion composition and its implications for solubility.

Main Results:

• Exothermic heat of solution is observed only when a kosmotrope pairs with a chaotrope.
• A greater difference in absolute heats of hydration between cation and anion leads to a more positive heat of solution.
• Inner sphere ion pairs preferentially form between oppositely charged ions with similar absolute hydration enthalpies.

Conclusions:

• Biological organization likely arises from noncovalent associations of molecules with matching free energies of solution.
• Intracellular anions (kosmotropes) and cations (chaotropes) form soluble, solvent-separated ion pairs, maintaining cellular solution homeostasis.