Zwitterions

Zwitterions (the word is derived from the German for "hybrid ion") are ions that are electrically neutral overall but contain nonadjacent regions of positive and negative charges; they are sometimes referred to as "dipolar ions." The best-known examples of zwitterions are the free amino acids found in cells.

An examination of the general structure of an amino acid reveals that there are two parts, or groups, of the molecule that can function as an acid/base pair, the –COOH and –NH₂ groups. At pH values near neutrality, a protein transfer reaction takes place that results in the –COOH becoming –COO⁻ and the –NH₂ becoming –NH₃⁺. A large favorable (stabilizing) electrostatic interaction now develops between these two parts of the molecule. This interaction is favorable enough to shift the equilibrium constant for the proton transfer reaction toward the formation of the charged species, by a factor of between 10– and 50–fold. In addition to the favorable electrostatic interaction between the charged regions, these same charged regions have very favorable electrostatic interactions with surrounding water molecules. Water molecules solvate these regions of the amino acid in a manner very similar to their solvation of cations and anions.

The physical properties of crystalline amino acids are consistent with their existence as zwitterions. Their melting points are relatively high, often above 200°C (392°F), and they are far more soluble in water than in nonpolar solvents such as ether or chloroform. Measured dipole moments for crystalline amino acids are fairly large, reflecting the significant degree of charge separation.