Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Water
In aqueous solutions, amino acids are ionized

When dissolved in Water, Amino Acids undergo ionization and act as either acids or bases. Understanding the acid-base properties of amino acids is crucial for grasping many of the fundamental characteristics of Proteins. Furthermore, these very properties form The basis of all Separation, identification, and quantitative analysis techniques—essential steps involved in determining the Amino Acid Composition and sequence of proteins.

α-Amino acids containing a single amino group and a single carboxyl group crystallize from neutral aqueous solutions as fully ionized molecules known as dipolar ions or zwitterions (from the German word meaning "hybrid ions") (Fig. 5-9). Although these ions carry electrical charges of opposite sign at their "poles," they are electrically neutral as a whole and therefore do not migrate in an electric field. The hypothesis regarding the dipolar Structure of Amino acids was originally based on the observation that crystalline Amino acids have significantly higher melting points than other organic molecules of comparable size. The crystal lattice of amino acids is stabilized by electrostatic forces of attraction between oppositely charged Functional groups of adjacent molecules, resembling in this respect the robust ionic crystal lattice of NaCl (Sec. 4.3). Melting such an ionic crystal requires overcoming the strong interactions between positive and negative charges, which can only be achieved by heating the crystal to a very high Temperature. In contrast, most simple non-ionic Organic compounds of similar molecular weight melt at relatively low temperatures, consistent with their relatively weaker non-ionic crystal lattices.

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Fig. 5-9. Non-ionic and dipolar (zwitterionic) forms of amino acids. Note that in the dipolar form, the two charges are spatially separated, making the molecule an electrical dipole.