A zwitterion is a chemical molecule possessing an equal number of positively and negatively charged functional groups, resulting in an overall neutral electrical charge. The term “zwitterion” originates from the German word “Zwitter,” which translates to “hybrid” or “hermaphrodite,” aptly describing its dual ionic nature. This internal charge separation distinguishes zwitterions from other types of ions.
The Chemical Nature of a Zwitterion
Zwitterions arise from compounds classified as amphoteric, meaning they inherently possess both acidic and basic functional groups within their structure. For instance, a common acidic group is the carboxylic acid, while a typical basic group is an amine. In an aqueous environment, an intramolecular proton transfer occurs where the acidic group donates a proton to the basic group on the same molecule.
The specific pH at which a molecule exists predominantly in its zwitterionic form is known as its isoelectric point (pI). At pH values below the pI, the molecule tends to gain additional protons, resulting in a net positive charge. Conversely, when the pH is above the pI, the molecule loses protons, leading to a net negative charge.
Zwitterions in Biological Systems
Amino acids serve as prime examples of zwitterions and are fundamental to biological systems. These building blocks of proteins naturally exist in their zwitterionic state at physiological pH, which is typically around 7.4. This internal charge separation is important for protein structure, as it allows amino acids to engage in various electrostatic interactions that stabilize protein conformation.
The zwitterionic nature of amino acids also enables them to function as biological buffers, helping to maintain stable pH levels. They achieve this by readily donating protons to neutralize added bases or accepting protons to counteract added acids, thereby resisting significant changes in pH. This buffering capacity is important for processes requiring specific pH ranges, such as enzyme activity. Furthermore, the charged amino and carboxyl groups of zwitterionic amino acids are directly involved in the formation of peptide bonds, the covalent links that join amino acids together to create polypeptide chains and proteins.
Properties and Practical Applications
The unique ionic structure of zwitterions imparts distinct physical properties. They exhibit high melting points, which is considerably higher than similarly sized neutral organic molecules. This elevated melting point is attributed to the strong ionic attractions between individual zwitterionic molecules in the solid state, which require substantial energy to overcome. Due to their highly polar nature, zwitterions are also highly soluble in polar solvents like water. The strong attractions between water molecules and the charged regions of the zwitterions facilitate their dissolution.
Conversely, zwitterions display poor solubility in nonpolar organic solvents, as there is insufficient attraction between the solvent molecules and the zwitterions to break apart the internal ionic lattice. These properties are leveraged in various practical applications. Zwitterions are widely employed in laboratory settings to maintain stable pH conditions for biochemical experiments. Their distinct charge characteristics also make them useful in separation techniques, such as hydrophilic interaction liquid chromatography (HILIC), where they interact weakly with solutes to facilitate separation. Moreover, zwitterionic materials are increasingly developed for advanced antifouling coatings. Inspired by the cell membranes of living organisms, these coatings resist the unwanted adhesion of proteins, bacteria, and other biological matter, finding use in medical implants and marine coatings for ship hulls.