A zwitterion is a molecule that carries both a positive and a negative electrical charge simultaneously within its structure. Although the molecule possesses these internal charges, they are perfectly balanced, resulting in a net electrical charge of zero. This internal charge separation creates a highly polarized molecule, differentiating it from a simple uncharged molecule.
The Chemistry of Dual Charge
The formation of a zwitterion requires the presence of two distinct functional groups on the same molecule: an acidic group and a basic group. The acidic group acts as a proton donor, and the basic group acts as a proton acceptor, allowing for an internal acid-base reaction. This process involves the transfer of a proton (a positively charged hydrogen ion, \(text{H}^+\)) from the acidic site to the basic site within the molecule.
When this internal proton transfer occurs, the original acidic group becomes negatively charged, and the original basic group simultaneously becomes positively charged. For example, a carboxylic acid (\(text{COOH}\)) group donates a proton to become a negatively charged carboxylate (\(text{COO}^-\)) group. Concurrently, an amine (\(text{NH}_2\)) group accepts that proton to become a positively charged ammonium (\(text{NH}_3^+\)) group. The presence of these two opposite charges is why zwitterions are sometimes referred to as “inner salts” or dipolar ions.
Amino Acids as Prime Examples
The most recognized biological examples of zwitterions are the alpha-amino acids, which are the building blocks of proteins. An amino acid structure features both a carboxylic acid group and an amine group attached to the central alpha-carbon atom. These two groups are precisely the required acidic and basic components for zwitterion formation.
At physiological pH, which is typically around 7.4, the intramolecular acid-base reaction is highly favored, causing the amino acid to exist in its zwitterionic form. The carboxylic acid group is deprotonated to form the carboxylate anion (\(text{COO}^-\)), while the amine group is protonated to form the ammonium cation (\(text{NH}_3^+\)). This zwitterionic structure, \(text{NH}_3^+-text{CHR}-text{COO}^-\), is the dominant form of amino acids in aqueous solutions.
Unique Physical and Chemical Properties
The dual-charge structure of zwitterions results in unique physical properties. Because they are essentially ionic compounds, zwitterions exhibit strong intermolecular electrostatic forces between adjacent molecules in the solid state. These strong attractions necessitate a large amount of energy to break the lattice structure, which is why zwitterions are typically crystalline solids with significantly higher melting points, often exceeding \(200^circtext{C}\). For instance, the zwitterion alanine melts at \(297^circtext{C}\), whereas its non-zwitterionic structural analogs have much lower melting points.
Zwitterions are highly soluble in polar solvents, especially water, because the charged regions allow for strong hydrogen bonding and electrostatic interactions with water molecules. Conversely, their ionic character makes them insoluble in non-polar organic solvents. Zwitterions are also amphoteric, meaning they can react as both an acid and a base, depending on the surrounding pH. This property is central to their function as effective buffers, which resist changes in solution \(text{pH}\). The \(text{pH}\) at which a zwitterion has a net zero charge is known as its isoelectric point (\(text{pI}\)), and zwitterions are particularly effective at buffering in the region around this \(text{pI}\).
Zwitterions in Everyday Materials
The dual-charge structure of zwitterions has led to their widespread application in science, industry, and consumer products. One prominent application is in biological research, where zwitterionic compounds are used to create high-performance biological buffers, such as the family known as Good’s buffers. Compounds like HEPES are zwitterionic and maintain a neutral \(text{pH}\) range that is stable and suitable for cell culture and biochemical assays. Their non-toxic nature and minimal interference with biological processes make them reliable tools for maintaining \(text{pH}\) balance in laboratory environments.
Zwitterions function effectively as surfactants and detergents, particularly in the form of betaines, such as cocamidopropyl betaine. These molecules contain a positively charged quaternary ammonium group and a negatively charged carboxylate or sulfonate group. Zwitterionic surfactants are prized in shampoos and cosmetics because they are mild, stable across a wide \(text{pH}\) range, and are compatible with both anionic and cationic ingredients, reducing irritation while still providing excellent cleaning properties.
In advanced materials science, zwitterionic polymers are being developed for applications requiring exceptional biocompatibility. These polymers feature repeating zwitterionic units along their chains, are highly hydrophilic, and attract a strongly bound layer of water molecules. This tightly bound hydration layer acts as a barrier that physically prevents proteins, cells, and bacteria from adhering to the surface. This resistance to fouling makes zwitterionic coatings desirable for medical implants, contact lenses, and drug delivery systems, where compatibility with the body’s environment is paramount.