A zwitterion is a molecule that carries both a positive and a negative electrical charge within its structure, while remaining electrically neutral overall. This characteristic distinguishes zwitterions from simple ions, which possess a net positive or negative charge. The term “zwitterion” originates from the German word “zwitter,” meaning “hybrid” or “hermaphrodite.” These molecules are also referred to as “inner salts” or “dipolar ions” due to their internal charge separation. Understanding zwitterions helps in comprehending fundamental biological and chemical processes.
What Makes a Zwitterion Unique?
A zwitterion contains both an acidic group and a basic group within the same molecule. An acidic group can donate a proton, while a basic group can accept one. An internal proton transfer occurs, where the acidic part of the molecule donates a proton to its basic part, forming distinct positive and negative charges localized on different atoms or functional groups. Despite these individual charges, the molecule remains electrically neutral overall. This internal charge separation distinguishes zwitterions from simple ionic compounds, and the charges are held together by covalent bonds, contributing to their stability.
Where Do We Find Zwitterions?
Amino acids are common examples of zwitterions, existing predominantly in this form at physiological pH (around 7.4). Each amino acid contains an amino group (basic) and a carboxyl group (acidic). At physiological pH, the carboxyl group loses a proton to become negatively charged (-COO-), while the amino group gains a proton to become positively charged (-NH3+), forming the zwitterionic structure.
Other compounds can also exist as zwitterions. Betaines are a class of zwitterions that maintain their charged state. Examples include sulfamic acid, tricine, bicine, and certain alkaloids like psilocybin. Phosphatidylcholine, a component of cell membranes, is another biological example.
How Zwitterions Behave
Zwitterions exhibit distinct physical and chemical properties due to their internal charge distribution. They have high melting points, behaving more like crystalline solids than simple molecular compounds. This is because strong electrostatic attractions between the oppositely charged groups within and between molecules require significant energy to overcome. These intermolecular forces are much stronger than the weaker forces found in uncharged molecules of similar size.
Zwitterions are highly soluble in polar solvents, especially water. Their positive and negative charges allow them to form strong interactions, such as hydrogen bonds and ion-dipole interactions, with water molecules. This strong interaction with water facilitates their dissolution. While they do not conduct electricity in their solid state, they can in solution due to their ionic character.
Zwitterions are also amphoteric, meaning they can act as both an acid (donating a proton) and a base (accepting a proton) depending on the surrounding pH. This dual reactivity allows them to function as buffers, resisting changes in pH. The pH at which a zwitterion has a net charge of zero is known as its isoelectric point.
The Role of Zwitterions in Everyday Life and Science
Zwitterions are important in biological systems. Amino acids, in their zwitterionic form, are the building blocks of proteins, which are fundamental for cellular processes. The charged groups within zwitterionic amino acids facilitate interactions like hydrogen bonds and ionic bonds, which are important for protein folding and maintaining their three-dimensional structures required for biological functions.
Zwitterions also function as buffers in biological fluids, helping to maintain stable pH levels. For example, the zwitterionic nature of amino acids contributes to buffering capacity in blood, supporting enzyme function and metabolic activities.
In scientific and medical applications, zwitterions are used in drug delivery systems, enhancing drug solubility and improving their ability to cross biological membranes. They are also used in anti-fouling coatings for medical implants, preventing the adhesion of biological matter and improving device safety. Other applications include protein separation techniques, biosensors, and materials for tissue engineering.