In chemistry, a zwitterion, the word which came from German word ‘zwitter’, meaning hybrid, is a molecule with a positive and a negative electrical charge but with a net charge of zero at different locations within a molecule. Zwitterions are sometimes also known as “inner salts”. The prefix ‘zwi’ means double, similar to old English prefix ‘twi’.
An ion is an atom or molecule with a positive or negative electrical charge. Atoms are composed of positively charged subatomic particles called protons, negatively charged electrons and neutral particles called neutrons. When the number of protons is not equal to the number of electrons, an atom will have a positive or negative charge. Negatively charged atoms or molecules are called anions, whereas positively charged atoms or molecules are known as cations. Acids and bases may exist as ions, or as uncharged molecules. At neutral pH levels, both acidic and basic groups exist as ions.
Due to its electrical charges, a zwitterion will be highly soluble in water and quite soluble in organic solvents. Water molecules are polar because of the electrical differences between hydrogen and oxygen molecules in water; that is, the oxygen atoms in water are attracted to cations and hydrogen atoms in water are attracted to anions. When a zwitter ion is dissolved in water, the hydrogen atoms in water immediately surround the negatively charged group of zwitter ion and the oxygen atoms in water immediately surround the positively charged group of zwitter ion, thereby completely dissolving in water.
Most zwitter ions are ampholytes meaning molecules containing both acidic and basic groups. Amino acids are the well-known examples of zwitterion. These compounds have one ammonium (basic) and one carboxylate (acidic) groups. Amino acids are the building blocks used to make proteins and peptides. The different amino acids have interesting properties because they have a variety of structural parts which result in various polarities and solubilities. The physical properties of amino acids show a “salt-like” behavior. Amino acids are crystalline solids with relatively high melting points and most are quite soluble in water and insoluble in non-polar solvents. In solution, the amino acid molecules appear to have a charge which changes in pH. An intra molecular neutralization reaction leads to a salt-like ion called a zwitter ion. The accepted practice is to show the amino acids in the zwitter ion form.
The zwitter ion has no net charge; there is 1 positive COO- and 1 negative NH3+ charge.
In addition to amino acids, some other common types of zwitterions are buffers (an ionic compound that resist changes in pH), detergents and drugs. The fact that these types of molecules are zwitterions, and also they are soluble in water, makes them so useful. When a zwitterion is stored in solid, dry form like in a pill, it will be in the form of a neutral molecule or a salt. When the zwitter ion is eaten or place in water, it takes on its ionic or charged form and becomes dissolved in water.