An amino acid is a compound that has both an amino group, \mathrm{{-}NH_2}, and a carboxy group, \mathrm{{-}COOH}. In an α-amino acid, both groups are bonded to the same carbon atom, and the general formula is \mathrm{R{-}CH(NH_2){-}COOH}.
The part R is called the side chain. About 20 kinds of α-amino acids make up the proteins of living things, and they differ only in their side chains. In glycine, the simplest, R is a hydrogen atom.
Glutamic acid, whose side chain has another carboxy group, is an acidic amino acid, and lysine, whose side chain has another amino group, is a basic amino acid. Cysteine has a side chain that contains sulfur.
Except in glycine, the α carbon atom of an α-amino acid is bonded to four different atoms or groups. Such a carbon atom is called an asymmetric carbon atom.
A molecule with an asymmetric carbon atom has two forms that are mirror images of each other and cannot be placed on top of each other. These are called optical isomers, and for amino acids the two forms are called the L form and the D form.
The two forms have almost the same chemical properties, but almost all the amino acids in the proteins of living things are in the L form.
In crystals and in water, the carboxy group of an amino acid passes a hydrogen ion to the amino group, giving \mathrm{H_3N^+{-}CHR{-}COO^-}, which carries both a positive and a negative charge. Such an ion is called a zwitterion.
Because amino acids exist as zwitterions, they dissolve easily in water and melt only at high temperatures, many of them decomposing first.
In acidic solution, the \mathrm{{-}COO^-} group takes a hydrogen ion and the amino acid becomes a cation. In basic solution, the \mathrm{{-}NH_3^+} group loses a hydrogen ion and the amino acid becomes an anion.
The pH at which the positive and negative charges balance overall is called the isoelectric point. For glycine it is about 6.0.
When a voltage is applied across filter paper soaked in a buffer solution, amino acids with an overall positive charge move towards the cathode, and those with an overall negative charge move towards the anode. This is called electrophoresis.
At pH 6, glutamic acid, whose isoelectric point is 3.2, is an anion and moves towards the anode. Lysine, whose isoelectric point is 9.7, is a cation and moves towards the cathode, while glycine, whose isoelectric point is 6.0, hardly moves.
In this way, amino acids can be separated by the differences in their isoelectric points.
When an amino acid solution is warmed with ninhydrin solution, it turns red-violet to blue-violet. This is the ninhydrin reaction. It is a reaction of the amino group, so proteins give it too.
Amino acids are colourless, so after a mixture has been separated by paper chromatography, the paper is sprayed with ninhydrin and warmed to show where each amino acid has moved.
Sweat contains amino acids, so the ninhydrin reaction is also used to reveal fingerprints.