When the carboxy group of one amino acid and the amino group of another lose a water molecule, the two amino acids are joined by a \mathrm{{-}CO{-}NH{-}} bond. A bond like this between amino acids is called a peptide bond.
The compound formed from two amino acids is a dipeptide, and one formed from three is a tripeptide. A chain of many amino acids joined in this way is called a polypeptide.
Proteins are polypeptides with large numbers of amino acids, usually from about 50 up to a few thousand.
The order of the amino acids in a polypeptide chain is called the primary structure of the protein. It is set by the genetic information in DNA.
Hydrogen bonds between the \mathrm{C{=}O} of one peptide bond and the \mathrm{N{-}H} of another fold parts of the chain into regular shapes. These shapes are called the secondary structure.
In an α-helix, the chain coils into a spiral, with a hydrogen bond between each amino acid and the fourth one along. In a β-sheet, stretched sections of chain lie side by side and are joined by hydrogen bonds.
The whole chain then folds into a particular three-dimensional shape, called the tertiary structure. The shape is held by bonds between side chains, such as disulfide bonds \mathrm{{-}S{-}S{-}} between cysteine units, ionic bonds between \mathrm{{-}COO^-} and \mathrm{{-}NH_3^+}, and hydrogen bonds.
Some proteins are made of several chains fitted together, and this arrangement is called the quaternary structure. Haemoglobin, which carries oxygen in red blood cells, is made of two α chains and two β chains, each holding a haem group that contains iron.
What a protein does depends on its three-dimensional shape, and the shape is decided by the primary structure.
Heat, acids and bases, heavy metal ions and alcohol break the bonds that hold the three-dimensional shape of a protein, and the chain unfolds. This is called denaturation.
A denatured protein usually cannot return to its natural shape, and it loses its function. When egg white is heated, its proteins denature and it turns white and sets.
Disinfecting with alcohol and sterilizing by heat both work partly because they denature the proteins of bacteria.
Adding sodium hydroxide solution and a little copper(II) sulfate solution to a protein solution gives a red-violet colour. This is the biuret reaction, and it is given by compounds with two or more peptide bonds, that is, tripeptides and larger.
Heating a protein with concentrated nitric acid gives a yellow colour, which turns orange-yellow when ammonia is added. This xanthoproteic reaction comes from the nitration of benzene rings in side chains such as that of tyrosine.
Heating a protein that contains sulfur with sodium hydroxide and then adding lead(II) acetate solution gives a black precipitate of lead(II) sulfide, \mathrm{PbS}. This shows the sulfur in side chains such as that of cysteine.