A peptide is a compound in which α-amino acids are linked together by peptide bonds, and it is also an intermediate product of protein hydrolysis.
Generally, the number of amino acids contained in the peptide is two to nine. According to the number of amino acids in the peptide, the peptide has various different names: a compound obtained by dehydration condensation of two amino acid molecules is called a dipeptide, and the analogy is analogous. Peptides, tetrapeptides, pentapeptides, etc., up to the nonapeptide. Compounds usually dehydrated by 10 to 100 amino acid molecules are called polypeptides. Their molecular weights are less than 10,000 Da (Dalton, Dalton), and they can pass through the semipermeable membrane and are not precipitated by trichloroacetic acid and ammonium sulfate. There are also literatures that refer to peptides consisting of 2 to 10 amino acids as oligopeptides (small peptides); peptides of 10 to 50 amino acids are called peptides; peptides composed of more than 50 amino acids are called proteins, in other words, Protein is sometimes referred to as a polypeptide. Polypeptides, also referred to as peptides, were discovered in the 20th century.