Peptide Links

Amino acids join together by condensation reactions. The -COOH group of one amino acid reacts with the -NH₂ group of another, a molecule of water is eliminated, and an amide link, -CONH-, forms. In proteins this is called a peptide link.H₂NCH₂COOH + H₂NCH(CH₃)COOH → H₂NCH₂CONHCH(CH₃)COOH + H₂OTwo amino acids joined by one peptide link form a dipeptide; three form a tripeptide. Long chains are polypeptides, and a protein is made of one or more polypeptide chains.A dipeptide still has a free -NH₂ group at one end and a free -COOH group at the other, so more amino acids can be added at either end.With two different amino acids there are two possible dipeptides, because either one can provide the -COOH. Glycine and alanine can give Gly-Ala (the equation above, using glycine's -COOH) or Ala-Gly, H₂NCH(CH₃)CONHCH₂COOH. These are different compounds.Worked example. How many different tripeptides can be made using one molecule each of three different amino acids? The first position can be any of 3, the second any of the remaining 2 and the third the last one: 3 × 2 × 1 = 6.A tripeptide contains two peptide links, and forming it eliminates two molecules of water. In general, a chain of n amino acids has (n − 1) peptide links.