One of the main goals of this chapter has been to bring us gradually to amino acids, and thus prepare the way for the discussion of proteins in Chapter 22. Amino acids are bifunctional, with an -NH amine group at one end of the molecule and a -COOH acid group at the other. The general expression for an amino acid is






in which -R can be one of many side groups with widely different chemical character. The of the amine group is around 9.5 and that of the carboxyl is approximately 2.2, so at a neutral pH of 7, both ends of the molecule are charged:






This is known as a zwitterion (German: "hybrid ion")