Although most of the known amino acids were identified and isolated (sometimes in impure form) during the nineteenth century, the chemical structures of many of them were not known until much later. Understanding their importance in the formation of proteins, the basis of the structure and function of all cells is of even more recent origin, dating to the first part of the twentieth century. Only about 20 amino acids are common in humans, with two others present in a few animal species. There are over 100 other lesser known ones that are found mostly in plants.
Each of the common amino acids has, in addition to its chemical name, a more familiar name and a three-letter abbreviation that is frequently used to identify it. They are often grouped by similarities in the chemical properties of the side chains. The side chain of glycine (gly) consists of a single hydrogen atom; alanine (ala), valine (val), leucine (leu), and isoleucine (ile) all have hydrocarbon (containing only hydrogen and carbon) side chains; proline (pro) has a hydrocarbon that is part of a ring structure; serine (ser) and threonine (thr) have an alcohol (-OH) side chain; cysteine (cys) and methionine (met) both have sulfur atoms as part of the -R group; phenlyalanine (phe), tyrosine (tyr), and tryptophan (trp) all contain an aromatic ring (related to the benzene ring) as part of the side chain; aspartic acid (asp) and glutamic acid (glu) have a second carboxylic acid group while asparagine (asn) and glutamine (gln) have a carboxylic acid derivative (a -CONH2) group; and lysine (lys), arginine (arg), and histidine (his) have an -R group that contains a second amino group.
Although amino acid molecules contain an amino group and a carboxyl group, certain chemical properties are not consistent with this structure. Unlike the behavior of molecules with amino or carboxylic acid functional groups alone, amino acids exist mostly as crystalline solids that decompose rather than melt at temperatures over 392°F (200°C). They are quite soluble in water but insoluble in non-polar solvents like benzene or ether. Their acidic and basic properties are exceptionally weak for molecules that contain an acid carboxyl group and a basic amino group.
This problem was resolved when it was realized that amino acids are better represented as dipolar ions, sometimes called zwitterions (from the German, meaning hybrid ions). Although the molecule as a whole does not have a net charge, there is a transfer of an H+ ion from the carboxyl group to the amino group; consequently, the amino group is present as an -NH3+ and the carboxyl group is present as a -COO- (Fig. 1). This reaction is an acid-base interaction between two groups in the same molecule and occurs because the -COOH group is a rather strong acid and the -NH2 group is a rather strong base. As a result of this structure, amino acids can behave as acids in the presence of strong bases or they can behave as bases in the presence of strong acids.
One other property of amino acids that is important to their chemical behavior is that all of the amino acids except glycine can exist as mirror images of each other; that is, right- or left-handed versions of the molecule. Like the positions of the thumb and fingers of a glove, the right hand being the mirror image of the left hand, the positions of the functional groups on the a carbon can be mirror images of each other. Interestingly, nearly all of the amino acids occurring in nature are the left-handed versions of the molecules. Right-handed versions are not found in the proteins of higher organisms but are present in some lower forms of life such as in the cell walls of bacteria. They are also found in some antibiotics such as streptomycin, actinomycin, bacitracin, and tetracycline. These antibiotics can kill bacterial cells by interfering with the formation of protein necessary for maintaining life and for reproducing.