Why might it be useful to use the R-group in the classification of amino acids?
Amino acids are the building blocks to proteins. Proteins are imperative to the structure and function of many parts of organisms. There are 20 organic amino acids. All amino acids contain a carboxyl group and an amino group. They are distinguished by a third group called the "R-group". The R-group is what makes one amino acid different from another. For example, of the 20 amino acids, 6 contain R-groups made of hydrocarbons, 7 have neutral R-groups, and 6 have R-groups that act as acids and bases. These side chains contain different structural features that result in differing reactivities.
The features of the R-groups also result in their unique physical properties such as state, solubility, mechanical strength, etc. These properties affect how the peptide chains fold to form proteins. Thus, these properties affect the shape of the protein. The shape of the protein then influences the amino acid's bonding abilities and function within the organism.