FIGURE 2-14 The 20 common amino acids used to build proteins. The side chain (R group; red) determines the characteristic properties of each amino acid and is the basis for grouping amino acids into three main categories: hydrophobic, hydrophilic, and special. Shown are the ionized forms that exist at the pH (∼7) of the cytosol. In parentheses are the three-letter and one-letter abbreviations for each amino acid.