In a quaternary protein structure, what type of interaction would you most likely expect between the R groups of the following pair of amino acids: lysine and aspartic acid?

• A. Hydrogen bonding due to their ability to form polar interactions at physiological pH
• B. Covalent bonding via a disulfide bridge between the side chains
• C. Ionic bonding due to electrostatic attraction between their charged R groups
• D. Hydrophobic interactions because of their nonpolar R groups

Answer: (C)