Final answer:
Interactions between side chains of amino acids are crucial for the tertiary level of protein structure, where various chemical interactions determine the three-dimensional shape essential for protein function.
Step-by-step explanation:
The interactions between the side chains (R groups) of amino acids are most important at the tertiary level of protein structure. The tertiary structure is the large-scale three-dimensional shape of a single polypeptide chain and is determined by a variety of chemical interactions. These include hydrophobic interactions, ionic bonding, hydrogen bonding, and disulfide linkages, all of which contribute to the protein's final three-dimensional shape that is crucial for its function. When proteins fold, the hydrophobic R groups tend to be tucked inside the protein, away from water, while the hydrophilic R groups are exposed to the aqueous environment, facilitating interactions that stabilize the protein's structure. Additionally, covalent disulfide bonds can form between cysteine side chains, further stabilizing the tertiary structure.