Final answer:
The interaction of nonpolar side chains within the inner edges of two protein sub-units is considered a feature of Quaternary structure, which concerns the arrangement of multiple polypeptide chains in a protein complex.
Step-by-step explanation:
The interactions of nonpolar side chains of amino acids within the inner edges of two protein sub-units are best considered a Quaternary structure feature. The quaternary structure of a protein refers to the arrangement and interaction of multiple polypeptide subunits in a multi-subunit complex. This is distinct from the tertiary structure, which involves the folding of a single polypeptide chain into its three-dimensional shape due to interactions between amino acid side chains. In the context of quaternary structure, nonpolar side chains often participate in hydrophobic interactions to stabilize the overall structure of the protein complex.