What does Edman degradation specifically target when sequencing proteins?

Edman degradation specifically targets the N-terminus of proteins because this method is designed to systematically identify each amino acid in a polypeptide chain starting from each protein's N-terminal end. The process involves chemically reacting the N-terminal residue with phenylisothiocyanate, which forms a cyclic product that can be cleaved to release the N-terminal amino acid in a stable form. This allows for the sequential identification of amino acids as they are removed one at a time.

The choice that indicates the C-terminus is incorrect since Edman degradation does not interact with the end of the protein chain that houses the carboxyl group. The option involving side chains does not align with the method's focus, as it does not analyze the side chains of the amino acids but rather pinpoints the N-terminus in the sequencing process. The notion of targeting the polypeptide backbone is also inaccurate because Edman degradation specifically assesses the terminal residues rather than the overall structure or backbone of the protein chain. Thus, the method's design fundamentally revolves around the N-terminus, validating the selection made in identifying the correct answer.