Which of the following is used to determine the sequence of a polypeptide via successive rounds of N-terminal modification, cleavage, and identification?

The process referred to in the question involves systematically determining the sequence of a polypeptide by modifying, cleaving, and identifying its N-terminal amino acids. Edman degradation is specifically designed for this purpose. It employs a method where phenyl isothiocyanate reacts with the N-terminal amino group of the polypeptide, allowing the first amino acid to be cleaved off. This amino acid can then be identified through chromatographic techniques. The process is repeated for successive amino acids, providing a sequential determination of the polypeptide chain.

Other techniques mentioned, like mass spectrometry, can determine molecular weights and aid in sequencing but do not directly perform the modifications and cleavage in the same manner as Edman degradation. SDS-PAGE is primarily used to separate proteins based on their size, and Western blotting is typically used to detect specific proteins after separation. Neither of these techniques provides detailed sequential information of the peptide chain. Thus, Edman degradation is indeed the correct choice for determining the sequence of a polypeptide as described in the question.