Final answer:
The N-terminus of a polypeptide chain refers to the free amino group at the start of the protein sequence, distinguishing it from the C-terminus which has a free carboxyl group at the end.
Step-by-step explanation:
The term N-terminus refers to the presence of a free amino group at the 5′ end of a polypeptide. A polypeptide chain has two ends: the N-terminus, where the amino group (-NH2) of the first amino acid is not participating in a peptide bond, and the C-terminus, where the last amino acid in the chain has a free carboxyl group (-COOH). The N- N-terminus is always written first, indicating the start of the peptide sequence, whereas the C-terminus represents the end. This convention reflects the direction of protein synthesis, which occurs from the N-terminus to the C-terminus. Various reagents such as Sanger's, Edmann's, and Dansyl chloride are used to determine the N-terminal amino acids, and Hydrazine is used to detect the C-terminal amino acid.