As part of the strategy for determination of the complete covalent structure of a human IgA immunoglobulin, 66 peptides were isolated from a thermolysin digest of reduced and carboxymethylated IgA alpha1 chain Bur and were purified. They range in length from 2 to 24 residues. Some of the peptides have been characterized and sequenced in order to supply needed information that was not obtained from the chymotryptic and tryptic peptides. These thermolysin peptides provide much necessary data to

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... oduce a rigorous proof for the primary structure of the human alpha1 chain. The remaining peptides from the thermolysin digest whose amino acid composition and NH2-terminal residues were sufficient to identify them unequivocally have also been assigned in the structure. They supply additional information that helps remove ambiguity in the structure, and they provide useful data about the profile of the peptide bonds that are susceptible to thermolysin digestion.