The amino acid sequence of a 65-residue segment from the second quarter of the heavy chain of guinea pig IgG(2) has been determined. This segment includes a 44-residue cyanogen bromide fragment (C-l-b) and a 28-residue tryptic peptide (T36-37). The tryptic peptide contained two contiguous methionine residues and was shown to extend the amino acid sequence of 72 chain from C-l-b toward the N terminus by 21 residues. T36-37 had a constant sequence and was placed at a position close to the switch point, that position in myeloma proteins N terminal to which heavy-chain subgroups have variable sequences. C-l-b was shown to be joined by an intrachain disulfide bond to another cyanogen bromide fragment, C-l-c.
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