FACILE ASSIGNMENT OF SEQUENCE IONS OF A PEPTIDE LABELED WITH O-18 AT THE CARBOXYL TERMINUS

The combination of collision-induced dissociation (CID) and linked-scan analysis was used for analysing the sequence ions from the precursor ion of a peptide, which had been labelled with O-18 at its carboxyl terminus (C-terminus) using 40 atom % (H2O)-O-18. The CID and linked-scan mass spectrum of the labelled peptide gave two series of sequence-ion signals: the one, originating from the C-terminus of the labelled peptide, showed a doublet signal due to the part-incorporation of O-18 into the carboxyl group at the C-terminus, while the other, originating from the amino terminus (N-terminus), has the natural isotopic ion distribution. From the distribution of the isotopic ions in a single CID spectrum, the sequence ions containing the C-terminus could be readily differentiated from those containing the N-terminus, allowing the facile assignment of sequence ions to the amino-acid sequence of a peptide by CID and linked-scan analysis. This method was successfully applied to determination of the amino-acid sequence of the light-chain of mouse anti-porphyrin monoclonal antibody.