Facile assignment of sequence ions of a peptide labelled with 18O at the carboxyl terminus

Abstract

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 ¹⁸O at its carboxyl terminus (C‐terminus) using 40 atom % H₂¹⁸O. 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 ¹⁸O 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 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.