1P-31 PDF
Intra-molecular proton abstraction in negative-ion CID of peptides
In collision-induced dissociation (CID) experiments of peptides, the formation of c-ions originating from N-Cα bond cleavage of the peptide backbone is unusual, whereas b- and y-ions are often observed in positive-ion CID spectra. Here we report that characteristic product c-ions can be observed in negative-ion CID spectra of peptides with Asp/Asn residues and those lacking any acidic amino acid residues. The negative c-ions are mainly generated at the N-Cα bond of Xxx-Asp and Xxx-Asn residues that are classified as flexible amino acids. In the absence of any acidic amino acid residues, negative-ion CID spectra show dominant c-ion peaks originating from cleavage at the N-Cα bond lying in between the 3rd and 4th residues from the C-terminus of analyte peptides. The c-ion formation can be explained by an intra-molecular proton abstraction from the backbone amide regions due to negatively charged acidic functional groups.