AMINO-ACID-SEQUENCE PREREQUISITES FOR THE FORMATION OF CN IONS

Amino acid sequence prerequisites are described for the formation of c (n) ions observed in high-energy collision-induced decomposition spect ra of peptides. It is shown that the formation of c(n) ions is promote d by the nature of the amino acid C-terminal to the cleavage site. A p ropensity for c(n) cleavage preceding threonine, and to a lesser exten t tryptophan, lysine, and serine, is demonstrated where fragmentation is directed N-terminally at these residues. In addition, the nature of the residue N-terminal to the cleavage site is shown to have little e ffect on c(n) ion formation. A mechanism for c(n) ion formation is pro posed and its applicability to the results observed is discussed.