Why b, y's? Sodiation-induced tryptic peptide-like fragmentation of non-tryptic peptides

Several examples of this fragmentation pattern are described, thus strongly suggesting that sodium cation may be complexed near or at the C-terminus even in the presence of other acidic residues within the peptide. This effect is especially pronounced in the case of the doubly charged non-tryptic peptides. This controlled modulation of the fragmentation behavior of non-tryptic peptides is shown to be advantageous for the de novo sequencing of non-tryptic bioactive peptides as it facilitates the differentiation between b and y ions.