To b or not to b: The ongoing saga of peptide b ions

Modern soft ionization techniques readily produce protonated or multiply protonated peptides. Collision-induced dissociation (CID) of these protonated species is often used as a method to obtain sequence information. In many cases fragmentation occurs at amide bonds. When the charge resides on the C-terminal fragment so-called y ions are produced which are known to be protonated amino acids or truncated peptides. When the charge resides on the N-terminal fragment so-called b ions are produced. Often the sequence of y and b ions are essential for peptide sequencing. The b ions have many possible structures, a knowledge of which is useful in this sequencing. The structures of b ions are reviewed in the following with particular emphasis on the variation of structure with the number of amino acid residues in the b ion and the effect of peptide side chain on b ion structure. The recent discovery of full cyclization of larger b ions results in challenges in peptide sequencing. This aspect is discussed in detail.