Scheme 1. Complementary c and z˙ fragments from N–Cα backbone bond cleavage between residues n and n + 1 in ECD, niECD, or ETD; z˙ fragment structures were confirmed by infrared ion spectroscopy,22c fragments with imidic acid structure can tautomerize to the amide form,23 and H˙ transferring from a c to its complementary z˙ fragment can produce a pair of complementary c˙ and z fragments.24,25 .