Scheme 2. Possible salt bridge (SB) and ionic hydrogen bond (IHB) network illustrated for K16, D11, and E12 that accounts for the formation of c11 and complementary z5˙ fragments from cleavage of the N–Cα backbone bond 11 in niECD of phosphopeptide anions. Similar structures that involve other residues, e.g., the protonated N terminus and the phosphate of pS3, could account for backbone cleavage at other sites.
