Probing the interaction of alkali and transition metal ions with bradykinin and its des-arginine derivatives via matrix-assisted laser desorption/ionization and postsource decay mass spectrometry

Abstract

The complexes of the peptides (Pep) bradykinin (RPPGFSPFR), des-Arg 1-bradykinin, and des-Arg 9-bradykinin with the metal (M) ions Na +, K +, Cs +, Cu +, Ag +, Co 2+, Ni 2+, and Zn 2+ are generated in the gas phase by matrix-assisted laser desorption/ionization and the structures of the corresponding [Pep + M +] + or [Pep − H + + M 2+] + cations are probed by postsource decay (PSD) mass spectrometry. The PSD spectra depend significantly on the metal ion attached; moreover, the various metal ions respond differently to the presence or absence of a basic arginine residue. The Na + and K + adducts of all three peptides mainly produce N-terminal sequence ions upon PSD; the fragments observed point out that these metal ions are anchored by the PPGF segment and not the arginine residue(s). In contrast, the adducts of Cu + and Ag + show a strong dependence on the position of Arg; complexes of des-Arg 1-Pep (which contains a C-terminal Arg) produce primarily y n ions whereas those of des-Arg 9-Pep generate exclusively a n and b n ions. These trends are consistent with Cu + ligation by Arg’s guanidine group. The [Pep + Cs +] + ions mainly yield Cs +; a second significant fragmentation occurs only if a C-terminal arginine is present and involves elimination of this arginine’s side chain plus water. This reaction is rationalized through a salt bridge mechanism. The most prominent PSD products from [Pep − H + + Co 2+] + and [Pep − H + + Ni 2+] + contain at least one phenylalanine residue, revealing a marked preference for these divalent metal ions to bind to aromatic rings; the fragmentation patterns of the complexes further suggest that Co 2+ and Ni 2+ bind to deprotonated amide nitrogens. The coordination chemistry of Zn 2+ combines features found with the divalent Co 2+/Ni 2+ as well as the monovalent Cu +/Ag + transition metal ions. Generally, the structure and fragmentation behavior of each complex reflects the intrinsic coordination preferences of the corresponding metal ion.