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

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+, Co2+, Ni2+, and Zn2+ are generated in the gas phase by matrix-assisted laser desorption/ionization and the structures of the corresponding [Pep + M+](+) or [Pep - H+ + Ni2+](+) 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 Naf 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+ + Co2+] and [Pep - H+ + Ni2+](+) 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 Co2+ and Ni2+ bind to deprotonated amide nitrogens. The coordination chemistry of Zn2+ combines features found with the divalent Co2+/Ni2+ 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. (C) 1999 Elsevier Science B.V.