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)-brad ykinin, 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-assist ed laser desorption/ionization and the structures of the corresponding [Pep + M+](+) or [Pep - H+ + Ni2+](+) cations are probed by postsource decay (P SD) mass spectrometry. The PSD spectra depend significantly on the metal io n attached; moreover, the various metal ions respond differently to the pre sence 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 fragm ents observed point out that these metal ions are anchored by the PPGF segm ent 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 t hose of des-Arg(9)-Pep generate exclusively a(n) and b(n) ions. These trend s 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 argini ne's side chain plus water. This reaction is rationalized through a salt br idge 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 Z n2+ combines features found with the divalent Co2+/Ni2+ as well as the mono valent Cu+/Ag+ transition metal ions. Generally, the structure and fragment ation behavior of each complex reflects the intrinsic coordination preferen ces of the corresponding metal ion. (C) 1999 Elsevier Science B.V.