Complexes of l-histidine with Fe 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+ studied by electrospray ionization mass spectrometry

Abstract

As a first building block towards the study of the active site of numerous metalloproteins, 1:1 complexes of l-histidine with Fe 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+, were studied by electrospray ionization MS/MS. It was found that either solvated or nonsolvated complexes could be formed depending upon source conditions. Low cone voltage conditions enabled the formation of four- to six-coordinated complexes that were a direct reflection of the species in solution. Using precursor ion scans, it was shown that these ions were the major precursors of the 1:1 l-histidine/metal complexes. The greater number of collisions existing with higher cone voltages resulted in the loss of labile ligands, and in the case of Cu 2+ in gas phase redox reactions to give MCu + adducts. The fragmentations of 1:1 l-histidine/metal(II) complexes (namely, [(M − H) − Cat 2+], using an ionic notation), were then studied by collisionally induced dissociation. All metal complexes were found to undergo decarboxylation. Although for Cu(II) and Zn(II), it is thought that the decarboxylated complexes, unstable, immediately dissociate by losing a reduced cation Cat 0, giving an immonium ion I His ( m/ z 110). For Fe(II), Co(II), and Ni(II), the next fragmentation reactions of the stable [(M − H) − Cat 2+ − CO 2] complexes could be either loss of NH 3, CH 2 = NH, HCN, or Cat 0 in various proportions. Loss of the neutral metal was observed eventually in the fragmentation sequence. This was interpreted in terms of a gradual reduction of the transition metal cation, through ligand-to-metal electron transfers, related to the diminution of the coordination sphere brought on by the fragmentations. For copper, which was the only case when a reduction to MCat + was observed, comparison of the fragmentations of Cu(II) and Cu(I) complexes showed the oxidation state influences the reactivity. Several mechanisms, consistent with deuterium labeling, are suggested and discussed.