Structures and fragmentations of Cobalt(II)–cysteine complexes in the gas phase

The electronebulization of a cobalt(II)/cysteine(Cys) mixture in water/methanol (50/50) produced mainly cobalt-cationized species. Three main groups of the Co-cationized species can be distinguished in the ESI-MS spectrum: (1) the cobalt complexes including the cysteine amino acid only (they can be singly charged, for example, [Co(Cys) n - H] + with n = 1-3 or doubly charged such as [Co + (Cys) 2 ] 2+ ); (2) the cobalt complexes with methanol: [Co(CH 3 OH) n - H] + with n = 1-3, [Co(CH 3 OH) 4 ]2 + ; and (3) the complexes with the two different types of ligands: [Co(Cys)(CH 3 OH) - H] + . Only the singly charged complexes were observed. Collision-induced dissociation (CID) products of the [co(cys) 2 ] 2+ , [Co(Cys) 2 - H] + and [Co(Cys) - H] + complexes were studied as a function of the collision energy, and mechanisms for the dissociation reactions are proposed. These were supported by the results of deuterium labelling experiments and by density functional theory calculations. Since [Co(Cys) - H] + was one of the main product ions obtained upon the CID of [Co(Cys) 2 ] 2+ and of [Co(Cys) 2 - H] + under low-energy conditions, the fragmentation pathways of [Co(Cys) - H] + and the resulting product ion structures were studied in detail. The resulting product ion structures confirmed the high affinity of cobalt(II) for the sulfur atom of cysteine.