Dissociation of deprotonated glycine complexes with Pb2+ and five transition-metal dications (Fe2+, Co2+, Ni2+, Cu2+, Zn2+): The importance of metal bond activation

Electrospray ionization has been used for engendering gas-phase 1:1 complexes of deprotonated glycine (Gly-H)− and the atomic transition metal dications Fe2+, Co2+, Ni2+, Cu2+ and Zn2+, as well as Pb2+ that we reported previously. The CID-resolved spectra of these complexes with (Gly-H)− could be compared with that of (Gly-H)− in the absence of the metal cation in order to assess the importance of metal bond activation by M2+ within [M(Gly-H)]+. Four channels were distinguished in the dissociation of the transition metal dication complexes: CO loss, H2O loss, CO2 loss and H2O + CO loss. The results were rationalized in terms of the properties of the metal center. The capacity to activate bonds was found to decrease from Fe2+ to Zn2+ leading to an increase in CO2 loss. Metal dications with empty valence orbitals have high capacities to activate the NH bond and induce a proton transfer from NH2 to COO−, favoring carboxyl structures. In essence, the number of electrons in the valence shell of the metal dication influences its capacity to activate bonds and determines the position of the proton in the complexes at eiher nitrogen or oxygen, generating carboxylate and carboxyl structures, respectively. Secondary loss of CO2 was found to be influenced by the ionization energy or proton affinity of the metal.

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► We electrosprayed 1:1 complexes of deprotonated glycine (Gly-H)− and the metal dications Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+.
► These metal complexes and bare (Gly-H)− were exposed to multi-collisional dissociation.
► We discovered trends in the extent of dissociation and the nature of ion products formed.
► This provided insights into the capacity of chemical bond activation by metal dications and dissociation mechanisms.
► The number of electrons in the valence shell of the metal dication appears to be important.