Bond activation in complexes of Pb(II) with 15 deprotonated amino acids: correlation with gas-phase acidity

Deprotonated amino acids and their complexes with the lead dication have been electrosprayed from solution and then subjected to collision-induced dissociation in a tandem mass spectrometer. The nature of the observed primary dissociation products and the relative onset energies for dissociation in the absence and presence of the lead dication were used as a signature and measure of bond activation by the lead dication. Bond-activation by Pb2+ appears to be the largest with deprotonated glycine and is also large with the other deprotonated amino acids containing hydrocarbon sidechains (alanine, proline and valine). Deprotonated methionine, aspartic and glutamic acid also exhibit significant bond activation with dissociation occurring at the same bond in the absence and presence of Pb2+. A correlation was uncovered between the dissociation onset energies of the eigth Pb2+ complexes that exhibit backbone dissociation and the gas-phase acidities of the corresponding amino acids. Backbone dissociation typically involves C-αC bond scission of the lead complex leading to loss of CO2 + Pb or H2O + CO. The activation of the C-αC bond by Pb2+ was found to be tempered by the same factor that influences the gas-phase acidities of the amino acids, namely the presence of electron-donating groups on the sidechain. In the other seven Pb2+ complexes, which exhibit sidechain dissociation through CC, CN, CS and CO bond scission, no such correlation was found.

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► Electrospray tandem mass spectrometry of gas-phase metal–dication complexes.
► Complexes of Pb2+ with 15 different deprotonated biological amino acids.
► Role of metal dication-induced bond activation in ion dissociation.
► Correlation of dissociation onset energies with amino acid gas-phase acidity.