An approach to estimate the activation energies of fragmentation occurring in quadrupole collision cell of the mass spectrometer

Competition between One- and Two-Electron Unimolecular Reactions of Late 3d-Metal Complexes [(Me3SiCH2)nM]-(M = Fe, Co, Ni, and Cu; n = 2-4)

Although organometallic complexes of the late 3d elements are known to undergo both one-and two-electron reactions, their relative propensities to do so remain poorly understood. To gain direct insight into the competition between these different pathways, we have analyzed the unimolecular gas-phase reactivity of a series of well-defined model complexes [(Me3SiCH2)nM]- (M = Fe, Co, Ni, and Cu; n = 2-4). Applying a combination of tandem-mass spectrometry, quantum-chemical computations, and statistical rate-theory calculations, we find several different fragmentation reactions, among which the homolytic cleavage of metal-carbon bonds and radical dissociations are particularly prominent. In all cases, these one-electron reactions are entropically favored. For the ferrate and cobaltate complexes, they are also energetically preferred, which explains their predominance in the corresponding fragmentation experiments. For [(Me3SiCH2)4Ni]- and, even more so, for [(Me3SiCH2)4Cu]-, a concerted reductive elimination as a prototypical two-electron reaction is energetically more favorable and gains in importance. [(Me3SiCH2)3Ni]- is special in that it has two nearly degenerate spin states, both of which react in different ways. A simple thermochemical analysis shows that the relative order of the first and second bond-dissociation energies is of key importance in controlling the competition between radical dissociations and concerted reductive eliminations.

Intact Transition Epitope Mapping - Thermodynamic Weak-force Order (ITEM - TWO)

We have developed an electrospray mass spectrometry method which is capable to determine antibody affinity in a gas phase experiment. A solution with the immune complex is electrosprayed and multiply charged ions are translated into the gas phase. Then, the intact immune-complex ions are separated from unbound peptide ions. Increasing the voltage difference in a collision cell results in collision induced dissociation of the immune-complex by which bound peptide ions are released. When analyzing a peptide mixture, measuring the mass of the complex-released peptide ions identifies which of the peptides contains the epitope. A step-wise increase in the collision cell voltage difference changes the intensity ratios of the surviving immune complex ions, the released peptide ions, and the antibody ions. From all the ions´ normalized intensity ratios are deduced the thermodynamic quasi equilibrium dissociation constants (K_Dm0g^#) from which are calculated the apparent gas phase Gibbs energies of activation over temperature (ΔG_m0g^#T). The order of the apparent gas phase dissociation constants of four antibody - epitope peptide pairs matched well with those obtained from in-solution measurements. The determined gas phase values for antibody affinities are independent from the source of the investigated peptides and from the applied instrument. Data are available via ProteomeXchange with identifier PXD016024. SIGNIFICANCE: ITEM - TWO enables rapid epitope mapping and determination of apparent dissociation energies of immune complexes with minimal in-solution handling. Mixing of antibody and antigen peptide solutions initiates immune complex formation in solution. Epitope binding strengths are determined in the gas phase after electrospraying the antibody / antigen peptide mixtures and mass spectrometric analysis of immune complexes under different collision induced dissociation conditions. Since the order of binding strengths in the gas phase is the same as that in solution, ITEM - TWO characterizes two most important antibody properties, specificity and affinity.