The gas-phase deprotonation reactions of multiply protonated ubiquitin ions have been studied in a Fourier-transform ion cyclotron resonance mass spectrometer. Electrospray ionization was used to generate ubiquitin ions with attachment of 7-13 protons. Rate constants were measured for the reactions of these protein ions with four amines: n-propylamine, di-n-propylamine, tri-n-propylamine, and N,N,N',N'-tetramethyl-1,4-diaminobutane. The gas-phase basicities of the amines ranged from 210.1 kcal/mol to 232.6 kcal/mol. The rate constants were found to increase as the charge state of the ion increased and as the basicity of the amine increased. Several reactions proceed at near the collision rate and have rate constants in excess of 10(-8) cm3 molecule-1 s-1. With the more basic reactants, multiple protons could be stripped sequentially from ubiquitin ions at roughly equivalent rates, suggesting that these protons are attached to sites with similar basicities. In general, deprotonation occurs if the gas-phase basicity of the amine is within 10 kcal/mol of the intrinsic gas-phase basicity of the amino acid residue being deprotonated. For [M+nH]n+, n = 4-6, nonlinear pseudo-first-order kinetic behavior indicated the presence of multiple ion structures. Kinetic, structural and thermodynamic aspects of these reactions are discussed.
Toward a goal of dideoxy sequencing DNA utilizing electrophore labels, we prepared four electrophore-labeled DNA oligonucleotide primers. Each primer has a different electrophore and DNA sequence but a common glycol keto (alpha,beta-dihydroxyketo) release group. Cleavage of this latter group by either periodate oxidation or a thermal retroaldol reaction releases the electrophores for detection by mass spectrometry. Successful sequencing data with these primers was obtained by capillary electrophoresis on an ABI Model 310 after fluorescence dideoxy terminator cycle sequencing reactions were conducted. In a separate experiment, it was demonstrated that a cocktail of the four electrophore DNA primers could be detected as a dried sample spot by CO2 laser desorption/capillary collection/gas chromatography electron capture mass spectrometry. These results establish some feasibility for our long-term goal of high-speed multiplex electrophore mass tag dideoxy DNA sequencing. Ultimately we plan to use a higher number of electrophore mass tags and to rely on direct detection of the desorbed electrophores by electron capture time-of-flight mass spectrometry.
The kinetics of reaction between Cr' produced by 70-eV electron impact on Cr(C0)6 are described. Approximately 26% of the Cr+ ions react at a rate 4.4 times slower than the remaining Cr+ ions. This is interpreted in terms of an excited state that is long lived on the time scale of the experiment (2 s). The variation of ion abundance with time in a mixture of Cr(CO), and CH4 following a pulse of 70-eV electrons is analyzed. The analysis indicates that the excited state reacts with CHI to form CrCH2+ but that the primary result of collision between the excited Cr+ and CH4 is quenching of the excited state. If the quenching process produces ground-state Cr+ rather than another excited state, it is spin forbidden. Collision-induced decomposition of CrCH4+ supports the involvement of an excited state in the reaction which produces CrCH2+. The CrCH2+ ion was not observed to react with CO. The possibility that Fe+ formed by electron impact on Fe(CO), includes ions in metastable states is discussed.
IntroductionRecently we described a study of the reactions of first-row transition-metal atomic ions with butanes in the gas phase.' Cr+ and Mn+ generated by electron impact ionization on Cr(C0)6 and Mnz(CO)lo were found to differ in their reactivity from the group 8 atomic metal ions. Fe+, Co+, and Ni+ generated by electron impact on Fe(CO)5, Co2(CO),, and Ni(CO), react with the butanes to form various metal-olefin Mn+ undergoes no observable reactions.' Cr+ reacted with the butanes to form metakolefin complexes, but it does not react with isobutane to form a metal-propene complex.' A metal-propene complex is the dominant product in the reactions of the group 8 metal ions with isobutane.' It is somewhat surprising that Cr+ with its very stable d5 electronic configuration should react at all. In fact, collision-induced decomposition studies of CrC,H complexes suggest a very weak interaction between Cr+ and the butane.]The observed reactivity of Cr+ led to studies to determine whether an excited state of Cr+ might be involved. Several conclusions of these studies were added to the report of the study of the atomic metal ion chemistry with butane as a "note added in proof'.' A full description of these studies is the purpose of the present report. First, the kinetics of reactions of Cr+ with Cr(C0)6 are considered. Next, the reaction of Cr+ with CHI and the collision-induced decomposition of CrCH4+ are discussed. The reaction of excited Cr+ with CH, produces CrCH2+. Since this reaction is endothermic for ground-state species, it is particularly useful for characterizing the excited state. Evidence that the
Electrospray ionization (ESI) in combination with Fourier-transform mass spectrometry (FTMS) gives both highly accurate mass analysis of peptides with external calibration and high resolution for charge-state determination of an ion from the naturally occurring isotopic peaks. Liquid chromatographic (LC) separation of peptides is important for the analysis of complex mixtures. High-resolution, high accuracy measurement of the molecular ions of the mix of peptides has been achieved with LC/ESI-FTMS. Examples are given to show both the reproducibility of mass accuracy over replicate analyses and to show the utility of broadband analysis of a gradient separation of peptides. The high mass accuracy and resolution of FTMS was uncompromised for the direct analysis of chromatography peaks.
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