Laser Electrospray Mass Spectrometry Minimizes Ion Suppression Facilitating Quantitative Mass Spectral Response for Multicomponent Mixtures of Proteins

A comparison of the mass spectral response for myoglobin, cytochrome c, and lysozyme is presented for laser electrospray mass spectrometry (LEMS) and electrospray ionization–mass spectrometry (ESI–MS). Analysis of multicomponent protein solutions using nonresonant femtosecond (fs) laser vaporization with electrospray postionization mass spectrometry exhibited significantly reduced ion suppression effects in comparison with conventional ESI analysis, enabling quantitative measurements over 4 orders of magnitude in concentration. No significant charge reduction was observed in the LEMS experiment while the ESI measurement revealed charge reduction for myoglobin and cytochrome c as a function of increasing protein concentration. Conventional ESI–MS of each analyte from a multicomponent solution reveals that the ion signal detected for myoglobin and cytochrome c reaches a plateau and then begins to decrease with increasing protein concentration preventing quantitative analysis. The ESI mass spectral response for lysozyme from the mixture initially decreased, before increasing, with increasing multicomponent solution concentration

Quantitative Measurements of Small Molecule Mixtures Using Laser Electrospray Mass Spectrometry

Quantitative measurements of atenolol, tioconazole, tetraethylammonium bromide, and tetrabutylammonium iodide using laser electrospray mass spectrometry (LEMS) reveal monotonic signal response as a function of concentration for single analytes, two- and four-component equimolar mixtures, and two-component variable molarity mixtures. LEMS analyses of single analytes as a function of concentration were linear over ∼2.5 orders of magnitude for all four analytes and displayed no sign of saturation. Corresponding electrospray ionization (ESI) measurements displayed a nonmonotonic increase as saturation occurred at higher concentrations. In contrast to the LEMS experiments, the intensity ratios from control experiments using conventional ESI-MS deviated from expected values for the equimolar mixture measurements due to ion suppression of less surface active analytes, particularly in the analysis of the four-component mixture. In the analyses of two-component nonequimolar mixtures, both techniques were able to determine the concentration ratios after adjustment with response factors although conventional ESI-MS was subject to a greater degree of saturation and ion suppression at higher analyte concentrations

Reactive Pendant MnO in a Synthetic Structural Model of a Proposed S₄ State in the Photosynthetic Oxygen Evolving Complex

The molecular mechanism of the Oxygen Evolving Center of photosystem II has been under debate for decades. One frequently cited proposal is the nucleophilic attack by water hydroxide on a pendant MnO moiety, though no chemical example of this reactivity at a manganese cubane cluster has been reported. We describe here the preparation, characterization, and a reactivity study of a synthetic manganese cubane cluster with a pendant manganese-oxo moiety. Reaction of this cluster with alkenes results in oxygen and hydrogen atom transfer reactions to form alcohol- and ketone-based oxygen-containing products. Nitrene transfer from core imides is negligible. The inorganic product is a cluster identical to the precursor, but with the pendant MnO moiety replaced by a hydrogen abstracted from the organic substrate, and is isolated in quantitative yield. ¹⁸O and ²H isotopic labeling studies confirm the transfer of atoms between the cluster and the organic substrate. The results suggest that the core cubane structure of this model compound remains intact, and that the pendant MnO moiety is preferentially reactive