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A simple and rapid means of enzyme kinetic analysis was achieved using electrospray ionization mass spectrometry and a one-point normalization factor. The model system used, glutathione S-transferase from porcine liver, is a two-substrate enzyme catalyzing the conjugation of glutathione with a variety of compounds containing an electrophilic center. An internal standard that is structurally similar to the product was added to the reaction quench solution, and a single-point normalization factor was used to determine the product concentration without the need of a calibration curve. Kinetic parameters, such as [K.sub.m], [V.sub.max] and [K.sub.i] (for thyroxine), obtained by electrospray mass spectrometry agreed with those obtained from traditional UV--vis spectroscopy, and competitive vs non-competitive inhibition reactions could be delineated via mass spectrometry. These results suggest that our method can be applied to enzymatic processes in which spectrophotometric or spectrofluorometric assays are not feasible or when the relevant substrates do not incorporate chromophores or fluorophores. This new method is competitive with traditional UV assays in that it is facile and it involves very little analysis time.

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We report the observation of chemical reactions in gas-phase Rhn(N2O)m+ complexes driven by absorption of blackbody radiation. The experiments are performed under collision-free conditions in a Fourier transform ion cyclotron resonance mass spectrometer. Mid-infrared absorption by the molecularly adsorbed N2O moieties promotes a small fraction of the cluster distribution sufficiently to drive the N2O decomposition reaction, leading to the production of cluster oxides and the release of molecular nitrogen. N2O decomposition competes with molecular desorption and the branching ratios for the two processes show marked size effects, reflecting variations in the relative barriers. The rate of decay is shown to scale approximately linearly with the number of infrared chromophores. The experimental findings are interpreted in terms of calculated infrared absorption rates assuming a sudden-death limit. [ABSTRACT FROM AUTHOR]

Highlights: [•] Nitromethane, acetaldehyde and benzaldehyde form radical anions with (H2O) n −. [•] With CO2 −(H2O) n , the electron is transferred, and neutral CO2 evaporates. [•] Nanocalorimetry yields thermochemistry of electron transfer in gas phase clusters. [•] Electron transfer from O2 −(H2O) n is endothermic, and is not observed. [•] Evidence is found that the electron is valence-bound in CH3CHO−(H2O) n . [Copyright &y& Elsevier]

Molecular cluster ions, which are stored in an electromagnetic trap under ultra-high vacuum conditions, undergo blackbody infrared radiative dissociation (BIRD). This process can be simulated with master equation modeling (MEM), predicting temperature-dependent dissociation rate constants, which are very sensitive to the dissociation energy. We have recently introduced a multiple-well approach for master equation modeling, where several low-lying isomers are taken into account. Here, we experimentally measure the BIRD of CO4●–(H2O)1,2 and model the results with a slightly modified multiple-well MEM. In the experiment, we exclusively observe loss of water from CO4●–(H2O), while the BIRD of CO4●–(H2O)2 leads predominantly to loss of carbon dioxide, with water loss occurring to a lesser extent. The MEM of two competing reactions requires empirical scaling factors for infrared intensities and the sum of states of the loose transition states employed in the calculation of unimolecular rate constants so that the simulated branching ratio matches the experiment. The experimentally derived binding energies are ΔH0(CO4●––H2O) = 45 ± 3 kJ/mol, ΔH0(CO4●–(H2O)–H2O) = 41 ± 3 kJ/mol, and ΔH0(CO2–O2●–(H2O)2) = 37 ± 3 kJ/mol. Quantum chemical calculations on the CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVDZ level, corrected for the basis set superposition error, yield binding energies that are 2–5 kJ/mol higher than experiment, within error limits of both experiment and theory. The relative activation energies for the two competing loss channels are as well fully consistent with theory. [ABSTRACT FROM AUTHOR]

Ion-molecule reactions of Mg+(H2O) n ( n≈20-60) with CH3CN are studied by Fourier-transform ion-cyclotron resonance mass spectrometry. Collision with CH3CN initiates the formation of MgOH+(H2O) n−1 together with CH3CHN. or CH3CNH., which is similar to the reaction of hydrated electrons (H2O) n− with CH3CN. In subsequent reaction steps, three more CH3CN molecules are taken up by the clusters, to form MgOH+(CH3CN)3 after a reaction delay of 60 seconds. Density functional theory (DFT) calculations at the M06/6-31++G(d,p) level of theory suggest that the bending motion of CH3CN allows the unpaired electron that is solvated out from the Mg center to localize in a π*(CN)-like orbital of the bent CH3CN.−, which undergoes spontaneous proton transfer to form CH3CNH. or CH3CHN., with the former being kinetically more favorable. The reaction energy for a cluster with the hexacoordinated Mg center is more exothermic than that with the pentacoordinated Mg. The CH3CNH. or CH3CHN. is preferentially solvated on the cluster surface rather than at the first solvation shell of the Mg center. By contrast, the three additional CH3CN molecules taken up by the resulting MgOH+(H2O) n clusters coordinate directly to the first solvation shell of the MgOH+ core, as revealed by DFT calculations. [ABSTRACT FROM AUTHOR]