Solution to spectroscopy challenge 5.
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Journal Article↗
Biomedical subjects
Renato Zenobi
Publications and source records attributed to Renato Zenobi.
Matrix-dependent cationization in MALDI mass spectrometry.
The matrix dependence in cationization processes, the competition between cationization and protonation and the question of whether gas-phase cation transfer or attachment of free cations dominates in matrix-assisted laser desorption/ionization mass spectrometry were studied. Two different sample preparation methods were employed, the dried-droplet sample preparation and a mixture of solid matrix, analyte and salt. The latter ensures that the formation of cation adducts takes place in the gas phase. By monitoring the suppression of matrix signals for different matrices, it was found that matrices with high gas-phase metal ion binding energies require high analyte concentrations for matrix suppression to occur. By comparing the mass spectra obtained using sinapinic acid or sinapinic methyl ester as a matrix, a correlation between cationization and deprotonation of matrix molecules was found. It is also demonstrated that attachment of free gas-phase cations, rather than cation transfer from the cationized matrix, is the predominant process in cationization.
Anti-Bacterial Agents↗
Role of electrons in laser desorption/ionization mass spectrometry.
A nonmetallic sample support for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry enhances the positive ion yield by 2 orders of magnitude and generally affects the charge balance in the desorption plume. We interpret the effects of the target material and of the sample preparation on MALDI mass spectra as a result of photoelectrons emitted upon laser irradiation of a metal target covered by a thin sample layer. These electrons are shown to play an important role in MALDI and laser desorption/ionization because they decrease the yield of positive ions, reduce ions with higher oxidation states, and affect the ion velocity distribution as well as the mass resolution. Understanding the role of these photoelectrons helps to clarify previously obscure aspects of the ion formation mechanism in MALDI.
Animals↗
Direct detection of particles formed by laser ablation of matrices during matrix-assisted laser desorption/ionization.
We report the detection of nanoparticles formed by irradiating matrix-assisted laser desorption/ionization (MALDI) matrix samples. This is direct evidence for the ejection of large size aggregates in the MALDI process. Nanometer-size particles were generated via a tunable solid-state UV laser, irradiating a sample placed in a nitrogen atmosphere. Size distribution measurements were performed using a differential mobility analyzer and a condensation particle counter. Particles in the 10-1000 nm size range were detected. The dependence of the particle size distribution on the laser fluence, wavelength and matrix was investigated. The observed effects are discussed and related to the MALDI ablation dynamics and gas-phase processes.
Journal Article↗
Production and fragmentation of multiply charged ions in 'electron-free' matrix-assisted laser desorption/ionization.
An unusually large fraction of multiply charged ions is observed in 'electron-free' matrix-assisted laser desorption/ionization (MALDI). Here we investigate how the yield of multiply charged ions depends on experimental parameters in MALDI. It is found to increase if measures are taken to limit the number of electrons in the plume, for example, by using non-metallic MALDI targets or low laser pulse energies. The ionization energy of the matrix is another important parameter that affects the yield of multiply charged ions: matrices with high ionization energies lead to greater intensities of multiply charged ions. It is furthermore proposed that some of the fragment ions observed in MALDI are due to reactions of analyte with electrons in the plume. The possibility of electron capture dissociation of multiply charged ions produced by MALDI is shown.
Animals↗
Quantitative evaluation of noncovalent chorismate mutase-inhibitor binding by ESI-MS.
Electrospray time-of-flight mass spectrometry was used to quantitatively determine the dissociation constant of chorismate mutase and a transition state analogue inhibitor. This system presents a fairly complex stoichiometry because the native protein is a homotrimer with three equal and independent substrate binding sites. We can detect the chorismate mutase trimer as well as chorismate mutase-inhibitor complexes by choosing appropriate conditions in the ESI source. To verify that the protein-inhibitor complexes are specific, titration experiments with different enzyme variants and different inhibitors were performed. A plot of the number of bound inhibitors versus added inhibitor concentration revealed saturation behavior with 3:1 (inhibitor:functional trimer) stoichiometry for the TSA. The soft ESI conditions, the relatively high protein mass of 43.5 kDa, and the low charge state (high m/z) result in broad peaks, a typical problem in analyzing noncovalent protein complexes. Due to the low molecular weight of the TSA (226 Da) the peaks of the free protein and the protein with one, two or three inhibitors bound cannot be clearly resolved. For data analysis, relative peak areas of the deconvoluted spectra of chorismate mutase-inhibitor complexes were obtained by fitting appropriate peak shapes to the signals corresponding to the free enzyme and its complexes with one, two, or three inhibitor molecules. From the relative peak areas we were able to calculate a dissociation constant that agreed well with known solution-phase data. This method may be generally useful for interpreting mass spectra of noncovalent complexes that exhibit broad peaks in the high m/z range.
Bacillus subtilis↗
Reduction of Cu(II) in matrix-assisted laser desorption/ionization mass spectrometry.
The mechanisms of the reduction of Cu(II) in matrix-assisted laser desorption/ionization mass spectrometry (MALDI) are studied. In MALDI mass spectra, ions cationized by copper mostly contain Cu(I) even if Cu(II) salts are added to the sample. It was found that Cu(II) was reduced to Cu(I) by gas-phase charge exchange with matrix molecules, which is a thermodynamically favorable process. Under some conditions, large amounts of free electrons are present in the plume. Cu(II) can be even more efficiently reduced to Cu(I) by free electron capture in the gas phase. The matrices studied in this work are nicotinic acid, dithranol, and 2,5-dihydroxybenzoic acid.
Journal Article↗
Mass spectrometric determination of association constants of adenylate kinase with two noncovalent inhibitors.
Noncovalent complexes between chicken muscle adenylate kinase and two inhibitors, P(1),P(4)-di(adenosine-5')tetraphosphate (Ap4A) and P(1),P(5)-di(adenosine-5') pentaphosphate (Ap5A), were investigated with electrospray ionization mass spectrometry under non-denaturing conditions. The nonconvalent nature and the specificity of the complexes are demonstrated with a number of control experiments. Titration experiments allowed the association constants for inhibitor binding to be determined. Problems with concentration dependent ion yields are circumvented by a data evaluation method that is insensitive to the overall ionization efficiency. The K(a) values found were 9.0 x 10(4) M(-1) (Ap4A) and 4.0 x 10(7) M(-1) (Ap5A), respectively, in very good agreement with available literature data.
Adenylate Kinase↗
Sampling gaseous oxidation products of aromatic compounds in gas/particle separation systems.
In this study we performed a direct comparison between two different ambient air samplers to characterize their performance in sampling oxidized gaseous organic compounds, known as oxidation products of aromatics. We investigated compounds with a variety of functional groups and vapor pressures. A polyurethane foam (PUF) adsorbent and an annular diffusion denuder sampler were operated along with particle filters. In both systems the sampling devices were liquid-extracted, followed by derivatization and analysis by GC-MS. The PUF system works very well for aromatic as well as non-aromatic compounds, whereas the denuder shows smaller collection efficiencies for highly volatile non-aromatic compounds. In addition, the sampling efficiencies in the PUF set-up are in good agreement with the calculated vapor pressures of the compounds and also the particle phase is not affected by most compounds.
Adsorption↗