Multi-anode detection in electrospray ionization time-of-flight mass spectrometry

An electrospray ionization ion source coupled to a time-of-flight mass analyzer incorporating a multi-anode time-to-digital converter is described. High-speed data acquisition (kHz mass spectral acquisition) rates are achieved. The four-anode detector produces a significant increase in detection/counting efficiency over that for a single-anode detector. In this work a 2.5 times increase in detection efficiency is demonstrated. The multi-anode detector is also used as a diagnostic tool to optimize transmission of the ion optics.     

Molecular Structure of 1,3,5-Tris (Trimethylstannyl) Benzene: An Electron Diffraction Study

The molecular structure of 1,3,5-tris (trimethylstannyl) benzene has been determined by gas-phase electron diffraction. The C — C bond length is in good agreement with that in benzene. In agreement with the somewhat electron-releasing character of the substituents, the endocyclic bond angles at the substituents are somewhat smaller than 120°. The mean value of Sn — C bond lengths is greater than that in tetraphenyltin and tetramethyltin. The SnMe₃ groups appear freely rotating around the C_aryl — Sn bonds. The following bond lengths (r _g) and bond angles were determined: (Sn — C)_mean 2.150 ± 0.007 Å, C — C 1.399 ± 0.005 Å, (C — H)_mean 1.105 ± 0.006 Å, < C — C(Sn) — C 117.7 ± 1.7º, < C_aryl — Sn — C_methyl 106.7 ± 0.7º < Sn — C — H 111.5 ± 0.9º.     

Selection rules for helicate ligand component self-assembly: steric, pH, charge, and solvent effects

The reaction between 1,10-phenanthroline-2,9-dicarboxaldehyde, copper(I), and certain primary amines was found to give quantitatively a dicopper double-helicate product (two of which were crystallographically characterized) by imine self-assembly around Cu(I) templates. The parameters of this reaction were investigated, and important roles were found to be played by (i) the steric bulk of the amine, (ii) the charge of the amine, (iii) the solvent used, and (iv) the pH of the solution. Water was found to allow the broadest range of structures to form, and ligand-component exchange reactions (involving the substitution of an aromatic for an aliphatic amine) were demonstrated to proceed readily in this solvent.     

Porphyrin distortion during affinity maturation of a ferrochelatase antibody, monitored by Resonance Raman spectroscopy

Resonance Raman (RR) spectra are reported for mesoporphyrin IX bound to the Fab fragment of the ferrochelatase antibody 7G12. Binding induces activation of a Raman band at 680 cm(-1), which is assigned to an out-of-plane porphyrin vibration, gamma15. This is exactly the predicted effect of distorting mesoporphyrin to the geometry of N-methylmesoporphyrin IX, the 7G12 hapten, based on DFT/CIS modeling of the RR spectrum. The modeling also shows that the pyrrole ring that is tilted out of the porphyrin plane bears a nitrogen lone pair, which is therefore available for coordination by an incoming metal ion. The 680 cm(-1) band intensity is approximately 3 times higher for the affinity-matured antibody than for the germline precursor antibody, while intermediate values are found for variants in which germline residues are mutated to mature residues or vice versa. Thus, RR spectroscopy reveals an evolution from weak substrate distortion in the germline antibody to strong substrate distortion in the affinity-matured antibody, and supports the view that catalysis involves a substrate strain mechanism.     

Evaluation of the VIDAS Listeria (LIS) immunoassay for the detection of Listeria in foods using demi-Fraser and Fraser enrichment broths, as modification of AOAC Official Method 999.06 (AOAC Official Method 2004.06)

A multilaboratory study was conducted to compare the VIDAS LIS immunoassay with the standard cultural methods for the detection of Listeria in foods using an enrichment modification of AOAC Official Method 999.06. The modified enrichment protocol was implemented to harmonize the VIDAS LIS assay with the VIDAS LMO2 assay. Five food types--brie cheese, vanilla ice cream, frozen green beans, frozen raw tilapia fish, and cooked roast beef--at 3 inoculation levels, were analyzed by each method. A total of 15 laboratories representing government and industry participated. In this study, 1206 test portions were tested, of which 1170 were used in the statistical analysis. There were 433 positive by the VIDAS LIS assay and 396 positive by the standard culture methods. A Chi-square analysis of each of the 5 food types, at the 3 inoculation levels tested, was performed. The resulting average Chi square analysis, 0.42, indicated that, overall, there are no statistical differences between the VIDAS LIS assay and the standard methods at the 5% level of significance.     

Ketene functionalized polyethylene: control of cross-link density and material properties

The functionalization and cross-linking of polyethylene is synthetically challenging, commonly relying on highly optimized radical based postpolymerization strategies. To address these difficulties, a norbornene monomer containing Meldrum's acid is shown to be effectively copolymerized with polyethylene using a nickel α-iminocarbaxamidato complex, providing high-melting, semicrystalline polymers with a tunable incorporation of the functional comonomer. Upon heating the copolymer to common polyethylene processing temperatures, the thermolysis of Meldrum's acid to ketene provides the desired reactive group. This simple and versatile methodology does not require small molecule radical sources or catalysts, and the dimerization of the in situ generated ketenes is shown to provide tunable cross-linking densities in polyethylene. Subsequent rheological and tensile experiments illustrate the ability to tune cross-linked polyethylene properties by comonomer incorporation and elucidate valuable structure/property relationships in these materials. This study illustrates the power of well-defined and synthetically accessible functional groups in polyolefin synthesis and functionalization.     

Genetic Incorporation of a Reactive Isothiocyanate Group into Proteins

Methods for the site‐specific modification of proteins are useful for introducing biological probes into proteins and engineering proteins with novel activities. Herein, we genetically encode a novel noncanonical amino acid (ncAA) that contains an aryl isothiocyanate group which can form stable thiourea crosslinks with amines under mild conditions. We show that this ncAA (pNCSF) allows the selective conjugation of proteins to amine‐containing molecular probes through formation of a thiourea bridge. pNCSF was also used to replace a native salt bridge in myoglobin with an intramolecular crosslink to a proximal Lys residue, leading to increased thermal stability. Finally, we show that pNCSF can form stable intermolecular crosslinks between two interacting proteins.