Post-source decay in the analysis of polystyrene by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Various secondary series are observed in matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectra of polystyrene. The number and positions of the series depend on the choice of matrix and added cation. For a given treatment, series observed in linear mode are not necessarily observed in reflectron mode, and vice versa. Post-source decay analysis was used to determine that the secondary series arise at least in part from formation and decay of adducts of polystyrene with matrix species. There is some treatment-to-treatment variation, but adduct formation and decay were observed for all tested treatments. The multiplicity of secondary series makes it unclear whether post-source decay occurs for the main series (polystyrene + cation)⁺ ions under the conditions normally used for polystyrene analysis. Such ions do undergo post-source decay at laser fluences greater than normally used. Although only polystyrene was investigated in this work, other polymers may also produce adduct and decay series in MALDI analysis. Their presence can mask the presence of minor components in a sample, but at least as observed here, do not have a strong influence on molecular mass determinations.     

THE EFFECT OF 1,4-DIAZABICYCLO[2.2.2]OCTANE ON THE RADIOSENSITIVITY OF BACTERIA

Abstract— Hydroxyl radicals ('OH) are scavenged by 1,4-diazabicyclo[2.2.2]octane (DABCO) at a diffusion-controlled rate of 1.25 ± 0.1 × 10⁹ M ^-1s^-1. Unlike other efficient 'OH scavengers which exhibit protection of bacteria against irradiation both in oxic and hypoxic conditions, DABCO has been shown to protect Serratia marcescens and various strains of Escherichia coli only in oxic conditions.
DABCO appears to eliminate a component of the sensitization afforded by oxygen in all strains of E. coli tested. The level of this protection increases from ∼15% in the wild type AB 1157 to ∼100% in the recA uvrA mutant AB 2480. It is suggested that DABCO protects against lethal events that can occur on macromolecules other than DNA such as the cell membrane.
Results with added glycerol, as well as work in D₂O solution, indicate that DABCO is more likely to be acting by scavenging radicals rather than by quenching ¹O₂. If ¹O₂ is a component of the sensitization afforded by oxygen, then it is unlikely to be formed in a hydrophilic environment in the cell.     

Comparison of cartesian and lobe function Gaussian basis sets

The lobe function and cartesian (spherical harmonic) gaussian are compared with reference to calculations for second-row atoms. Single and grouped gaussian basis sets which have been reported for cartesian functions are taken over directly to construct corresponding lobe function bases with identical sets of exponents and with lobe separations chosen by a scaling procedure. Total and orbital energies and SCF coefficients resulting from calculations on the second-row atoms using the two types of functions for both primitive and grouped gaussian basis sets are seen to be in excellent agreement, thereby emphasizing the essential equivalence of lobe functions and cartesian gaussians, at the very least with respect to calculation of energy surfaces.

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Zusammenfassung Die Lobe-Funktionen und kartesischen (Kugelfunktions-) Gaußfunktionen werden in Berechnungen von Atomen der zweiten Reihe des periodischen Systems verglichen. Schon bekannte einfache und gruppierte Gaußfunktionsbasissätze für kartesische Funktionen werden direkt übernommen, um entsprechende Lobe-Funktionsbasen mit identischen Exponenten zu konstruieren, wobei die zugehörigen Lobe-Abstände nach einer Koordinatenstreckungs-(scaling) Methode berechnet werden. Gesamt- und Orbitalenergien sowie SCF-Koeffizienten für die Atome der zweiten Reihe stimmen bei Benutzung der beiden verschiedenen Funktionstypen, sowohl bei einfacher als auch bei gruppierter Gaußfunktionsbasis, außerordentlich gut überein, wodurch die wesentliche Gleichwertigkeit von Lobe-Funktionen und kartesischen Gaußfunktionen betont wird, zum allermindesten hinsichtlich der Berechnung von Energieflächen.

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Résumé La fonction de lobe et la gaussienne cartésienne (harmonique sphérique) sont comparées sur des calculs d'atomes de la seconde ligne. Des bases de gaussiennes simples et groupées utilisées pour les fonctions cartésiennes sont directement employées pour construire les bases correspondantes de fonctions à lobes avec des ensembles d'exposants identiques et des séparations de lobes choisis par calibrage. L'emploi de ces deux types de fonctions donne des résultats concordants pour l'énergie totale, les énergies orbitales et les coefficients SCF, mettant l'accent sur l'équivalence essentielle des fonctions à lobe et des gaussiennes cartésiennes, tout au moins pour le calcul des surfaces d'énergie.

     

Theoretical study of the electronic spectrum of diimide by AB initio methods

A series of ab initio calculations is reported for the ground and low-lying valence and Rydberg states of diimide N₂H₂. Symmetric bending potential curves for both the cis and trans forms of this system have been obtained at the SCF level of treatment. In addition Cl calculations have been carried out for the trans-diimide ground state equilibrium nuclear conformation, using a configuration selection procedure described elsewhere; an associated energy extrapolation scheme is also employed which enables the effective solution of secular equations with orders of up to 40000. The ensuing Cl wavefunctions are interpreted in the discussion and the corresponding calculated energy differences between the various electronic states are compared with experimental transition energy results for both diimide and for related systems such as trans-azomethane. A more detailed analysis of the observed absorption bands in the ¹B_g-X¹A_g transition in N₂H₂ is also given, making use of calculated potential curve data as well as the pertinent Cl vertical energy difference. The dipole-forbiddenness of the excitation process is thereupon concluded to result in a distinct non-verticality for this electronic band system, causing its absorption maximum to occur at a position some 0.6 eV to the blue of the so-called vertical transition, i.e., that for which maximum vibrational overlap is obtained.     

Molecular weight distribution studies. I. Radiation-induced polymerization of liquid α-methylstyrene

The concentration of water in purified and BaO-dried α-methylstyrene was found to be 1.1 × 10^?4M. The radiation-induced bulk polymerization of the α-methylstyrene thus prepared was studied in the temperature range of ?20°C to 35°C. The polymerization rate varied as the 0.55 power of the dose rate. The theoretical molecular weights and molecular weight distribution were calculated from a proposed kinetic scheme and these values were then compared with those found experimentally. The agreement between these two was reasonably close, and therefore it was concluded that, from the molecular weight distribution point of view, the proposed kinetic scheme for the cationic polymerization of α-methylstyrene is an acceptable one. The rate constant for chain transfer to monomer k_f changed with temperature and was found to be responsible for the decrease in the molecular weight of the polymer with increase in temperature. k_f and k_p at 20°C were found to be 0.95 × 10⁴ l./mole-sec and 0.99 × 10⁶ l./mole-sec, respectively.     

Preparation and reactions of [2,2-bis(carbethoxy)propyl]cyclopentadienyldicarbonyliron

Although

, (Fp  η-C₅H₅Fe(CO)₂) (I) is stable and characterizable, the lower homologue FpCH₂CH(CO₂CH₃)₂ (II) is not; this we attribute to a facile elimination reaction resulting from the relatively acidic β-hydrogen of II. Formation of I from Fp^? and XCH₂CE₂CH₃ (X  Br, Cl; E  CO₂CH₂CH₃) and cleavage of its FeC bonds (using H⁺, Br₂, Ce^IV and Hg^II) occur without major amounts of ester group migrations, even though ·CH₂CE₂CH₃ radicals are involved in some of these reactions.     

Stacking due to ionic transport number mismatch during sample sweeping on microchips

Sample stacking can occur in isoconductive buffer systems as a result of ion transport mismatches that cause changes in buffer conductivity during electrophoresis. Fluorescence imaging was used to examine this effect in the sweeping of hydrophobic dyes with sodium dodecyl sulfate (SDS) on microchips. Imaging revealed the occurrence of a stacking effect in a sodium borate buffer system in which the sample buffer and SDS-containing run buffer had the same initial conductivity. Injected sample plugs were first swept by SDS micelles and the swept band was then stacked at the trailing end of the sample zone. This effect is due to changes in conductivity at both the front and back interfaces of the injected sample plug and can be modeled by moving boundary equations. Maximum signal enhancements of 86-, 160- and 560-fold were obtained for Rhodamine 560, Rhodamine B and Rhodamine 6G, respectively, by the combination of sweeping and stacking within a 1 cm section of microchannel. Based on sample sweeping/stacking and manipulation of the electric field polarity, a method of trapping and concentrating analyte from multiple injections was also demonstrated.     

Unexpected conversion of a polycyclic thiophene to a macrocyclic anhydride

Oxygenation of 2,5,9,12-tetra(tert-butyl)diacenaphtho[1,2-b:1′,2′-d]-thiophene (1, C₄₀H₄₄S) by peracids gave the cyclic sulfonic ester 4 (2,7,10,13-tetra(tert-butyl)diacenaphtho[1,2-c:1′,2′-e]oxathiin 5,5-dioxide, C₄₀H₄₄O₃S) which, when heated in nitrobenzene, is converted into a complex, macrocyclic anhydride 3 (C₈₀H₈₈O₃), which is derived from two molecules of 4. Further investigation found a likely intermediate in this reaction, 4,4′,7,7′-tetra(tert-butyl)-1,1′-biacenaphthylenylidene-2,2′-dione (5, C₄₀H₄₄O₂), apparently formed from 4 by additional oxidation. Anhydride 3 plausibly arises by Diels-Alder reaction of 4 and 5 followed by several ring fragmentations. The structures of 3, 4, and 5 were unambiguously established by X-ray crystallography.     

Toward the synthesis of reidispongiolide a: stereocontrolled synthesis of the C17-C22 and C23-C35 degradation fragments

[structure: see text] By relying on the asymmetric aldol reactions of chiral ketones, a highly stereocontrolled synthesis of each of the C(17)-C(22) and C(23)-C(35) degradation fragments of reidispongiolide A has been achieved. This permits a configurational assignment of the complete C(17)-C(36) region of this antimitotic macrolide, along with providing advanced intermediates for a projected total synthesis.     

Upper and lower bounds for a serial dilution test

A Poisson-binomial model estimates the concentration of a target microbe from a serial dilution test. The maximum likelihood procedure gives an equation whose solution equals the estimate of the concentration. This paper gives bounds for the solution to this equation that require only minimal calculations.