Investigation of apparent mass deviations in electrospray ionization tandem mass spectrometry of a benzophenone-labeled peptide

In a previous study utilizing benzophenone-based topological probes to study conformationally dependent changes in mouse muscle nicotinic acetylcholine receptor (nAChR) topology, electrospray ionization tandem mass spectrometric (ESI-MS/MS) analysis led to a consistent -2.0 Da mass deviation from expected values. In the present study a synthetic peptide, corresponding to nAChR alpha1 subunit residues 130-139, was photolabeled. MS/MS analysis of this peptide using an ion trap confirmed the previously observed mass deviation, associated only with fragment ions that contain the incorporated benzophenone moiety. Analysis of peak profiles for the photolabeled ions does not indicate the typical 'peak fronting' that produces a mass shift when labile ions are prematurely ejected from the ion trap. Rather, hydrogen/deuterium (H/D) exchange experiments support the hypothesis that a chemical rearrangement involving phenyl migration and ketone formation has formed an unexpected oxidized peptide, with molecular mass 2 Da less than that expected, that is isolated for collision-induced dissociation in the ion trap together with the predicted precursor due to the broad ion isolation window specified.     

Formation of Metal Oxyfluorides from Specific Metal Reactions with Oxygen Difluoride: Infrared Spectroscopic and Theoretical Investigations of the OScF2 Radical and OScF with Terminal Single and Triple Sc?O Bonds

The scandium oxydifluoride free radical, OScF₂, is produced by the spontaneous, specific reaction of laser ablated Sc atoms with OF₂ in solid argon and characterized by using matrix infrared spectroscopy and theoretical calculations. The OScF₂ molecule is predicted to have C_2v symmetry and a ²B₂ ground state with an unpaired electron located primarily on the terminal oxygen atom, which makes it a scandium difluoride molecule coordinated by a neutral oxygen atom radical in forming the Sc? O single bond. The closed shell singlet OScF molecule with an obtuse bent geometry has a much shorter Sc? O bond of 1.682 Å than that of the OScF₂ radical (1.938 Å) on the basis of B3LYP calculations. The Sc? O bond in OScF consists of two covalent bonds and a dative bond in which the oxygen 2p_π lone pair donates electron density into an empty Sc 3d orbital thus forming a triple oxo bond. Density functional calculations suggest it is highly exothermic for fluorine transfer from OF₂ to scandium, which favors the formation of the OScF₂ radical species as well as the OScF molecule after fluorine loss.     

From aromaticity to self-organized criticality in graphene

The unique properties of graphene are rooted in its peculiar electronic structure where effects of electron delocalization are pivotal. We show that the traditional view of delocalization as formation of a local or global aromatic bonding framework has to be expanded in this case. A modification of the π-electron system of a finite-size graphene substrate results in a scale-invariant response in the relaxation of interatomic distances and reveals self-organized criticality as a mode of delocalized bonding. Graphene is shown to belong to a diverse class of finite-size extended systems with simple local interactions where complexity emerges spontaneously under very general conditions that can be a critical factor controlling observable properties such as chemical activity, electron transport, and spin-polarization.     

Tetraacyldiborates: selective and efficient acylation reagents suitable for multiple parallel synthetic applications

Boron-based mixed anhydrides are rapidly reactive, easy to prepare, cheap, efficient, and general acylating reagents capable of selectivity when chelation is possible. High yields of various esters, amides and thioesters are quickly obtainable and the products are easy to isolate in high purity. The method is readily used under multiple parallel synthesis conditions and is readily scaleable.     

A class of new silylating agents. II. A highly reactive reagent for introduction of the trimethylsilyl group.

Trimethylsilyl enol ethers of pentane-2,4-dione and methyl acetoacetate react rapidly with alcohols at room temperature without the need for catalytic assistance to give high isolated yields of trimethylsilyl ethers.     

A THEORETICAL STUDY OF EXCITONS IN CHLOROPHYLL a PHOTOSYSTEMS ON A PICOSECOND TIMESCALE

Abstract— The creation and decay of singlet excitons in structurally-explicit Chi a photosystems were investigated by numerical integration of the appropriate Master Equations. The antenna chlorophylls were excited by a simulated 6ps Gaussian light pulse. Positions and orientations for the antenna chlorophylls were randomly generated within regions whose size and shape were chosen to be appropriate for the particles observed in freeze-fractured photosynthetic membranes. Among the variables considered were chlorophyll concentration, depth of the trap (i.e. P680 or P700), and the R₀ parameter for Förster transfer. Among the properties considered were antenna fluorescence lifetime; detrapping rates; the instantaneous and integrated quantum yields for antenna fluorescence and intersystem crossing; and the instantaneous and integrated quantum yields for trap fluorescence, intersystem crossing, and photochemistry. A variety of experimental evidence was used as input to the computational model.     

The structure of ce-doped Bi2(MoO4)3 as determined by neutron profile refinement

The structure of Bi_1.8Ce_0.2(MoO₄)₃ has been refined with powder neutron diffraction data by the Rietveld method. The structure can be derived by severely distorting the scheelite structure (AMO₄) and is perhaps better written $\text{A}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{Ø}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{M}\text{O}{}_4$, where Ø = cation vacancy. Of the two bismuth atom sites, cerium preferentially occupies the more symmetric of the two (Bi(2) in the structure) with some cerium found in the scheelite subcell vacancies also. This site preference is understood by examining the symmetries of the two Bi sites. Crystal data: monoclinic, space group $\text{P2}{}_1\text{c}\text{, Z = 4, a = 7.697(2), b = 11.535(3), c = 11.944(3)}$, β = 115.19.     

Infrared spectrum and structure of the Hf(OH)4 molecule

Laser-ablated Hf atoms react with H2O2 and with H2 + O2 mixtures in solid argon to form the Hf(OH)2 and Hf(OH)4 molecules, which are identified from the effect of isotopic substitution on the matrix infrared spectra. Electronic structure calculations at the MP2 level varying all bond lengths and angles converge to nearly linear and tetrahedral molecules, respectively, and predict frequencies for these new product molecules and mixed isotopic substituted molecules of lower symmetry that are in excellent agreement with observed values, which confirms the identification of these hafnium hydroxide molecules. This work provides the first evidence for a metal tetrahydroxide molecule and shows that the metal atom reaction with H2O2 in excess argon can be used to form pure metal tetrahydroxide molecules, which are not stable in the solid state.     

Alternative antifouling biocides

In response to increasing scientific evidence on the toxicity and occurrence of organotin residues from antifouling paints in the aquatic environment, the use of triorganotin antifouling products was banned on boats of less than 25 m length in many countries during 1987. The use of tributyltin (TBT) products on small boats was superseded by products based on copper, containing organic booster biocides to improve the efficacy of the formulation. Available information and evidence on the occurrence, fate and toxicity of these biocides is reviewed. It is concluded that increased copper concentrations in the aquatic environment, due to the increased use of copper‐based antifoulants, do not have significant effects on marine ecosystems. However, lack of validated analytical methods, limited monitoring data, and very little information about the fate and toxicity of the booster biocides in the aquatic environment, make accurate risk assessments in relation to these compounds difficult. Copyright © 1999 John Wiley & Sons, Ltd.     

Analytical method development for the determination of four biocides used in antifouling paints in estuarine waters and sediments by gas chromatography-mass spectrometry

Summary Irgarol 1051, chlorothalonil, dichlofluanid and diuron are biocides utilised in antifouling paints as alternatives to organotin compounds, after restrictions imposed in the use of tributyltin in 1987. Effective analytical methods for the simultaneous determination of these four biocides in waters and sediments have been developed and method performance data are presented. Quantification is by gas chromatography—electron ionisation mass spectrometry (GC-EI-MS). The method involves the simultaneous extraction of the biocides from water with dichloromethane or from sediments with dichloromethane and acetone. For the determination of diuron the extract was methylated prior to GC analysis to avoid thermal degradation in the instrument. The method was applied to water and sediment samples spiked with the biocides. The highest detection sensitivities were achieved in the selected-ion monitoring (SIM) mode. Recovery studies were performed at 20, 2 and 1 ppb for all biocides in water and 400, 40 and 20 ppb in sediments. The percentage recoveries ranged between 90 and 100% for waters and 80 and 90% for sediments. Seven determinations were made at each concentration level along with a procedural blank. The quantification limit of the method was around 0.2 ppb for water and 5 ppb for sediments depending on the individual compound.