Delayed extraction experiments using a repulsive potential before ion extraction: evidence of clusters as ion precursors in UV-MALDI. Part I: dynamical effects with the matrix 2,5-dihydroxybenzoic acid

Delayed extraction experiments were undertaken to precise the dynamical effects involved in the ion formation in ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI). Careful examination of the ion time-of-flight variation with the extraction delay time were performed with a repulsive potential before ion extraction. Depending on the mass of the ion (matrix 2,5-dihydroxybenzoic acid and peptides) and on the repulsing potential, some deviations from the linear relationship between the ion time-of-flight and the delay time were observed. Simulations of the ion time-of-flight clearly show that ions are not directly produced on the target surface but originate from the gas-phase decomposition of higher mass precursors. The size of the precursor, composed of the analyte surrounded by matrix molecules, increases with that of the analyte. Complete desolvation of the cluster-precursor could be likely induced by the high electric field transient during the pulse extraction. The existence of clusters as precursor of the ion production in MALDI highlights a new global frame to explain the analyte protonation in UV-MALDI.     

Computational study of matrix-peptide interactions in MALDI mass spectrometry: Interactions of 2,5- and 3,5-dihydroxybenzoic acid with the tripeptide valine-proline-leucine

The mechanism of matrix-to-analyte proton transfer in matrix-assisted laser desorption and ionization mass spectrometry (MALDI-MS) has been investigated computationally by modeling the matrix-analyte interaction of potential MALDI matrixes such as 2,5-dihydroxybenzoic acid (2,5-DHB) and 3,5-DHB with the tripeptide valine-proline-leucine (VPL). A combination of molecular dynamics/simulated annealing calculations followed by density functional theory geometry optimization using a reasonably large basis set has been done on a large number of clusters in an attempt to study the ionization energy of each matrix in the cluster environment and the intracluster proton transfer from the matrix to the tripeptide. The calculations show a substantial reduction in the IP for both matrixes in their cluster environments. In the 2,5-system, proton transfer can sometimes occur in the neutral clusters (preformed ions), whereas proton transfer in the cationic clusters, which is actually a double proton transfer, is spontaneous and exoergic. Even though it is more acidic from a thermodynamic perspective, the radical cation of 3,5-DHB is a less efficient proton donor to VPL. The thermodynamics of proton transfer in the cationic clusters is discussed in detail.     

Positive ion formation in the ultraviolet matrix-assisted laser desorption / ionization analysis of oligonucleotides by using 2,5-dihydroxybenzoic acid

The development of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry and its demonstrated performance with large proteins has generated substantial interest in utilizing this technique as an alternative to gel electrophoresis for DNA sequence analysis. However, a lack of understanding of the desorption and ionization processes has greatly hampered advances in this field. This article explores the formation of positively charged oligonucleotides in UV (355-nm) MALDI analysis by using the matrix 2,5-dihydroxybenzoic acid. Whereas substantial fragmentation is observed in the positive-ion mode by using the short oligomer d(TAGGT), no fragmentation is evident in the negative-ion mode under identical conditions. The fragmentation products are consistent with a previously published model in which base protonation initiates base loss, which leads to subsequent cleavage of the phosphodiester backbone. Several polydeoxythymidilic acids containing modified nucleosides were used to investigate positive-ion formation. The results support the hypothesis that positive ions are formed by protonation of the nucleobases. Because base protonation initiates base loss, fragmentation is intrinsic to positive-ion formation in the MALDI analysis of oligonucleotides. This result explains the dramatic difference in fragmentation observed in positive-ion compared to negative-ion UV-MALDI mass spectra of oligonucleotides.     

Calcium complex of 2,5-dihydroxybenzoic acid (gentisic acid): synthesis,crystal structure,and spectroscopic properties

The synthesis, crystal structure, and spectral characteristics (IR, Raman, and ¹H and ¹³C NMR) of a monomeric hexa(aqua)calcium compound, viz. [Ca(C₇H₅O₄)₂(H₂O)₆]·H₂O (C₇H₅O₄ = 2,5-dihydroxybenzoate), are reported. The central Ca(II) located on a twofold axis is eight coordinate in an approximate square antiprismatic environment, bonded to two monodentate 2,5-dihydroxybenzoate ligands via the carboxylate oxygen, and six waters. The adjacent monomeric units are linked with the aid of several O–H?O hydrogen bonds.     

Vibrational studies in aqueous solutions: Part II. The acid oxalate ion and oxalic acid

Assignments for oxalic acid in solution are re-examined. A detailed assignment of the IR and Raman spectra of the acid oxalate ion is presented for the first time. Raman spectroscopy is used to study the first ionization of oxalic acid.     

The mechanism of matrix to analyte proton transfer in clusters of 2,5-dihydroxybenzoic acid and the tripeptide VPL

Intracluster proton transfer from the matrix-assisted laser desorption/ionization matrix 2,5-dihydroxybenzoic acid (DHB) to the peptide valyl-prolyl-leucine has been investigated as a function of excitation laser wavelength and power. Ionization laser power studies at 308 nm indicate that cluster ionization occurs with a two-photon dependence, whereas matrix-to-analyte proton transfer and cluster dissociation requires an additional photon. At 266 nm, two-photon absorption leads to both cluster ionization and cluster dissociation/proton transfer. A consideration of these results clearly indicates that analyte protonation occurs following ionization of the cluster to produce a radical cation matrix/analyte cluster. Mass spectral features also indicate that mixed DHB/peptide cluster ionization can occur via two-photon ionization at wavelengths as long as 355 nm. These results suggest a reduction in the ionization potential of larger mixed DHB/peptide clusters of greater than 1 eV. The reduced ionization potential seen in these clusters suggests that radical cation initiated proton transfer remains a viable mechanism for analyte protonation in matrix-assisted laser desorption/ionization at these longer wavelengths.     

Dissociation of salicylic acid, 2,4-, 2,5-and 2,6-dihydroxybenzoic acids in 1-propanol-water mixtures at 25°C

The dissociation constants and molar conductivities at infinite dilution of salicylic acid, 2,4-, 2,5-, and 2,6-dihydroxybenzoic acids have been determined in 1-propanol-water mixtures at 25°C. The experimental data have been analyzed by means of the Lee and Wheaton equation.     

Topography and field effects in secondary ion mass spectrometry Part II: insulating samples

A study is conducted on the effects of sample topography on the secondary ion mass spectrometry (SIMS) analysis of insulating samples, using poly(ethylene terephthalate) fibres (100 µm diameter) as a model system and simulations of the ion extraction field using finite element analysis. We focus on two significant issues: topographic field effects caused by the penetration of the extraction field into the sample, and the effect of charge compensation on the secondary ion images. Guidance is provided for setting the reflector voltage correctly for insulating fibres in reflectron SIMS instruments. The presence of the topographic sample distorts the extraction field, causing the secondary ions to be deflected laterally. This results in the severe loss of ion signals from the sides of the fibres because of the limited angular acceptance of the analyser. Strategies to reduce topographic field effects, including alternative sample mounting methods, are discussed. We also find that, in general, insulating samples are charged by the flood gun electrons resulting in a negative surface potential. This causes large variations in the SIMS images depending on the electron current, electron energy, raster mode and secondary ion polarity. Recommendations are given for analysts to obtain more reproducible images and reduce the effect of differential electron charging, for example by using a lower electron flood beam energy. © 2011 Crown copyright.     

Silica surface controversies,strong adsorption sites,their blockage and removal. Part II

Summary In this second part of the review heterogeneity of the silica surface is described and evidence is presented for the existence of a low population of strong adsorption sites. Methods of detection and determination of these strongly interacting sites are discussed. The last part of the review is devoted to the suppression of unwanted adsorption activity. Methods of blockage and special methods for the preparation of HPLC packings are described.     

Studies in cluster ion formation. Part II. Concentration and matrix effects on the cluster ions of tetraalkylammonium halide salts

The roles of the matrix and salt concentration on the formation of cluster ions during fast atom bombardment of tetraalkylammonium halide salts have been studied. The occurrence of anomalous ion intensity regions at certain cluster numbers (n) is found to depend strongly on the salt concentration in the matrix. In addition the time-dependent nature of the spectra is examined and cluster ion formation is explained by postulating that equilibrium processes occur during bombardment of the salt solution.     

Mechanistic study of electrochemical oxidation of 2,5-dihydroxybenzoic acid and 3,4-dihydroxybenzaldehyde in the presence of 3-hydroxy-1H-phenalene-1-one

The mechanism of the electrochemical oxidation of 2,5-dihydroxybenzoic acid and 3,4-dihydroxybenzaldehyde in the presence of 3-hydroxy-1H-phenalene-1-one as a nucleophile has been studied in water/acetonitrile (80/20 v/v) solution using cyclic voltammetry and controlled-potential coulometry methods. The results indicate that the quinones derived from oxidation of 2,5-dihydroxybenzoic acid and 3,4-dihydroxybenzaldehyde participate in Michael addition reactions with 3-hydroxy-1H-phenalene-1-one and via ECE and ECEC mechanisms convert to the different products, with good yield under controlled potential conditions, at carbon electrode.     

Noncovalent association phenomena of 2,5-dihydroxybenzoic acid with cyclic and linear oligosaccharides. A matrix-assisted laser desorption/ionization time-of-flight mass spectrometric and X-ray crystallographic study

D-Glucose and 19 glucose derivatives were investigated by positive and negative ion matrix assisted laser desorption/ionization time-of-flight mass spectrometry using 2,5-dihydroxybenzoic acid (DHB) as the matrix. The set of substrates includes oligomers of amylose and cellulose, native alpha-, beta-, and gamma-cyclodextrin, and chemically modified beta- and gamma-cyclodextrins. These analytes were chosen to modulate molecular weight, polarity, and capability of establishing noncovalent interactions with guest molecules. In the negative-ion mode, the DHB matrix gave rise to charged multicomponent adducts of type [M + DHB - H]- (M = oligosaccharide) selectively for those analytes matching the following conditions: (i) underivatized chemical structure and (ii) number of glucose units > or = 4. In the positive-ion polarity, only some amylose and cellulose derivatives and methylated beta-cyclodextrins provided small amount of cationized adducts with the matrix of type [M + DHB + X]+ (X = Na or K), along with ubiquitous [M + X]+ ions. The results are discussed by taking into account analyte-matrix association phenomena, such as hydrogen bond and inclusion phenomena, as a function of the molecular structure of the analyte. The conclusions derived by mass spectrometric data are compared with the X-ray diffraction data obtained on a single crystal of the 1:1 alpha-cyclodextrin - DHB noncovalent adduct.     

Irradiation effects on thermal,surface, and physicomechanical properties of ethylene and vinyl acetate copolymer

The structural and thermal transitions for ethylene and vinyl acetate copolymer (EVA) samples irradiated by fast electrons at doses in the range of 2.5–25 Mrad were investigated by DSC and X-ray diffraction analysis. The parameters of chemical bonds in the amorphous phase of copolymer were determined. The change in the degree of crystallinity, melting temperature, and crystallite sizes before and after radiochemical modification were estimated. The obtained results were analyzed and corresponded to the physicomechanical properties of copolymers. The surface energy of copolymers before and after irradiation was defined. The strength of adhesive joints based on EVA from PET substrates and the influence of radiochemical modification of adhesive before joint formation on its strength were analyzed.     

Characterization of surface modifications of stainless steel samples after electron and ion bombardment by SEM, AES, and XPS

The interaction of inert or reactive gas plasmas with the surface of stainless steel has been investigated with the aim, to modify the surface and hence to reduce the outgassing rate of the material, an important factor for the production of an ultrahigh vacuum. The plasma treatments investigated may be an alternative to the common used in situ baking. The samples have been exposed to electrons, argon and oxygen ions either in a DC glow discharge or in a microwave discharge. The DC glow discharge in Ar/O₂, the most effective plasma treatment reduces the outgassing rate by a factor of 10. After this treatment the surfaces of the samples have been investigated with respect to the topography and the chemical composition (depth profile) by Secondary Electron Microscopy (SEM), Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS), respectively. The surface modifications resulting from the different treatments of the samples have been correlated to the outgassing rate.     

Synthesis, characterization, and molecular structure of Ru(II) complex containing 2,5-pyridinedicarboxylic acid

This article presents a combined experimental and computational study of Ru(II) complex containing 2,5-pyridinedicarboxylic acid ligand. The novel complex [Ru(py-2,5-COOH)₂(PPh₃)₂]·3H₂O has been obtained in the reaction of [RuCl₂(PPh₃)₃] with 2,5-pyridinedicarboxylic acid in methanol and has been studied by IR, ¹H, ³¹P NMR, UV–Vis spectroscopy, and X-ray crystallography. The electronic structure of [Ru(py-2,5-COOH)₂(PPh₃)₂] has been calculated with the density functional theory (DFT) method. The spin-allowed electronic transitions of the complex have been calculated with the time-dependent DFT method, and the UV–Vis spectrum has been discussed on this basis and rationalized by determination of ligand field splitting (10Dq) and Racah’s parameters from the experimental spectrum. The luminescence property of the complex has been examined.     

The protonation and dimerization of 2,4-, 2,5-, 2,6-and 3,5-dihydroxybenzoic acids in 0.5M sodium perchlorate medium

The protonation equilibria of 2,4-, 2,5-, 2,6- and 3,5-dihydroxybenzoic acids were studied by means of potentiometric titrations at I = 0.5 (NaClO(4)) and 25 degrees . The dimeric species H(6)L(2) and H(5)L(2) were found to form as well as the monomeric species H(p)L in the acidic solutions of 2,4- and 3,5-dihydroxybenzoic acids under the conditions studied. For the other two acids, the protonation scheme can be expressed exclusively in terms of the species H(p)L (p = 1, 2 or 3).     

On the admittance of lipid bilayer membranes: Part II. Uncouplers and ion carriers

Rangarajan's modular formalism has been applied to those cases in which ion transport across an ultra-thin membrane is associated with membrane permeation of a neutral species. A single connectivity diagram can be used to represent the admittance in the presence of an ion carrier or of either a monomeric or a dimeric uncoupler.     

Electrochemical and spectroscopic studies of 2,3-dihydroxybenzoic acid,its oxidation products and their interaction with manganese(II), in dimethyl sulfoxide solutions

The electrochemical and spectroscopic behaviour of 2,3-dihydroxybenzoic acid (2,3-DHBA) and its oxidized forms have been studied in dimethyl sulfoxide solutions under aerobic and anaerobic conditions. The products resulting from the reaction with manganese(II) (in dimethyl sulfoxide) are also studied by cyclic voltammetry, u.v–vis., n.m.r. and e.s.r. spectroscopies. Under anaerobic conditions the anions of the ligand form stable complexes with manganese(II) and (III) of MnL₂ type, while in the presence of air the oxidized forms of the ligand react with manganese(II) to give mixed-valence species. The chemical stability of the semiquinone and its manganese complexes in addition to its photosensitivity is noteworthy. Calculations show that the electrogenerated manganese(III)–(2,3-DHB–semiquinone) system is stable, but redox-active and can undergo a two-electron exchange (per monomer). The dimeric (or oligomeric) species should be good candidates for water oxidation studies.     

Magnetite surface area titration using nitric oxide, II. Pretreatment effects and silica-supported samples

The use of NO adsorption for determination of Fe₃O₄ surface areas has been examined and extended, from earlier work on unsupported samples, to silica-supported Fe₃O₄. Gravimetric studies confirm earlier suggestions that the irreversible NO adsorption uptake on unsupported Fe₃O₄ is dependent upon sample pretreatment. Long (ca. 24 h) vacuum treatment at 650 K leads to increased apparent NO adsorption uptakes due to reduction of the surface. Magnetite particle size estimates were made using irreversible NO adsorption uptakes on silica-supported Fe₃O₄ samples that had received a standard 1 h vacuum pretreatment at 650 K. These estimates agreed well with average particle sizes calculated using x-ray line-broadening and magnetization measurements, indicating that NO adsorption can, in fact, be used to measure magnetite surface areas on silica-supported samples.

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Camille and Henry Dreyfus Foundation Teacher-Scholar; to whom all correspondence should be addressed.     

2,5-Furandicarboxylic acid production via catalytic oxidation of 5-hydroxymethylfurfural:Catalysts,processes and reaction mechanism

Biomass conversion to value-added chemicals has received tremendous attention for solving global warming issues and fossil fuel depletion.5-Hydroxymethylfurfural(HMF)is a key bio-based platform molecule to produce many useful organic chemicals by oxidation,hydrogenation,polymerization,and ring-opening reactions.Among all derivatives,the oxidation product 2,5-furandicarboxylic acid(FDCA)is a promising alternative to petroleum-based terephthalic acid for the synthesis of biodegradable plastics.This review analytically discusses the recent progress in the thermocatalytic,electrocatalytic,and photocatalytic oxidation of HMF into FDCA,including catalyst screening,synthesis processes,and reaction mechanism.Rapid fundamental advances may be possible in non-precious metal and metal-free catalysts that are highly efficient under the base-free conditions,and external field-assisted processes like electrochemical or photoelectrochemical cells.