On the mechanism of ferrocene oxidation under the conditions of electrospray ionization

Russian Chemical Bulletin - The formation of molecular ions of ferrocene is detected under the «inverse» conditions of electrospray ionization where the pure solvent alone is...     

Ferrocenylalkylation processes under electrospray ionization conditions

The electrospray ionization behavior of some ferrocenylalkylazoles CpFeC₅H₄CH(R)Az (AzH are derivatives of imidazole, pyrazole, triazole and their benzo analogs; R = H, Me, Et, Ph), ferrocenylalkanols CpFeC₅H₄CH(R)OH (R = H, Me), and mixtures of the latter with azoles was studied. The electrospray ionization mass spectra of these compounds, in addition to the molecular ion [M]^+·, the protonated molecule [M + H]⁺, and ferrocenylalkyl cation [FcCHR]⁺ peaks, exhibit also intensive peaks for the binuclear ions [(FcCHR)₂X]⁺ (X = Az or O), resulting from ferrocenylalkylation of the initial compounds with the ferrocenylalkyl cations. Electrospray ionization of an equimolar mixture of ferrocenylmethanol FcCH₂OH and imidazole gives the protonated ferrocenylmethylimidazole molecule [FcCH₂Im + H]⁺ and the [FcCH₂(Im)₂ + H]⁺ dimer, apart from the ions typical of each component, i.e., ferrocenylalkylation of azoles with the ferrocenylalkylcarbinols, known in the chemistry of solutions, takes place under electrospray conditions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1318–1321, August, 2006.     

Mass spectrometric behavior of levoglucosan under different ionization conditions and implications for studies of cellulose pyrolysis

Pyrolysis-mass spectrometric studies of cellulose indicate low abundances of levoglucosan in the product spectrum compared to the yield values determined in more conventional types of pyrolysis studies. To examine the reason for these conflicting observation, levoglucosan was examined under different ion source conditions and ionization modes to ascertain the relative contributions of thermal degradation and ionization fragmention to the low abundances of the levoglucosan molecular ion. Low-energy electron ionization using conventional sample volatilization and molecular-beam sampling is compared to chemical ionization using methane, isobutane, and ammonia as reagent gases, and to field ionization and desorption. The mass spectrometric fragmentation patterns under the various systems indicate that studies of cellulose pyrolysis underestimate the amount of levoglucosan formed due to ionization fragmentation and thermal rearrangement reactions in the ion source. Several peaks, including m/z 126 and 144, are dominated by the contribution from the fragmentation of levoglucosan.     

Investigation of the stabilities of neutral and ionic lead and lead-antimony clusters under single and multiphoton ionization conditions

The fluence dependence of lead cluster ion distributions at 222 nm and 308 nm reveal markedly different behavior. Results obtained at 308 nm display a simple uniform increase in intensity with higher laser fluence with little change in relative intensities. At 222 nm, however, a significant transformation is found from a markedly different low fluence distribution to a high fluence pattern, which is essentially indistinguishable from that observed at 308 nm. It is concluded that mass spectra obtained at 308 nm, regardless of fluence, or at 222 nm and high fluence contain appreciable contributions from fragmentation. Hence, under these conditions the mass spectra are found to be dominated by cluster ion stabilities. Magic numbers observed at both high and low fluence correspond well to those obtained using electron-impact ionization, and in many instances parallel the magic numbers characteristic of rare-gas clusters. This suggests the stabilities of both neutral and monovalent cationic lead clusters are largely determined by close-packing considerations, and are not appreciably influenced by electronic structure. Similar preferences for close-packed structures are also found for mixed lead-antimony clusters containing one or two antimony atoms that are ionized using high fluence 308 nm excitation.     

The reactivity of ferrocenylalkyl azoles under the conditions of electrospray ionization

Under the conditions of electrospray ionization of ferrocenylalkyl azoles FcCH(R)X (Fc-η⁵-C₅H₅Fe-η⁵-C₅H₄, R - H, Me, XH - 2-methyl imidazole, pirazole) the processes of oxidation, protonation, fragmentation and ferrocenylalkylation to form, molecular ions [М]⁺, protonated molecules [М+Н]⁺, ferrocenylalkyl cations [FсCHR]⁺ and bisferrocenylalkyl azole cations [(FcCHR)2X]⁺, respectively, take place. Using special experimental techniques (deuterated solvents, saturation of ionic source of an ESI mass-spectrometer by the vapors of solvents, the experiments under the “inverse” ESI conditions when the solvent is subjected to electrospray in the presence of ferrocenylalkyl derivative vapours) and quantum-chemical calculations at the level of the B3LYP/LanL2DZ theory the scheme of the formation of these ions in a gas phase according to the mechanism of “activating protonation” was suggested. it was found that all these ions are formed through the protonation stage, which is taking place mainly in a gas phase. The key stage is the exothermic process of the protonation of the initial compounds by hydroxonium ions giving rise to protonated [M+H]⁺ molecules which further oxidize and alkylate ferrocenylalkylazoles to form molecular radical cations and bisferrocenylalkyl azole ions [FcCH(Me)-X-CH(Me)Fc]⁺. The decomposition of protonated ions with the elimination of the azole molecule gives rise to ferrocenylalkyl cations [FсCHR]⁺ capable in turn of oxidizing and alkylating the initial compounds.     

Fragmentation patterns of novel dithiocarbamate derivatives with pharmaceutical activity under electrospray ionization tandem mass spectrometry conditions

The fragmentation patterns of a novel series of dithiocarbamate derivatives with pharmaceutical activity were investigated by positive ion electrospray ionization mass spectrometry in conjunction with tandem mass spectrometry (ESI-MS(n)). In the gas phase, the dithiocarbamate protonated molecules containing the piperazinium moiety undergo losses of bromide to form the piperazinium cation by ionization, followed by subsequent losses of methyl bromide, ring opening and rearrangement of piperazinium. Furthermore, the dithiocarbamate derivatives and their intermediates both undergo cleavage of the C-S bond to produce two common fragment ions. The different fragmentation observed for these compounds facilitated their identification and could be valuable in the further study of their metabolic pathways as prodrugs.     

Comparative studies on photo-ionization and electron-impact ionization of peptide derivatives

Photo-ionization (PI) mass spectrometry performed with a monochromatic photon beam was applied to a series of peptide derivatives. PI mass spectra of ten N-acylpeptide methyl esters containing two to four residues of glycine, alanine, valine, leucine, proline, tryptophan, tyrosine, phenylalanine, methionine, carboxymethylcysteine, lysine and ornithine were studied. Comparative analysis of PI (10.2 eV) and electron-impact ionization (EI) (70 eV) mass spectra shows the total number of peaks on PI to be much less than that obtained with EI, especially in the low m/e region (< 250 to 300). At the same time the relative abundance of ‘heavy’ ions, including molecular ions, is much higher in PI. The amino acid fragmentation pattern followed by N-acylpeptide esters in PI was found to be the same as for EI.     

Chiral discrimination of alpha-amino acids by DNA tetranucleotides under electrospray ionization conditions

A set of DNA tetranucleotides, which are 3'- or 5'-end extended versions of GCA, was used as chiral selectors for the discrimination of enantiomers of alpha-amino acids. The [X+Y-2H](2-) ions of the 1:1 complexes were generated by electrospraying a mixture of tetranucleotide (X) and amino acid (Y) solution. Chiral discrimination was achieved by studying the collision-induced dissociation spectra of the [X+Y-2H](2-) ion and the ratio of relative abundance of precursor ion to that of the product ion was used to measure the extent of discrimination. Among the tetranucleotides used, GCAA and GGCA exhibited better discrimination, in which GCAA showed D-selectivity and GGCA showed L-selectivity for the studied amino acids. In addition, binding constants were measured for the 1:1 complexes of phenylalanine enantiomers with GCAA and GGCA. Ltd.     

Erythrocyte electrokinetic behavior under low ionic strength conditions

Under low ionic strength conditions, a linear approximation of the electrostatic potential within the charged polyelectrolyte layer coating the membrane bilayer leads to overestimation of the red cell electrokinetic mobility. Considering the full non linearized Poisson-Boltzmann and Navier-Stokes equations relative to the erythrocyte system, the deviation at low ionic strength between theoretical and experimental values for the cell mobility then almost disappears. A weak glycocalyx expansion may account for the remaining differences. However, this result is in contrast with previously suggested explanations involving an important stretching of the polyelectrolyte chains to interpret the “artificial” erythrocyte aggregation at low ionic strength. Under such conditions cell aggregation is probably induced by haemolysate adsorption onto the erythrocyte membrane and is unlikely to be the consequence of a strong interfacial rearrangement.     

二茂铁在离子液体1-甲基-3-丁基咪唑鎓盐中的电化学行为

在电化学领域,由于离子液体有较宽的电化学窗口、高的电导率而吸引着众多的研究者。伏安法常被用于测定离子液体的电化学行为,但一般都是采用金属丝或者不常用的氧化还原物质作参比电极,如I^-／I3^-[8]副和Li^＋／Li^[9],而这些参比并不常用于传统的有机溶剂中,与在传统的有机溶剂中测得的结果不具有可比性,为了克服这个问题,IUPAC推荐的一些电势标准物质作为电势测定标准。本文采用玻碳圆盘电极测定了二茂铁（Fc）在离子液体1-甲基3-丁基咪唑铃盐BMIMBF4中的伏安行为,实验表明,在GC电极上还原过程表现为可逆单电子氧化还原过程,其可逆性良好,可将二茂铁应用于离子液体MBIMBF4中作为准参比系统。     

Differentiation of diastereomeric conduramine derivatives under electron ionization and chemical ionization mass spectral conditions

Diastereomeric conduramine derivatives, i.e., (1R,2S,3R/S,6S)-6-(N-carbomethoxyamino) 1,2-O-isopropylidenecyclohex-4-ene-1,2,3-triol (1 and 2) and their O-acetyl derivatives (3 and 4), were studied using gas chromatography (GC) with electron ionization (EI) and chemical ionization (CI). The EI mass spectra of diastereomeric pairs show consistent differences in the relative abundances of characteristic ions. The EI fragmentation patterns are based on precursor/product ion spectra, high-resolution mass spectrometry (HRMS) and deuterium labelling. The CI spectra show differences from the EI spectra, and the isobutane/CI spectra are much simpler than the methane/CI spectra. The differences shown in the CI spectra are similar to those shown in the product ion spectra of [M+H](+) ions generated under electrospray ionization (ESI) conditions. Theoretical calculations are performed to understand the observed differences. The differences in the relative stabilities of molecular ions, or protonated molecules at different sites, can explain the observed differences in the spectra.     

Probe electrospray ionization of mixture solutions using metal needles with different tip conditions

Time-dependent mass spectra of mixture solutions of cytochrome c and cholesterol were measured by probe electrospray ionization (PESI) mass spectrometry and the PESI characteristics for several metal needles were investigated with different tip conditions. For a mixture solution of cytochrome c and cholesterol in 0.1% formic acid methanol/water (1/1) at a motion frequency of 1 Hz, an acupuncture needle with a sharp tip and 0.5-mm-diameter titanium and stainless-steel wire needles gave strong cytochrome c and extremely weak cholesterol ion signals. When the frequency was lowered, and the high-voltage duration increased, the titanium and stainless-steel needles provided both cytochrome c and cholesterol ion signals with high intensities well separated over time.     

Transformation of neutral rhenium compounds during electrospray ionization mass spectrometry

Four neutral rhenium compounds were examined by electrospray ionization mass spectrometry. Acetonitrile solutions of (Ind)Re(CO)₃ (Ind = indenyl) and (Cp)Re(CO)₃ (Cp = cyclopentadienyl) gave rise to [Re(CO)₃(CH₃CN)₃]⁺ ions. This is indicative of a reaction with the solvent, although these compounds do not react with acetonitrile under regular laboratory conditions. In contrast, (Ind)Re(CO)₂(butyne) and (Cp)Re(CO)₂(butyne) did not lose their aromatic hydrocarbon ligand upon ionization; the predominant product ions generated upon electrospray ionization were [(Ind)Re(CO)(CH₃CN)(butyne)]⁺ and [(Cp)Re(CO)(CH₃CN)(butyne)]⁺, respectively.     

Study of the electrospray ionization tandem mass spectrometry of sildenafil derivatives

A detailed analysis of the product ion spectrum generated from the protonated molecule of sildenafil (Viagra(R)) under multiple tandem mass spectrometry (ESI-MS(n)) conditions using an ion-trap mass spectrometer is reported. Molecular composition data of the fragment ions were obtained with the aid of comparisons of the multiple tandem mass spectra of eight sildenafil derivatives in two series, the structures of which are identical except for some substituted alkyl groups. Attempts have been made to provide rational pathways for the formation of the fragment ions from these protonated sildenafil derivatives. The structure-fragmentation relationships will facilitate the characterization of the structures of other sildenafil analogs.     

Estimation of proton affinity of proline and tryptophan under electrospray ionization conditions using the extended kinetic method

The relative order of the proton affinity (PA) of 20 naturally occurring amino acids has been determined under electrospray ionization conditions and compared with earlier studies of different research workers. The order we obtained is similar to that reported by other groups except in three cases viz., valine--aspartic acid, asparagine--glutamic acid and tryptophan--proline. The PA values of proline and tryptophan are determined by the extended kinetic method using amino acids themselves as reference bases. The PA values we thus obtained for proline and tryptophan are 219.9 and 221.6 kcal/mol, respectively.     

Simple ammonium ionic liquid catalyses the 1,5-benzodiazepine derivatives under mild conditions

Abstract

An incredibly elementary procedure for the effective synthesis of 1,5-benzodiazepine derivatives using triethyl ammonium acetate (TEAA) [Et₃NH][CH₃COO] ionic liquid has been developed. TEAA was found to be practical, inexpensive, reusable and has a simple work-up procedure, which is absent in other catalyst. The role of TEAA as the reaction medium as well as the catalyst has shown the benefits of the use of TEAA in organic synthesis. The effect of [Et₃NH][CH₃COO] on the synthesis of 1,5-benzodiazepine derivatives has been studied and results are provided herein.     

Electrochemical studies and potential anticancer activity in ferrocene derivatives

Several ferrocene derivatives (five mononuclear and two binuclear), including the new N-(p-chlorophenyl)-carboxamidoferrocene (1), were synthesized and their anticancer activity investigated. Two of them, 3 and 7, bearing a benzimidazole backbone were the most active against HeLa cells achieving IC₅₀ values of ~5 μM along with 4 with a dipyridylamine ligand (~6 μM). Complex 6, also with a benzimidazole backbone, displayed slightly higher values (~11 μM). Cyclic voltammetry studies show that while the non-cytotoxic ferrocene derivatives 1, 2, and 5 follow a ferrocene-based redox behavior, derivatives 3, 4, 6, and 7 exhibit a more complex mechanism. These complex mechanisms are consistent with a more effective cytotoxic activity. Mössbauer spectroscopy parameters reflect a very small influence of the substituents.     

Reactivity of benzenetricarbonylchromium derivatives under conditions of dissociative ionization in the gas phase

The reactivity of benzenetricarbonylchromium derivatives under conditions of their fragmentation by electron impact was studied. The generalized structural and mass spectral characteristics formed on the basis of the information theory, molecular graph theory, and theory of absolute reaction rates were used to establish correlations between some of these characteristics.     

Mass spectral analysis of N-oxides of Chemical Weapons Convention related aminoethanols under electrospray ionization conditions

N,N'-Dialkylaminoethanols are the hydrolyzed products or precursors of chemical warfare agents such as V-agents and nitrogen mustards, and they are prone to undergo oxidation in environmental matrices or during decontamination processes. Consequently, screening of the oxidized products of aminoethanols in aqueous samples is an important task in the verification of chemical weapons convention-related chemicals. Here we report the successful characterization of the N-oxides of N,N'-dialkylaminoethanols, alkyl diethanolamines, and triethanolamine using positive ion electrospray ionization mass spectrometry. The collision-induced dissociation (CID) spectra of the [M+H](+) and [M+Na](+) ions show diagnostic product ions that enable the unambiguous identification of the studied N-oxides, including those of isomeric compounds. The proposed fragmentation pathways are supported by high-resolution mass spectrometry data and product/precursor ion spectra. The CID spectra of [M+H](+) ions included [MH-CH(4)O(2)](+) as the key product ion, in addition to a distinctive alkene loss that allowed us to recognize the alkyl group attached to the nitrogen. The [M+Na](+) ions show characteristic product ions due to the loss of groups (R) attached to nitrogen either as a radical (R) or as a molecule [R+H or (R-H)] after hydrogen migration.