Evaluation of eluents in thermospray liquid chromatography-mass spectrometry for identification and determination of pesticides in environmental samples.

The influence of different eluents in positive and negative ion mode thermospray liquid chromatography-mass spectrometry was studied with several groups of pesticides, including carbamates, chlorotriazines, phenylureas, phenoxy acids and organophosphorus and quaternary ammonium compounds, and the corresponding degradation products. Using the positive ion mode in combination with reversed-phase eluents the base peaks generally corresponded either to [M + H]+ for the chlorotriazines and their hydroxy metabolites or to [M + NH4]+ for the carbamates, the phenylureas, the organophosphorus pesticides and their oxygen analogues. In the negative ion mode different processes such as (dissociative) electron-capture and anion attachment mechanisms occurred. Fragment ions such as [M - CONHCH3]- for the carbamates, [M - H]- for the chlorotriazines, phenylureas and chlorinated phenoxy acids and [M].-, [M - R]- (R being a methyl or ethyl group) for organophosphorus pesticides were usually formed. Depending on the eluent additive used (ammonium acetate, ammonium formate and/or chloroacetonitrile), three different adduct ions were formed: [M + CH3COO]-, [M + HCOO]- and [M + Cl]-. Normal-phase eluents with cyclohexane, n-hexane and/or dichloromethane provided more structural information and enhanced the response of several compounds. The positive ion mode was useful for the detection of chlorinated phenoxy acids and chlorophenols which could not be detected in the positive ion mode using reversed-phase systems. The base peaks generally corresponded to [M].+, [M + H]+ or [M - Cl]+. For the characterization of difenzoquat, a quaternary ammonium pesticide of which trace level analysis is troublesome, a post-column ion-pair extraction system was used. An aqueous mobile phase with a sulphonate-type counter ion was applied and an extraction solvent containing cyclohexane-dichloromethane-n-butanol (45:45:10) was used in thermospray liquid chromatography-mass spectrometry. Illustrative examples of the determination of residue levels of pesticides in soil matrices are shown.     

Influence of reversed- and normal-phase liquid chromatographic eluents in the fragmentation of selected chlorinated herbicides in thermospray liquid chromatography-mass spectrometry

The effect of two completely different mobile phase compositions, reversed-phase acetonitrile-water + ammonium acetate and normal-phase cyclohexane, were compared in filament-on thermospray liquid chromatography-mass spectrometry (LC-MS) for the determination of selected chlorinated herbicides such as chloroatrazines and chlorinated phenoxyacetic acids. By using acetonitrile-water + 0.05 M ammonium acetate mixtures in positive ion mode thermospray LC-MS, the chloroatrazine herbicides showed the acetonitrile adduct ion [M + (CH₃CN)H]⁺ as the base peak, whereas the chlorinated phenoxyacetic acids showed no signal. In contrast, when cyclohexane, which is reported for the first time as an eluent in the thermospray technique, was used as the mobile phase the chlorinated phenoxyacetic acid herbicides exhibited [M – H]⁺, [M – Cl]⁺ and M⁺˙ as the main ions. Negative ion mode thermospray LC-MS showed [M – H]^? as the base peak for the chloroatrazines in the different mobile phases, whereas the chlorinated phenoxyacetic acids exhibited [M + H]^?, [M + Cl]^? or [M – HCl]^? as the base peaks in cyclohexane and [M + acetate]^? in acetonitrile-water-ammonium acetate.     

Analysis of pyridoquinoline derivatives by liquid chromatography/atmospheric pressure chemical ionization mass spectrometry

A method using liquid chromatography/atmospheric pressure chemical ionization mass spectrometry (LC/APCI-MS) has been developed for the characterization and determination of pyridoquinoline derivatives 4,6-bis(dimethylaminoethylamino)-2,8,10-trimethylpyrido[3,2-g]quinoline, 4,6-bis(dimethylaminoethoxy)-2,8,10-trimethylpyrido[3,2-g]quinoline and 4,6-bis[(dimethylaminoethyl)thio]-2,8,10-trimethylpyrido[3,2-g] quinoline, all with potential antitumor properties. LC separation was performed on a conventional C18 column using a binary mobile phase composed of acetonitrile and 50 mM aqueous ammonium formate at pH 3. The APCI mass spectra obtained showed that proton addition giving [M + H]+ was the common mode of ionization to the amino- and thiopyridoquinolines, whereas the alkoxypyridoquinoline was identified by the main formation of the [M - (C2H3)N(CH3)2 + H]+, followed by the [M + H]+ ion. The LC separation conditions and MS detection parameters were optimized for the determination. The analytical method was also applied to the determination of these pyridoquinoline derivatives in fetal calf serum using liquid-liquid extraction with dichloromethane. Acceptable recovery values were obtained, ranging between 45 and 98%.     

Improved protonation, collision-induced decomposition efficiency and structural assessment for 'red tide' brevetoxins employing nanoelectrospray mass spectrometry

Brevetoxins are a group of natural neurotoxins found in blooms of red tide algae. Previous electrospray mass spectrometry (ES-MS) studies show that all brevetoxins have high affinities for sodium ions, and they form abundant sodium adduct ions, [M + Na]+, in ES-MS, even when trace contamination is the only source of sodium ions. Attempts to obtain informative product ions from the collision-induced decomposition (CID) of [M + Na]+ brevetoxin precursor ions resulted only in uninformative sodium ion signals, even under elevated collision energies. In this study, a nano-ES-MS approach was developed wherein ammonium fluoride was used to form cationic [M + NH4]+ adducts of brevetoxin-2 and brevetoxin-3; a significant increase in the abundance of protonated brevetoxin molecules [M + H]+ also resulted, whereas the abundance of sodium adducts of brevetoxins [M + Na]+ was observed to decrease. Under CID, both [M + NH4]+ and [M + H]+ gave similar, abundant product ions and thus underwent the same types of fragmentation. This indicated that ammonium ions initially attached to brevetoxins forming [M + NH4]+ easily lose neutral ammonia in a first step in the gas phase, leaving protonated brevetoxin [M + H]+ to readily undergo further fragmentation under CID.     

Reactivity of anomeric 1alpha- and 1beta-pentofuranosyl azide derivatives in ammonia chemical ionization

Electron impact (EI), fast atom bombardment (FAB) and ammonia chemical ionization [CI(NH3)] mass spectrometry were applied with the aim of differentiating between the anomeric 1alpha- and 1beta-azidopentofuranosyl derivatives. Calculated ammonium affinities [AA(M)] and proton affinities [PA(M)] show that beta-anomers have higher affinities for H+ and NH4+ ions than alpha-azides. Protonated molecules, obtained by CI(NH3) of azidofuranosyl derivatives, lose HN3 giving abundant furanosyl (S+) ions. Ammonia solvation of MH+ ions competes with the previous reaction producing the [SNHN2NH3]+ ion, a competitive product to the ammonium-attached [SN3NH4]+ ion. The fragmentation pathways of the stable and metastable [MNH4]+, MH+ ions, and several other important fragment ions, were determined using mass analyzed ion kinetic energy spectrometry (MIKES). The abundance of the [SN3NH4]+ and/or [SNHN2NH3]+ ions was found to correlate inversely with the exothermicity of ammonia solvation of the MH+ ion. The abundance of the fragment ions [SNHNH3]+, [SNH3]+ and SNH+ in some examples correlates with the exothermicity of the corresponding [MNH4]+ and MH+ parent ion formation. The fragment ions SNH3+ and SNHNH3+ can be formed, at least in part, in the ammonia solvation reaction of the S+ and SNH+ ions taking place within the high-pressure region of the CI ion source.     

Speciation of cyclo(Pro-Gly)<Subscript>3</Subscript> and its divalent metal-ion complexes by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) was used to study the binding of selected group II and divalent transition-metal ions by cyclo(Pro-Gly)3 (CPG3), a model ion carrier peptide. Metal salts (CatXn) were combined with the peptide (M) at a molar ratio of 1:10 M/Cat in aqueous solvents containing 50% vol/vol acetonitrile or methanol and 1 or 10 mM ammonium acetate (NH4Ac). Species detected include [M+H]+, [M+Cat-H]+, [M2+Cat]2+, [M+Cat+Ac]+, and [M+Cat+X]+. The relative stabilities of complexes formed with different cations (Mg2+, Ca2+, Sr2+, Mn2+, Co2+, Ni2+, Cu2+, and Zn2+) were determined from the abundance of 1:1 and 2:1 M/Cat species relative to that of the unbound peptide. The largest metal ions (Ca2+, Sr2+, and Mn2+) formed the most stable complexes. Reducing the buffer concentration increased the overall extent of metal binding. Results show that the binding specificity of CPG3 depends upon the size of the metal ion and its propensity for electrostatic interaction with oxygen atoms. Product ion tandem mass spectrometry of [M+H]+ and [M+Cu-H]+ confirmed the cyclic structure of the peptide, although the initial site(s) of metal attachment could not be determined.     

Detection of oxidative species for 4-phenoxyphenol derivatives during the electrospray ionization process

Analyses by flow injection as well as liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS) were performed with four 4-phenoxyphenol derivatives. When ambient temperature nitrogen gas was used to facilitate solvent evaporation, [M + H]+, [M + NH4]+, and [2M + NH4]+ ions were observed as the major ions. As the nitrogen gas temperature increased from ambient to 250 and 450 degrees C, [M]+*, [M - 1]+ and [M + 15]+ ions were the predominant ions. Heat-induced oxidation was found to be the primary source for the formation of oxidative species. Aqueous solvents were found to be essential for the formation of the [M + 15]+ ions. The [M]+* and [M + 15]+ ions were further characterized by tandem mass spectrometry. Based on the MS/MS data, it was proposed that the [M + 15]+ ions were the in-source generated 1,2-quinone ions.     

Electrospray and atmospheric pressure chemical ionization tandem mass spectrometric behavior of eight anabolic steroid glucuronides

Mass spectrometric and tandem mass spectrometric behavior of eight anabolic steroid glucuronides were examined using electrospray (ESI) and atmospheric pressure chemical ionization (APCI) in negative and positive ion mode. The objective was to elucidate the most suitable ionization method to produce intense structure specific product ions and to examine the possibilities of distinguishing between isomeric steroid glucuronides. The analytes were glucuronide conjugates of testosterone (TG), epitestosterone (ETG), nandrolone (NG), androsterone (AG), 5alpha-estran-3alpha-ol-17-one (5alpha-NG), 5beta-estran-3alpha-ol-17-one (5beta-NG), 17alpha-methyl-5alpha-androstane-3alpha,17beta-diol (5alpha-MTG), and 17alpha-methyl-5beta-androstane-3alpha,17beta-diol (5beta-MTG), the last four being new compounds synthesized with enzyme-assisted method in our laboratory. High proton affinity of the 4-ene-3-one system in the steroid structure favored the formation of protonated molecule [M + H]+ in positive ion mode mass spectrometry (MS), whereas the steroid glucuronides with lower proton affinities were detected mainly as ammonium adducts [M + NH4]+. The only ion produced in negative ion mode mass spectrometry was a very intense and stable deprotonated molecule [M - H]- . Positive ion ESI and APCI MS/MS spectra showed abundant and structure specific product ions [M + H - Glu]+, [M + H - Glu - H2O]+, and [M + H - Glu - 2H2O]+ of protonated molecules and corresponding ions of the ammonium adduct ions. The ratio of the relative abundances of these ions and the stability of the precursor ion provided distinction of 5alpha-NG and 5beta-NG isomers and TG and ETG isomers. Corresponding diagnostic ions were only minor peaks in negative ion MS/MS spectra. It was shown that positive ion ESI MS/MS is the most promising method for further development of LC-MS methods for anabolic steroid glucuronides.     

Evaluation of the potential of thermospray liquid chromatography-mass spectrometry in neurochemistry

Optimized operating conditions previously developed for the determination of neuroactive indoleamines and metabolites were adapted to meet the requirements of thermospray liquid chromatography-mass spectrometry (LC-MS) in terms of the ammonium acetate buffer system needed in this technique. Mass spectra were obtained for nineteen indolic compounds in both the positive and negative ion modes. The positive thermospray mass spectra of indoles with a free primary amino group are characterized by the base peak at [M + H]+, whereas the alcohol and acid metabolites show the base peak at [M + NH4]+. In the negative mode only amino acids and acids give good mass spectra with base peaks at [M - H + ACOOH]-. Detection limits by selected ion monitoring were of the order of 50-100 pg SIM on-column, allowing the direct determination of endogenous serotonin in an extract from rat hypothalamus. Quantitation was performed by isotope dilution MS. In the same way 5-hydroxyindoleacetic, indoleacetic, indolepropionic and indolelactic acids in urine were directly determined in an ethyl acetate extract from acidified urine samples. Likewise, gamma-aminobutyric acid and tricyclic antidepressants gave detection limits of 10 pg whereas only nanogram sensitivity could be achieved with catecholamines.     

2-rhodaoxetanes: their formation of oxidation of [RhI(ethene)]+ and their reactivity upon protonation

New cationic, pentacoordinate complexes [(TPA)Rh1(ethene)]+, [1a]+, and [(MeTPA)Rh1(ethene)]+, [1b]+, have been prepared (TPA = N,N,N-tri(2-pyridylmethyl)amine, MeTPA = N-[(6-methyl-2-pyridyl)-methyl]-N,N-di(2-pyridylmethyl)amine). Complex [1a]+ is selectively converted by aqueous HCl to [(TPA)RhIII-(ethyl)Cl]+, [2a]+. The same reaction with [1b]+ results in the [(MeTPA)RhIII-(ethyl)Cl]+ isomers [2b]+ and [2c]+. Treatment of [1a]+ and [1b]+ with aqueous H2O2 results in a selective oxygenation to the unsubstituted 2-rho-da(III)oxetanes (1-oxa-2-rhoda(III)cyclo-butanes) [(TPA)RhIII(kappa2-C,O-2-oxyethyl)]+, [3a]+, and [(MeTPA)RhIII(kappa2-C,O-2-oxyethyl)]+, [3b]+. The reactivity of 2-rhodaoxetanes [3a]+ and [3b]+ is dominated by the nucleophilic character of their 2-oxyethyl oxygen. Reaction of [3a]+ and [3b]+ with the non-coordinating acid HBAr(f)4 results in the dicationic protonated 2-rhodaoxetanes [(TPA)RhIII(kappa2-2-hydroxyethyl)]2+, [4a]2+, and [(MeTPA)RhIII(kappa2-2-hydroxyethyl)]2+, [4b]2+. These eliminate acetaldehyde at room temperature, probably via a coordinatively unsaturated kappa1-2-hydroxyethyl complex. In acetonitrile, complex [4a]2+ is stabilised as [(TPA)-RhIII(kappa1-2-hydroxyethyl)(MeCN)]2+, [5a]2+, whereas the MeTPA analogue [4b]2+ continues to eliminate acetaldehyde. Reaction of [3a]+ with NH4Cl and Mel results in the coordinatively saturated complexes [(TPA)RhIII(kappa1-2-hydroxyethyl)(Cl)]+, [6a]+, and [(TPA)-RhIII(kappa1-2-methoxyethyl)(I)+, [7a]+, respectively. Reaction of [3a]+ with NH4+ in MeCN results in formation of the dicationic metallacyclic amide [(TPA)-RhIII [kappa2-O,C-2-(acetylamino)ethyl]]2+, [9]2+, via the intermediates [4a]2+, [5a]2+ and the metallacyclic iminoester [(TPA)RhIII[kappa2-N,C-2-(acetimidoyloxy)ethyl]]2+, [8]2+. The observed overall conversion of the [Rh(I)(ethene)] complex [1a]+ to the metallacyclic amide [9]2+ via 2-rhodaoxetane [3a]+, provides a new route for the amidation of a [RhI(ethene)] fragment.     

Two chain compounds of [M(N3)2(HCOO)][(CH3)2NH2] (M = Fe and Co) with a mixed azido/formato bridge displaying metamagnetic behavior

Two new magnetic chain compounds, where the anionic [M(N3)2(HCOO)-]n chains with a 3-fold bridge of two end-on azido and one syn-syn formato ligands are isolated by the cations of [(CH3)2NH2]+, exhibit metamagnetism because of the strong intrachain ferromagnetic coupling versus the weak interchain antiferromagnetic coupling.     

Direct oxidation of L-cysteine by [FeIII(bpy)2(CN)2]+ and [FeIII(bpy)(CN)4]-

The oxidation of L-cysteine by the outer-sphere oxidants [Fe(bpy)2(CN)2]+ and [Fe(bpy)(CN)4]- in anaerobic aqueous solution is highly susceptible to catalysis by trace amounts of copper ions. This copper catalysis is effectively inhibited with the addition of 1.0 mM dipicolinic acid for the reduction of [Fe(bpy)2(CN)2]+ and is completely suppressed with the addition of 5.0 mM EDTA (pH<9.00), 10.0 mM EDTA (9.010.0) for the reduction of [Fe(bpy)(CN)4]-. 1H NMR and UV-vis spectra show that the products of the direct (uncatalyzed) reactions are the corresponding Fe(II) complexes and, when no radical scavengers are present, L-cystine, both being formed quantitatively. The two reactions display mild kinetic inhibition by Fe(II), and the inhibition can be suppressed by the free radical scavenger PBN (N-tert-butyl-alpha-phenylnitrone). At 25 degrees C and micro=0.1 M and under conditions where inhibition by Fe(II) is insignificant, the general rate law is -d[Fe(III)]/dt=k[cysteine]tot[Fe(III)], with k={k2Ka1[H+]2+k3Ka1Ka2[H+]+k4Ka1Ka2Ka3{/}[H+]3+Ka1[H+]2+Ka1Ka2[H+]+Ka1Ka2Ka3}, where Ka1, Ka2, and Ka3 are the successive acid dissociation constants of HSCH2CH(NH3+)CO2H. For [Fe(bpy)2(CN)2]+, the kinetics over the pH range of 3-7.9 yields k2=3.4+/-0.6 M(-1) s(-1) and k3=(1.18+/-0.02)x10(6) M(-1) s(-1) (k4 is insignificant in the fitting). For [Fe(bpy)(CN)4]- over the pH range of 6.1-11.9, the rate constants are k3=(2.13+/-0.08)x10(3) M(-1) s(-1) and k4=(1.01+/-0.06)x10(4) M(-1) s(-1) (k2 is insignificant in the fitting). All three terms in the rate law are assigned to rate-limiting electron-transfer reactions in which various thiolate forms of cysteine are reactive. Applying Marcus theory, the self-exchange rate constant of the *SCH2CH(NH2)CO2-/-SCH2CH(NH2)CO2- redox couple was obtained from the oxidation of L-cysteine by [Fe(bpy)(CN)4]-, with k11=4x10(5) M(-1) s(-1). The self-exchange rate constant of the *SCH2CH(NH3+)CO2-/-SCH2CH(NH3+)CO2- redox couple was similarly obtained from the rates with both Fe(III) oxidants, a value of 6x10(6) M(-1) s(-1) for k11 being derived. Both self-exchange rate constants are quite large as is to be expected from the minimal rearrangement that follows conversion of a thiolate to a thiyl radical, and the somewhat lower self-exchange rate constant for the dianionic form of cysteine is ascribed to electrostatic repulsion.     

Liquid ionization mass spectrometry of some triorganotin carboxylates.

and ESI, in which [M + H]+ were not observed or the spectra were complicated. The liquid ionization mass spectra of triorganotin carboxylates varied with solvents and sample concentrations. For instance, the fragment ions [M + (C4H9)3Sn]+ of dimeric ions were observed with chloroform used as a solvent, while the [M + H]+ were observed as the base peak using ethylene dichloride. Spectra useful for the differentiation of isomers [CgH7O3Sn(C4Hg)3] were obtained by the formation of characteristic adduct ions, such as [M + EA + H]+ and [M + 2EA + H]+, with a reagent like 2-aminoethanol. Collision-induced dissociation (CID) spectra observed by ESI and LPI mass spectrometry were similar and provided less information than adduct ions did.     

Ionization of anabolic steroids by adduct formation in liquid chromatography electrospray mass spectrometry

The ionization of 46 anabolic steroids has been studied. The absence of basic or acidic moieties in most of these analytes makes their direct ionization as [M + H]+ by atmospheric pressure interfaces difficult. The formation of adducts with different components of the mobile phase has been found to be an efficient way to ionize anabolic steroids by electrospray. Different mobile phases using methanol (MeOH) or acetonitrile as organic solvent and HCOOH, Na+ or NH4+ as additives have been tested to favor the adduct formation. A direct correlation between the chemical structure of the anabolic steroid and the possibility to ionize it in a particular chromatographic condition has been found. According to their ionization, anabolic steroids can be divided into seven different groups depending on both the nature and the relative position of their functional groups. The formation of different adducts such as [M + Na + MeOH]+ or [M + H + CH3 CN - H2O]+ is required in order to ionize some of these groups and the optimal mobile phase composition for each group of anabolic steroids is proposed. Despite the ionization limitations due to their chemical structure, most of tested anabolic steroids could be ionized using the adduct formation approach.     

Ion formation of N-Methyl carbamate pesticides in thermospray mass spectrometry: The effects of additives to the liquid chromatographic eluent and of the vaporizer temperature

The effects of three additives—ammonium acetate, ammonium formate, and nicotinic acid—to the liquid chromatographic (LC) eluent and of the vaporizer temperature on the ion formation of N-methyl carbamate pesticides in thermospray (TSP) mass spectrometry was investigated by using filament- or discharge-assisted ionization. Nineteen carbamates and 12 of their known environmental degradation products were used as model compounds. The additives cause a strong reduction in the abundance of the characteristic fragment ions [M + H ? CH₃NCO]⁺ and [M ? H ? CH₃NCO]^? for some of the carbamates. The addition of nicotinic acid reduces the quasimolecular ion intensity and, in most cases, produces nicotinic acid adduct ions. The addition of ammonium acetate or ammonium formate increases the intensity of the quasimolecular ion and in most cases produces a base peak for the ammonium adduct ion. The combination of a suppression of fragmentation and an enhancement of quasimolecular ion formation produces an overall gain in sensitivity. As to more specific effects, the addition of the ammonium salts reduces the intensity of M^?? with the chlorinated carbamate barban and suppresses the formation of “odd” adduct ions in the TSP mass spectra of most other carbamates. Monitoring the intensity of the fragment and the quasimolecular ion signal as a function of the probe stem temperature, and the related probe tip temperature, proved to be an easy method to study the thermal degradation of the carbamates. This monitoring procedure showed that methiocarb and its sulfone already suffer from thermal degradation at a stem temperature of 90°C and that these compounds will therefore present problems in quantitation with LC/TSP mass spectrometry.     

Multiple mixed-valence behavior in trans,trans-[(tpy)(Cl)2Os(III)(mu-1,3-N3)Os(III)(Cl)2(tpy)]+. An azido bridge from the reaction between trans-[Os(VI)(tpy)(Cl)2(N)]+ and NH3

Reaction between the Os(VI)-nitrido complex, trans-[OsVI(tpy)(Cl)2(N)]PF6 (tpy = 2,2':6',2' '-terpyridine), and ammonia (NH3) under N2 in dry CH3CN gives the mu-1,3-azido bridged [OsII-N3-OsII]- dimer, trans,trans-NH4[(tpy)(Cl)2OsII(N3)OsII(Cl)2(tpy)]. It undergoes air oxidation to give the [OsIII-N3-OsIII]+ analogue, trans,trans-[(tpy)(Cl)2OsIII(N3)OsIII(Cl)2(tpy)]PF6 ([OsIII-N3-OsIII]PF6), which has been isolated and characterized. The structural formulation as a mu-1,3-N3 bridged complex has been established by infrared and 15N NMR measurements on the 15N-labeled forms, [OsIII-14N=15N=14N-OsIII]+, [OsIII-15N=14N=15N-OsIII]+, and [OsIII-15N=15N=15N-OsIII]+. Cyclic voltammetric measurements in 0.2 M Bu4NPF6/CH3CN reveal the existence of five chemically reversible waves from 1.40 to -0.12 V for couples ranging from OsV-OsIV/OsIV-OsIV to OsIII-OsII/OsII-OsII. DeltaE1/2 values for couples adjacent to the three mixed-valence forms are 0.19 V for OsIII-OsII, 0.52 V for OsIV-OsIII, and >0.71 V for OsV-OsIV. In CH3CN at 60 degrees C, [OsIII-N3-OsIII]+ undergoes a [2 + 3] cycloaddition with CH3CN at the mu-N3- bridge followed by a solvolysis to give trans-[OsIII(tpy)(Cl)2(5-MeCN4)] and trans-[OsIII(tpy)(Cl)2(NCCH3)]PF6.     

Metal-controlled assembly and selectivity of a urea-based anion receptor

1-(3-[1,10]phenanthrolin-2-yl-phenyl)-3-(4-trifluoromethyl-phenyl)-urea (1) forms a solution-stable 1:2 complex with copper(I), [CuI(1)2]+, which behaves as an anion receptor in aprotic media. The [CuI(1)2]+ receptor may adopt a geometrical arrangement in which the two facing urea subunits give tetrafurcate H-bond interaction with a spherical anion (halides and H2PO4-). In the presence of the Y-shaped acetate, the [CuI(1)2]+ receptor rearranges to interact with two CH3COO- ions, according to two stepwise equilibria. Thus, in the presence of substoichiometric amounts of the anions, [CuI(1)2]+ discriminates H2PO4- over CH3COO-, inverting the basicity trend.     

Fragmentation chemistry of [M + Cu]+ peptide ions containing an N-terminal arginine

[M + Cu]+ peptide ions formed by matrix-assisted laser desorption/ionization from direct desorption off a copper sample stage have sufficient internal energy to undergo metastable ion dissociation in a time-of-flight mass spectrometer. On the basis of fragmentation chemistry of peptides containing an N-terminal arginine, we propose the primary Cu+ ion binding site is the N-terminal arginine with Cu+ binding to the guanidine group of arginine and the N-terminal amine. The principal decay products of [M + Cu]+ peptide ions containing an N-terminal arginine are [a(n) + Cu - H]+ and [b(n) + Cu - H]+ fragments. We show evidence to suggest that [a(n) + Cu - H]+ fragment ions are formed by elimination of CO from [b(n) + Cu - H]+ ions and by direct backbone cleavage. We conclude that Cu+ ionizes the peptide by attaching to the N-terminal arginine residue; however, fragmentation occurs remote from the Cu+ ion attachment site involving metal ion promoted deprotonation to generate a new site of protonation. That is, the fragmentation reactions of [M + Cu]+ ions can be described in terms of a "mobile proton" model. Furthermore, proline residues that are adjacent to the N-terminal arginine do not inhibit formation of [b(n) + Cu - H]+ ion, whereas proline residues that are distant to the charge carrying arginine inhibit formation of [b(n) + Cu - H]+ ions. An unusual fragment ion, [c(n) + Cu + H]+, is also observed for peptides containing lysine, glutamine, or asparagine in close proximity to the Cu+ carrying N-terminal arginine. Mechanisms for formation of this fragment ion are also proposed.     

Enhanced reactivity of 2-rhodaoxetanes through a labile acetonitrile ligand

New cationic, square-planar, ethene complexes [(Rbpa)RhI(C2H4)]+ [2a]--[2c]+ (Rbpa = N-alkyl-N,N-di(2-pyridylmethyl)amine; [2a]+: alkyl =R=Me; [2b]+: R = Bu; [2c]+: R = Bz) have been selectively oxygenated in acetonitrile by aqueous hydrogen peroxide to 2-rhoda(III)oxetanes with a labile acetonitrile ligand, [(Rbpa)RhIII(kappa2-C,O-CH2CH2O-)(MeCN)]+, [3a]+-[3c]+. The rate of elimination of acetaldehyde from [(Rbpa)RhIII(kappa2-C,O-CH2CH2O-)(MeCN)]+ increases in the order R = Me< R = Bu< R = Bz. Elimination of acetaldehyde from [(Bzbpa)RhIII(kappa2-C,O-CH2CH2O)(MeCN)]+ [3c]+, in the presence of ethene results in regeneration of ethene complex [(Bzbpa)RhI(C2H4)]+ [2c]+, and closes a catalytic cycle. In the presence of Z,Z-1,5-cyclooctadiene (cod) the corresponding cod complex [(Bzbpa)RhI(cod)]+ [6c]+ is formed. Further oxidation of [3c]+ by H2O2 results in the transient formylmethyl-hydroxy complex [(Bzbpa)RhIII(OH)[kappa1-C-CH2C(O)H]]+ [5c]+.     

Organo-Zintl clusters soluble in conventional organic solvents: setting the stage for organo-Zintl cluster chemistry

Organo-Zintl cluster ions were synthesized by reactions of alkynes with Ge94- clusters in ethylenediamine. The triple bonds are hydrogenated to double bonds during the process. The resulting dial-kenylated species [RHC=CHGe9CH=CHR]2- have lower charge, their alkali-metal cations can be exchanged for tetralkylammonium cations, and the resulting compounds are soluble in nonpolar solvents. This was demonstrated with [H2C=CHGe9CH=CH2]2-, which was structurally characterized with [K-(18-crown-6)]+, [Me4N]+, and [Pr4N]+ as countercations. The solubility of its salt with [Oc4N]+ in various conventional organic solvents was studied.