Resonant dissociative electron capture by the simplest amino acids and dipeptides

The formation of ions from amino acids (glycine and alanine) and dipeptides (glycylglycine, alanylalanine, and glycylalanine) under the resonant electron capture conditions was studied by negative ion resonant electron capture mass spectrometry. The isobaric ions were found, their effective yield curves were experimentally separated, and the elemental composition was determined. The thermochemical aspect of ion formation was considered, and probable dissociative channels of fragmentation ion formation and their structures were established on the basis of this aspect. Bond cleavage reactions only and H-shift processes were revealed. The rearrangements occur presumably through the stage of formation of intramolecular hydrogen bonds. The cross-sections of formation of ions [M − H]⁻ were measured in the energy range 1.1–1.3 eV. The metastable decay channels of ions [M − H]⁻ and [M − COOH]⁻ were found in the energy range 4.5–7.5 eV for dipeptides, which enabled establishing the genetic relationship between the parental and daughter ions and revealing hidden fragmentation pathways.     

Resonant electron capture by uridine

Processes of the resonant electron attachment to uridine molecules have been studied in the energy range 0–14 eV. The main fragmentation channels of negative molecular ions have been determined. Long-lived negative molecular ions have been detected in the thermal energy region. It was found that these ions form in a dipole-bound state. The lifetime of molecular ions was estimated at τ_a ～ 29 × 10^?6 s.     

Exotic amino acids. 4. Synthesis of methyl esters of some N-heteroamino-methylenemalonic acids

Isopropylidene N-hetarylaminomethylenemalonates, obtained from isopropylidene ethoxymethylenemalonate and 2-aminopyridine, 2-amino-5-methylpyridine, 2-aminopyrimidine, 7-amino-4-methylcoumarin, and 2-amino-3,5-diethoxycarbonyl-4-methylthiophene, undergo methanolysis at room temperature to give methyl esters of the corresponding N-hetarylaminomalonic acids. Isopropylidene N-hetarylaminomethylenenemalonate, containing a 2-aminothiazole group as a heterocyclic substituent, does not react under analogous conditions, but cyclizes upon heating at reflux to give 5-oxo-5H-thiazolo[3,2-a]pyrimidine-6-carboxylic acid. 4-Oxo-9-R-4H-pyridino[1,2-a]pyrimidines are obtained upon the sublimation of isopropylidene N-pyridyl- and N-5-methylpyridylaminomethylenemalonates.Communication 3, see ref. [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 857–860, June, 2000.     

Resonant electron capture by Captopril molecules

Russian Chemical Bulletin - The modified dipeptide, Captopril, was studied to determine the effect of lateral functional groups on the decay of biomolecules in resonant reactions of electrons with...     

Resonant electron capture by orotic acid molecules

Resonant electron attachment by orotic acid molecules (6-COOH-uracil) are studied in the energy range of 0–14 eV via negative ion mass spectrometry. Molecular ions, whose lifetimes relative to electron autodetachment are found to be ~300 μs are recorded in the region of thermal electron energies; they form in the valence state through a vibration-excited resonance mechanism. Unlike unsubstituted uracil, most dissociative processes occur in the low-energy region of <4 eV and are due to carboxylic anions. An absolute cross section of 2.4 × 10^?17 cm² is found for the most intense fragment ions [M–H]^– at an output energy of 1.33 eV. The kinetics of decarboxylation is considered for these ions. This could be a model reaction for the last stage of uridine monophosphate biosynthesis.     

Mass spectrometry of N-(Pyrimidin-2-yl)amino acids and their methyl esters

N(Pyrimidin-2-yl)-glycine, -alanine and -phenylalanine (1-3) and their methyl esters (4-6) were investigated using electron impact (EI) mass Spectrometry. The results showed that EI-induced decomposition occurs on the carboxylic group or involves the loss of R₂OH. In contrast to earlier investigations on N-(pyrimidin-4-yl)amino acids, elimination of water (in 1-3) or methanol (in 4-6) was found to be of EI-induced nature. The loss of 'COOH from M⁺ of ester 4 suggests the occurrence of a skeletal rearrangement leading to the isomeric N-methylamino acid.     

Thermal electron capture by some halopropanes

Thermal electron attachment rate constants for CF₃CHClCH₃, CF₂ClCFClCF₃ and CBrF₂CH₂CH₂Br have been measured with electron swarm method. Corresponding rate constants are equal to 7.6×10⁻¹¹, 5.5×10⁻⁹ and 1.5×10⁻⁸ cm³ molecule⁻¹ s⁻¹, respectively. The dissociative electron attachment (DEA) spectra for nine haloalkanes have been determined using negative ion mass spectrometry. The correlation between rate constants, position of the DEA peaks and vertical attachment energy (VAE) available in literature has been demonstrated.     

Conformational study of methyl esters of some aliphatic erythro- and threo-dichlorocarboxylic acids

The average conformations of methyl esters of some aliphatic erythro- and threo-dichlorocarboxylic acids in dilute carbon tetrachloride solutions have been determined from the vicinal proton–proton coupling constants and ¹H NMR shifts. The ¹³C shift differences between the erythro and threo forms are compared and discussed with regard to the differences in the average conformations.     

Friedel-Crafts alkylation of benzene by normal ω-chloroalkanoic acids and their methyl esters and nitriles

Conclusions In the alkylation of benzene by -chloroalkanoic acids Cl(CH₂)_nCOOH (where n=3–6) and their methyl esters and nitriles, in the presence of A1C1₃, the degree of isomerization of the alkyl chain is less than with the corresponding 1-chloroalkanes, depending on the increase in electron acceptor activity in the sequence HOOC-> CH₃OCO-> CN-.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 367–371, February, 1987.     

Azides of N-substituted aziridine-2-carboxylic acids. Reaction with methyl esters of amino acids

The corresponding azide was obtained by nitrosation of 1-benzylaziridine-2-carboxylic acid hydrazide. Reaction of the azide with methyl esters of amino acids gave N-(1-benzyl-2-aziridinylcarbonyl)-substituted methyl esters of amino acids.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1350–1352, October, 1980.     

Catalytic hydrogenation of methyl esters of some 1h-pyrazoline-3-carboxylic acids

Hydrogenation over Raney nickel of the methyl ester of 1H-pyrazoline-3-carboxylic acid and also of its 4-phenyl and 5-methoxycarbonyl-substituted analogs, leads respectively to 3-aminopyrrolidin-2-one, its 4-phenyl- and 5-methoxycarbonyl derivatives, predominantly to the trans isomer. Under the same conditions 1-amino-4-methoxycarbonylpyrrolidin-2-one was obtained from 3,4-di(methoxycarbonyl)-1H-pyrazoline, but 3,4,5-tri(methoxycarbonyl)-1H-pyrazoline did not react.     

Dissociative electron-capture mass spectrometry of fatty acids and their pyrrolidies and methyl esters

The regions of the energies of resonance capture of electrons by the molecules of fatty acids and their pyrrolidides and methyl esters have been established, which has permitted an explanation of the difference between the dissociative electron-capture mass spectra of these compounds and their chemical ionization and fast-atom bombardment negative-ion mass spectra. It has been shown that the dissociative electron-capture mass spectra of a high-energy resonance state characterized by the most far-reaching dissociation permit the unambiguous determination of the positions of multiple bonds in the initial molecules.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Scientific Center, Academy of Sciences of the USSR. Translated from Khimiya Priordnykh Soedinenii, No. 3, pp. 348–353, May–June, 1987.     

Gas-liquid chromatographic analysis of protein amino acids as N-isobutyloxycarbonylamino acid methyl esters.

A practical method for the quantitative determination of protein amino acids by gas-liquid chromatography (GLC) is described. All of the common protein amino acids except arginine can be readily converted into their N-isobutyloxycarbonyl (N-isoBOC) methyl ester derivatives by a simple procedure involving isobutyloxycarbonylation with isobutyl chloroformate in aqueous medium, followed by methylation with diazomethane. Arginine was converted into N-isoBOC ornithine methyl ester by treatment with arginase, followed by the above derivatization procedure. The resulting N-isoBOC methyl esters of the amino acids have good GLC properties. Complete resolution of the derivatives of 20 protein amino acids was achieved by using a dual-column system consisting of a 0.65% Poly-A-101A column and a 0.70% FFAP-Poly-A-101A (1:1, w/w) column. The reproducibility of response was found to be good for derivatives carried through the entire chemical and chromatographic procedure. The calibration graphs were linear and showed no statistical bias. The results of recovery experiments with synthetic mixtures containing known amounts of the amino acids were satisfactory, the recoveries ranging from 94.3 to 106.2%.     

Reactions of triphenylphosphine with 2,3-dihalo-2-methylpropionic acids and their methyl esters

Triphenylphosphine reacts with 2,3-dichloro-2-methylpropionic acid and its ester along the dehydrochlorination pathway with the participation of the less labile hydrogen atom from the methyl group. The subsequent reaction of the unsaturated product with triphenylphosphine yields 2-carboxypropane- and 2-methoxycarbonylpropane-1,3-diylbis(triphenylphosphonium) dichlorides, respectively. The unusual course of dehydrochlorination may be due to easier electron density transfer from the C-H bond of the methyl group as compared to the chloromethyl group in the carbocationoid intermediate. With the bromine analogs, the reaction pathways are different. The ester reacts similarly to dibromopropionic acid and its derivatives, following the debromination scheme, whereas the free acid gives the product of double nucleophilic substitution.     

Fingerprint vibrational spectra of protonated methyl esters of amino acids in the gas phase

Infrared spectra of the protonated monomers of glycine, alanine, valine, and leucine methyl esters are presented. These protonated species are generated in the gas phase via matrix assisted laser desorption ionization (MALDI) within the cell of a Fourier transform ion cyclotron resonance spectrometer (FTICR) where they are subsequently mass selected as the only species trapped in the FTICR cell. Alternatively, they have also been generated by electrospray ionization and transferred to a Paul ion-trap mass spectrometer where they are similarly isolated. In both cases IR spectra are then derived from the frequency dependence of the infrared multiple photon dissociation (IRMPD) in the mid-infrared region (1000-2200 cm(-1)), using the free electron laser facility Centre de Laser Infrarouge d'Orsay (CLIO). IR bands are assigned by comparison with the calculated vibrational spectra of the lowest energy isomers using density functional theory (DFT) calculations. There is in general good agreement between experimental IRMPD spectra and calculated IR absorption spectra for the lowest energy conformer which provides evidence for conformational preferences. The two different approaches to ion generation and trapping yield IRMPD spectra that are in excellent agreement.     

Alkali metal ion binding to amino acids versus their methyl esters: affinity trends and structural changes in the gas phase

The relative alkali metal ion (M(+)) affinities (binding energies) between seventeen different amino acids (AA) and the corresponding methyl esters (AAOMe) were determined in the gas phase by the kinetic method based on the dissociation of AA-M(+)-AAOMe heterodimers (M=Li, Na, K, Cs). With the exception of proline, the Li(+), Na(+), and K(+) affinities of the other aliphatic amino acids increase in the order AAAAOMe is already observed for K(+). Proline binds more strongly than its methyl ester to all M(+) except Li(+). Ab initio calculations on the M(+) complexes of alanine, beta-aminoisobutyric acid, proline, glycine methyl ester, alanine methyl ester, and proline methyl ester show that their energetically most favorable complexes result from charge solvation, except for proline which forms salt bridges. The most stable mode of charge solvation depends on the ligand (AA or AAOMe) and, for AA, it gradually changes with metal ion size. Esters chelate all M(+) ions through the amine and carbonyl groups. Amino acids coordinate Li(+) and Na(+) ions through the amine and carbonyl groups as well, but K(+) and Cs(+) ions are coordinated by the O atoms of the carboxyl group. Upon consideration of these differences in favored binding geometries, the theoretically derived relative M(+) affinities between aliphatic AA and AAOMe are in good overall agreement with the above given experimental trends. The majority of side chain functionalized amino acids studied show experimentally the affinity order AAAAOMe. The latter ranking is attributed to salt bridge formation.     

Resonant electron capture mass spectrometry of free fatty acids: examination of ion structures using deuterium-labeled fatty acids and collisional activation

The structures of [M-H](-) ions generated from free fatty acids in resonant electron capture at energies of 1.2 and 7.2 eV were investigated using deuterium-labeled isotopomers and collision-induced dissociation. The [M-H](- small middle dot) ions occur in both a carboxylate anion and a carbanion form. While the formation of the carboxylate anion at 1.2 eV involves the loss of a carboxylic hydrogen, that at 7.2 eV involves the loss of a hydrogen from different positions in the aliphatic chain followed by a rearrangement of a carboxylic hydrogen on to the radical site in the chain. The [M-H-H(2)O](-) ion which is a minor ion in the resonant electron capture spectrum at 7.2 eV is shown to be a precursor for the charge-remote fragment ions corresponding to formal losses of a hydrogen and elements of alkanes. The [M-H-H(2)O](-) ion corresponding to the second major ion in the resonant electron capture spectrum at 7.2 eV is demonstrated to be consistent with a cyclopentanone anion structure. On the basis of new insights obtained in the present study and taking into account previous results, an updated proposal is presented for the mechanism of charge-remote fragmentation which operates in resonant electron capture of free fatty acids at 7.2 eV.