Resonant electron capture by some amino acids and their methyl esters

Resonant electron capture mass spectra of aliphatic and aromatic amino acids and their methyl esters show intense [M-H](-) negative ions in the low-energy range. Ion formation results from a predissociation mechanism mediated by the low-energy pi*oo resonant state. Methylation in general has little influence on the electronic structure according to quantum chemical calculations, but the corresponding ions from the methyl esters, [M-Me](-), could be ascertained to arise only at higher resonance energies. Aromatic amino acids are characterized by an additional low-energy fragmentation channel associated with the generation of negative ions with loss of the side chain. The complementary negative ions of the side chains are more efficiently produced at higher energies. The results have significant implications in biological systems as they suggest that amino acids can serve as radiation protectors since they have been found to efficiently thermalize electrons.     

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.     

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...     

Total dissociative electron attachment cross sections of selected amino acids

Total dissociative electron attachment cross sections are presented for the amino acids, glycine, alanine, proline, phenylalanine, and tryptophan, at energies below the first ionization energy. Cross section magnitudes were determined by observation of positive ion production and normalization to ionization cross sections calculated using the binary-encounter-Bethe method. The prominent 1.2 eV feature in the cross sections of the amino acids and the closely related HCOOH molecule is widely attributed to the attachment into the -COOH pi* orbital. The authors discuss evidence that direct attachment to the lowest sigma* orbital may instead be responsible. A close correlation between the energies of the core-excited anion states of glycine, alanine, and proline and the ionization energies of the neutral molecules is found. A prominent feature in the total dissociative electron attachment cross section of these compounds is absent in previous studies using mass analysis, suggesting that the missing fragment is energetic H-.     

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.     

Resonant dissociative electron attachments to cysteine and cystine

Shape-resonant electron attachments to cysteine and cystine and the subsequent dissociation dynamics are investigated with the single-center expansion potential scattering calculations. Selectivity of the direct bond cleavage at a given resonant state or by the specific resonant state coupling is demonstrated with the one-dimensional complex potential energy curves of the temporary anion (cysteine)(-). The wave function of the lowest shape resonant state of the temporary anion (cystine)(-) distinctly shows the localized anti-bond (S-S)* character, implying that this disulfide bond can be easily broken due to the low-energy electron resonant attachment.     

Processes of dissociative electron capture by 20-hydroxyecdysone molecules

The peculiarities of dissociative electron capture by 20-hydroxyecdysone molecules with the formation of fragment negative ions were studied. In the region of high electron energies (5–10 eV), processes of skeleton bond rupture are accompanied by the elimination of H₂O and H₂ molecules. In the region of thermal energies of electrons (≈0 eV), the mass spectrum is formed mainly by the [M−nH₂O]^.− (n=1–3) and [M−H₂−nH₂O]^.− (n=0−3) ions that are generated exclusively by the rearrangement. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 709–712, April, 2000.     

Radiation-induced destruction of hydroxyl-containing amino acids and dipeptides

The yields of molecular products resulting from radiolysis of hydroxyl-containing amino acids and dipeptides under various conditions were determined. The possibility of a new radiation-induced destruction pathway has been shown for serine and threonine, as well as for the dipeptides having residues of these amino acids at the N-terminal part of the respective molecule. This process includes formation of N-centered radicals from the starting molecules followed by their decomposition with elimination of side substituents. On radiolysis, serine and threonine were also shown to undergo free-radical destruction to form acetaldehyde and acetone, respectively. A mechanism has been proposed including consecutive stages of fragmentation of α-hydroxyl-containing carbon-centered radicals with elimination of ammonia and decomposition of the secondary radicals with elimination of CO₂. The yields of CO₂ obtained on radiolysis of serine and threonine were significantly higher (except for solutions at pH 12) than those for alanine and valine, which have no hydroxyl groups in their structures. The obtained data indicate that the hydroxyl-containing amino acids occupy a special place among other amino acids as regards the variety of radiation-induced reactions which they may undergo due to their structural features.     

Guanidine hydrochloride aminomethylation with formaldehyde and simplest amino acids

Aminomethylation of guanidine hydrochloride with aqueous formaldehyde and simplest amino acids depending on the reagents ratio results in bicyclic hydrochlorides (1: 4: 2) or tricyclic iminium chlorides (1: 6: 3) that in their turn may be converted into internal salts: zwitter-ions or betaines respectively.     

Theory and application of dissociative electron capture in molecular identification

The coupling of an electron monochromator (EM) to a mass spectrometer (MS) has created a new analytical technique, EM-MS, for the investigation of electrophilic compounds. This method provides a powerful tool for molecular identification of compounds contained in complex matrices, such as environmental samples. In particular, EM-MS has been applied to the detection of nitrated aromatic compounds, many of which are potent mutagens and/or carcinogens and are considered environmental hazards. EM-MS expands the application and selectivity of traditional MS through the inclusion of a new dimension in the space of molecular characteristics-the electron resonance energy spectrum. EM-MS also enhances detection sensitivity as well because the entire electron flux of the proper energy can be delivered into the negative ion resonance that is analytically most useful to solving the problem at hand. However, before this tool can realize its full potential, it will be necessary to create a library of resonance energy scans from standards of the molecules for which EM-MS offers a practical means of detection. Unfortunately, the number of such standards is very large and not all of the compounds are commercially available, making this library difficult to construct. Here, an approach supplementing direct measurement with chemical inference and quantum scattering theory is presented to demonstrate the feasibility of directly calculating resonance energy spectra. This approach makes use of the symmetry of the transition-matrix element of the captured electron to discriminate between the spectra of isomers. As a way of validating this approach, the resonance values for 25 nitrated aromatic compounds were measured along with their relative abundance. Subsequently, the spectra for the isomers of nitrotoluene were shown to be consistent with the symmetry-based model. The initial success of this treatment suggests that it might be possible to predict negative ion resonances and thus create a library of EM-MS standards.     

Violation of frozen shell approximation in dissociative electron capture by halogenated anthraquinones

A series of halogenated anthraquinone (AQ) derivatives has been studied by means of electron capture negative ion (NI) mass spectrometry (ECNI-MS). 1Cl-AQ and 2Br-AQ display dramatically steep positive temperature dependencies of Hal(-) ion abundance in the low electron energy region. Molecular NI intensity decreases rapidly with increasing temperature in the case of 1I-AQ. In the case of 2Br-AQ, a metastable NI peak (m/z 22.9) corresponding to the process BrAQ(-) --> Br(-) + AQ(0) was recorded. This means that the characteristic dissociation lifetime of the molecular NI Br-AQ(-) is at least approximately 25 micros at the energy approximately 0.67 eV in the low-temperature spectrum (T approximately 80 degrees C), and at the energy approximately 0.13 eV in the hot spectrum (T approximately 290 degrees C). Together with the observed temperature dependence of the 2Br-AQ curves of effective yield (CEY), this proves that this anion dissociates according to Coulson's model. The same halogen anion behavior is observed in the case of 1Cl-AQ. There are three consecutive stages in the process of molecular NI dissociation of Cl- and Br-substituted AQ, namely, electron capture into the empty pi-orbital by means of the shape resonance mechanism, followed by a radiationless transition into the ground electronic pi-state of the anion, as predicted by Compton in the case of the parabenzoquinone molecule, and, finally, a fluctuative dissociation of the molecular NI accompanied by the transition from the pi-term into the sigma-term, so-called predissociation. Calculations show reasonable agreement with the experimental data. In the case of 1I-AQ, an effect of inversion of empty levels in the process of electron capture by the molecule takes place, a violation of the so-called frozen shell approximation. The phenomenon found may be of significance not only in the case of ECNI-MS, but also in other experimental investigations using low-energy electron-molecule and ion-molecule collisions.     

Chiral recognition of amino acids and dipeptides by a water-soluble zinc porphyrin

A chiral water-soluble zinc porphyrin was optically resolved on a chiral HPLC column, and the binding of chiral amino acids and peptides to each of the enantiomers was examined spectrophotometrically in basic aqueous solution. The binding data apparently indicated that the zinc porphyrin has chiral selectivity for amino acids and dipeptides. This was reasonably explained in terms of the triple cooperation of coordination, Coulomb, and steric interactions of the chiral amino carboxylates with the porphyrin. A compensatory relationship among the thermodynamic parameters for chiral recognition was also shown.     

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.     

Capillary gas chromatographic analysis of amino acids in geological environments with electron capture detection

Studies have been performed on the analysis of 21 amino acids using a fused-silica open tubular (FSOT) capillary column, and electron-capture detection (ECD) or flame-ionization detection (FID). It was shown with the N(O)-heptafluorobutyryl (HFB) amino acid isobutyl esters that the ECD response was several hundred times more sensitive than the FID response. The relative standard deviation (RSD) of retention relative to norleucine is determined with the ECD. RSD values for all N(O)-HFB amino acid isobutyl esters are relatively small (≦ 0.5%). This method has been successfully applied to trace analysis of most of the amino acids from fossil brachiopods and black shales.     

Charge exchange in tandem mass spectrometry: dissociative single electron capture by doubly-charged toluene cations

Single electron capture by doubly-charged toluene cations upon collision with various target gases has been investigated by sector tandem mass spectrometry. Both non-dissociative and dissociative charge transfer reactions leading to C(7)H(7)(+) + H and to C(5)H(5)(+) + [C(2),H(3)] are detected. Seven atomic or molecular target gases have been used with ionisation energies ranging from 8.8 eV to 14 eV. The branching ratios between the different non-dissociative and dissociative exit channels have been determined as well as the translational energy release on the dissociation products. The experimental data are compared to the predictions of a two-state semi-classical theoretical model that takes into account the non-adiabatic transition responsible for the charge transfer reaction. A wide reaction window shows up but the internal energies of the C(7)H(8)(+) cations produced by single electron capture are observed to be larger than expected. We assign this effect partly to the influence of the large density of vibrational states and to the multichannel nature of the process. Excited states of the dication are also most probably involved in the charge exchange reaction.     

Synthesis of o-glycosides of dipeptides containingβ-hydroxy amino acids

Conclusions The O--D-galactopyranosides of some serine-containing dipeptides were synthesized.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2092–2094, September, 1971.     

Local hydration environments of amino acids and dipeptides studied by X-ray spectroscopy of liquid microjets

The nitrogen K-edge spectra of aqueous proline and diglycine solutions have been measured by total electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at neutral and high pH. All observed spectral features have been assigned by comparison to the recently reported spectrum of aqueous glycine and calculated spectra of isolated amino acids and hydrated amino acid clusters. The sharp preedge resonances at 401.3 and 402.6 eV observed in the spectrum of anionic glycine indicate that the nitrogen terminus is in an "acceptor-only" configuration, wherein neither amine proton is involved in hydrogen bonding to the solvent, at high pH. The analogous 1s --> sigma(NH) preedge transitions are absent in the NEXAFS spectrum of anionic proline, implying that the acceptor-only conformation observed in anionic glycine arises from steric shielding induced by free rotation of the amine terminus about the glycine CN bond. Anionic diglycine solutions exhibit a broadened 1s --> pi(CN) resonance at 401.2 eV and a broad shoulder resonance at 403 eV, also suggesting the presence of an acceptor-only species. Although this assignment is not as unambiguous as for glycine, it implies that the nitrogen terminus of most proteins is capable of existing in an acceptor-only conformation at high pH. The NEXAFS spectrum of zwitterionic lysine solution was also measured, exhibiting features similar to those of both anionic and zwitterionic glycine, and leading us to conclude that the alpha amine group is present in an acceptor-only configuration, while the end of the butylammonium side chain is fully solvated.     

Electron capture detector response and dissociative electron attachment cross sections in chloroalkanes and chloroalkenes

Electron capture detectors (ECDs) are widely used in gas chromatography to detect electronegative compounds. In this work, we examine the connections between the ECD response and the cross sections for dissociative electron attachment (DEA) determined from low energy electron beam studies in the chloroalkane family, stressing in particular the role of temporary anion state energies. We show that attachment rate coefficients computed from these cross sections are well correlated with ECD response, and that the latter decreases exponentially with increasing energies of the lowest anion states. ECD measurements are also carried out in monochloroalkanes substituted with unsaturated ethenyl and phenyl moieties, and the response is shown to depend strongly on the mixing between the unsaturated pi* and the C-Cl sigma* temporary anions as exhibited by the vertical attachment energies (VAEs) of these states. The results show good correlations between the chloroalkene and phenyl chloride ECD responses and the VAEs for the mixed states.