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  • 1
    Electronic Resource
    Electronic Resource
    Charge transfer interactions between nucleic acid bases and strong acceptors (1979)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 16 (1979), S. 387-394 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The regularities of the donor-acceptor interaction of nucleic acid bases and some of their derivatives with the strong acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) in thin amorphous films at 77 K were studied using UV and visible spectroscopy. Purine and pyrimidine bases are shown to form charge transfer TCNQ complexes whose band energies hνct = Ip - EA - (H* - H0) are linearly dependent of the donor ionization potential Ip. Calculated differences between the interaction energies of uncharged molecules and ion radicals, ΔE ≈ 3.0 eV, are in good agreement with experimental variations of the complex dissociation energies in the ground and excited states, (H* - H0) = 2.73 eV. Annealing of the films of complexes containing cytosine, 1-methylcytosine, uracil, and caffeine leads to ordering of their structure and the formation of ion radical salts.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560160218
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  • 2
    Electronic Resource
    Electronic Resource
    Mass spectra of methylsubstituted monosila- and monogermacyclobutanes, 1,2- disila- and 1,2-digermacyclopentanes. Some correlations with thermal decomposition (1972)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 6 (1972), S. 309-316 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The mass spectra of 1,1-dimethyl-1-silacyclobutane (I - as reported by Cherniak et al.),6, 1,1-dimethyl-1-germacyclobutane (II), 1,1,2,2-tetramethyl-1,2-disilacyclopentane (III) and 1,1,2,2-tetramethyl-1,2-digermacyclopentane (IV) are compared and some correlations between electron-impact fragmentation and thermal decomposition are derived. The mass spectra of the germanium compounds with respect to the silicone compounds are enriched by light fragment ions and exhibit lower intensities of odd-electron ions. The composition of some ions and apparently of neutral fragments coincides with that of the unstable intermediates which are suggested in the thermal decomposition mechanism of some related compounds. The loss of C2H4 is more characteristic under electron-impact as well as in thermal decomposition of Si-compounds, while C3H6 is preferable eliminated by the Ge-compounds.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210060309
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  • 3
    Electronic Resource
    Electronic Resource
    Comparative studies on photo-ionization and electron-impact ionization of peptide derivatives (1972)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 6 (1972), S. 9-20 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210060103
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  • 4
    Electronic Resource
    Electronic Resource
    Electron-impact fragmentation of cyclocarbosiloxanes (1970)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 4 (1970), S. 195-201 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The fragmentation of six and eight membered methyl substituted cyclocarbosiloxanes of general formula (CH3)6Si3O3 — n(CH2)n (n = 1 or 2) and (CH3)8 Si4O4-n(CH2)n (n = 1, 2 or 3) under electron-impact (70 eV) is reported. The behaviour of these compounds under electron-impact is intermediate between that of the corresponding cyclosiloxanes and cyclocarbosilanes. In every case the most intense singly and doubly charged ions correspond to the loss of one and two methyl groups respectively. In going from the cyclosiloxanes to the cyclocarbosilanes the intensity of the primary ions is reduced and the intensity of those fragment ions whose formation involves the rearrangement or breakdown of the cyclic framework of the molecule is increased. These ions are the [M —CH3—(CH3)4Si]+ ions in the eight membered and the [(CH3)3Si]+ ions in both cases. The data on doubly charged and metastable ions in the mass spectra of the cyclocarbosiloxanes are presented and discussed.The fragmentation of six and eight membered methyl substituted cyclocarbosiloxanes of general formula (CH3)6Si3O3 — n(CH2)n (n = 1 or 2) and (CH3)8 Si4O4-n(CH2)n (n = 1, 2 or 3) under electron-impact (70 eV) is reported. The behaviour of these compounds under electron-impact is intermediate between that of the corresponding cyclosiloxanes and cyclocarbosilanes. In every case the most intense singly and doubly charged ions correspond to the loss of one and two methyl groups respectively. In going from the cyclosiloxanes to the cyclocarbosilanes the intensity of the primary ions is reduced and the intensity of those fragment ions whose formation involves the rearrangement or breakdown of the cyclic framework of the molecule is increased. These ions are the [M — CH3—(CH3)4Si]+ ions in the eight membered and the [(CH3)3Si]+ ions in both cases. The data on doubly charged and metastable ions in the mass spectra of the cyclocarbosiloxanes are presented and discussed.
    Additional Material: 5 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210040121
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  • 5
    Electronic Resource
    Electronic Resource
    Mass spectrometric studies on the effect of sidechains of N-acetylamino acid methyl esters and N-methylamides on their photoionisation (1974)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 9 (1974), S. 801-810 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Photoionisation mass spectra of N-methylamides of N-acetyl derivatives of the following amino acids have been studied: alanine, valine, leucine, serine, threonine, phenylalanine, tyrosine, tryptophan and proline. The study also included methyl esters of N-acetyl tyrosine, N-acetyl phenyl alanine and N-trideuteroacetyl phenylalanine. The photon energies were within the range of 7.5 to 13.0 eV. The appearance potentials of the main ions were also determined. The compounds under study contain CH—CO, CO—NH and N—Cα bonds as well as sidechains and may therefore be regarded as models of short peptides.Under photoionisation conditions, decomposition of compounds containing aliphatic sidechains involves mainly CH—CO bonds; the appearance potentials of the corresponding fragments decrease with an increase in the size of sidechain. The presence of hydroxyl in the sidechain results in an increase in the appearance potential of a fragment formed due to CH—CO bond rupture, whereas the presence of an aromatic moiety favours an even greater increase in the appearance potential and results in predominant rupture of the N—Cα bond.Analysis of the mass spectra obtained confirms the conclusion made previously that application of the photoionisation technique enables one to increase considerably the relative intensity of characteristic peaks and to suppress secondary process taking place upon ionisation by the electrons with energy values of the order of 70 eV.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210090809
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  • 6
    Electronic Resource
    Electronic Resource
    Photoionisation mass spectra of volatile derivatives of short peptides and appearance potentials of their characteristic ions (1974)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 9 (1974), S. 811-825 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Mass spectra of the volatile derivatives of short peptides were studied by the photoionisation method with the use of monochromatic photons. The dependence of the intensity of the main peaks on the photon energy was analysed from 7·5 to 13·0 eV. The data obtain reveal the influence of the chemical structure of amino acid residues on the relative probability of the decomposition of peptide molecular ions at the CH—CO and CO—NH bonds, resulting in the formation of positively charged aldimine and amino acid N-terminal fragments, respectively. These data may be used as a basis for the application of the photoionisation technique to mass spectrometric sequential studies in peptides.In peptides containing residues of aliphatic amino acid the decomposition results mainly in the formation of aldimine ions, the stability of which increase with the increase of the alkyl chain size. In peptides containing residues of aromatic amino acids the decomposition is usually observed leading to formation of the amino acid ions.Ionisation potentials, as well as photoionisation efficiency curves and appearance potentials were determined for characteristic ions. Comparison was made of the values of the appearance potentials of different fragments formed upon decomposition of molecular ions. It has been shown that for peptides containing aliphatic amino acid moieties the appearance potentials of aldimine fragments are always lower than those inherent to peptides containing aromatic amino acid residues. The larger the size of an aliphatic chain, the lower is the energy of formation of these fragments. For all the compounds studied, including the peptides containing aromatic amino acid residues, the appearance potentials of the aldimine ions did not exceed those of the amino acid ions. These data indicate that, contrary to the experiments with electron-impact with energy of about 70 eV, upon ionisation with photons with energy from 7·5 to 13·0 eV, the aldimine fragments appear directly due to primary decomposition of molecular ions, independent of the formation of the amino acid fragments. The photoionisation efficiency curves for peptides containing different types of amino acid residues facilitate the choice of an optimal photon energy providing unequivocal information on the amino acid sequence in the peptide under study.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210090810
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