Interpretation of alkyl diselenide and selenosulfenate mass spectra

Abstract

The mass spectral fragmentation of aliphatic diselenides and selenosulfenates is analyzed to gain a better understanding of the behavior of these species. The main fragmentation pathways of these species include the fragmentation along the Se™C bond, fragmentation along the Se™Se or Se™S bonds and intra-molecular rearrangements. In general, negative ionization favors the fragmentation along the Se™Se or Se™S bonds while positive ionization leads to stable molecular ions. Density functional theory calculations of bond dissociation energies and molecular orbital analysis was undertaken to explain the observed trends in molecular fragmentation. Besides the analysis of molecular fragmentation, a phenomenon of molecular association in negative electron impact and positive chemical ionization conditions was observed and investigated using a high resolution time-of-flight mass spectrometer. Molecular association that occurs during the ionization of species includes the formation of symmetrical diselenides from asymmetrical selenosulfenates and formation of alkylseleno adducts from the corresponding diselenides. For species which is hard to resolve by mass analysis, such as isobars of CHSe, CH 2 Se, and CH 3 Se, the isotope pattern superimposition procedure was applied to define the overlapping clusters. nterest in selenium species has recently increased because of their anti-oxidative and anti-carcinogenic properties. As a result, speciation of selenium metabolites in the environment has been undertaken by many research groups to gain more understanding about bio-transformations and the occurrence of the various selenium compounds. Inductively coupled plasma mass spectrometry (ICP-MS) is usually utilized for efficient detection and screening of the various selenium species at ultra-trace levels; however, the ultimate identification necessitates characterization by molecular mass spectrometry [1].Volatile selenium species are of interest as the end terminal in Se metabolism, and are emitted by plants as a means of self-detoxification [2]. Additionally, diselenides and selenosulfenates are red.-ox. active species and account for the anti-oxidative action of selenium [3]. Despite the interest in the selenium volatiles, very few reports are devoted to the investigation of mass spectral fragmentation of heavier dichalcogenidesdiselenides (R-SeSe-R) and selenosulfenates (R-SeS-R). Early mass spectrometric studies of organic selenium compounds were undertaken by Rebane [4 -6] and recently by Prabhakar et al. [7]. However, diselenides and selenosulfenates were not a subject of interest in either of the studies.In this study we analyzed the mass spectral fragmentation of methyl-, ethyl-and ethylmethyl-diselenides and selenosulfenates to gain a better understanding of the behavior of these species. In order to explain the fragmentation behavior of diselenides and selenosulfenates, density functional theory calculations were performed to obtain the bond dissociation energies and molecular orbital information. Experimental Reagen...

Deconvolution of isobaric interferences in mass spectra

“…It is known that the electron impact fragmentation of dimethyl diselenide does not render a pure cluster of CH 3 Se as the region at m/z ϭ 88 -98 u clearly shows a distorted isotope pattern of selenium [4] (see also Figure 1). The following example illustrates the deconvolution of this isobaric overlap.…”

“…The results were as follows (with the standard deviation given in the parenthesis): 0 ϭ 2.0(0.8)% (CH 0 Se ϩ ); 1 ϭ 49.8(0.6)% (CH 1 Se ϩ ); 2 ϭ 19.7(0.6)% (CH 2 Se ⅐ϩ ); 3 ϭ 22.7(0.6)% (CH 3 Se ϩ ) and 4 ϭ 5.9(0.6)% (CH 4 Se ⅐ϩ ), which is consistent with the 2:1:1 contribution from CHSe ϩ , CH 2 Se ⅐ϩ and CH 3 Se ϩ , respectively, and the result obtained is in good agreement with the experimentally observed CH z Se isotope pattern (Figure 1).…”

“…In particular, these approaches were demonstrated to be useful in the interpretation of diselenide and selenosulfenate mass spectra [4] and here we give a more technical report of the various isobar deconvolution approaches. These are isotope cluster reconstruction using isotope pattern superimposition combined with the least square optimization algorithm, signal deconvolution to its components in the mass domain, and mass shift analysis of the overlapping clusters.…”

Electron Capture Dissociation of Disulfide, Sulfur–Selenium, and Diselenide Bound Peptides

“…Previous studies have been mainly focused on using theoretical calculations to study the effects of electron capture on small diselenide compounds (XSeSeX'), such as CH 3 SeSeCH 3 , CH3SeSeOH, CH3SeSeF, and such [1,[32][33][34][35]. So, it is of interest to examine the ECD behavior of diselenide peptides compared with previous theoretical calculations, and also to examine the differences and similarities of the ECD fragmentation patterns among disulfide (S-S), sulfur-selenium (S-Se), and diselenide (Se-Se) peptides.…”

Is Se–Se bond cleavage the most favourable process in electron attachment to diselenides? The importance of asymmetry

“…26 Not surprisingly much less attention was devoted to Se-Se bond fission by electron capture to diselenides. Some studies carried out on dimethyldiselenide 27,28 indicate that electron attachment yields mainly the breaking of the Se-Se bond since the extra electron is accommodated in the s*(Se-Se) antibonding orbital, like for disulfides. However, the fact that both substituents attached to Se are identical prevents from getting a general picture of the electron capture process in these systems.…”