Development of new analytical methods for selenium speciation in selenium-enriched yeast material

A sequential extraction allowing the discrimination of water-soluble and non-soluble selenium fractions has been developed to evaluate the availability of selenium (Se) in an Se-enriched yeast candidate reference material. The fractionation of selenium-containing compounds in the extracts was achieved on preparative grade 200 Superdex 75 and columns. It showed that water-soluble selenium is present in several fractions with a large mass distribution. Low-molecular- (< or = 10,000) and high-molecular-mass selenocompounds (range 10,000-100,000) were considered separately for further experiments. The analytical approach for low-molecular-mass selenocompounds was based onanion-exchange HPLC with on-line inductively coupled plasma (ICP) MS for quantitative analysis. Selenocystine, selenomethionine, selenite and selenate were quantified in the fractions isolated in preparative chromatography. The study revealed the existence of various unidentified Se species in yeast material. The Se-containing proteins in the yeast material have been further separated and selenium quantified by the combination of gel electrophoresis and electrothermal vaporization-ICP-MS. This new approach allows the separation of the proteins with high resolution by sodium dodecylsulfate-polyacrylamide gel electrophoresis and the sensitive determination of selenium in the protein bands.     

On-line focused microwave digestion-hydride generation of inorganic and organic selenium: Total determination and inorganic selenium speciation by atomic absorption spectrometry

An on-line FIA pretreatment with HBr/KBrO₃, assisted by on-line focused microwave-induced digestion, has been coupled with hydride generation-atomic absorption spectrometry (HG-AAS) for final detection for total selenium determination. This total selenium determination is virtually independent of the different Se species investigated (selenite, selenate, selenomethionine, selenoethionine and selenocystine). Detection limits of 0.8 μg l⁻¹ of Se can be achieved by AAS with precisions better than 5%. This continuous flow system for selenium determination allows a high sample throughput (about 30 samples h⁻¹ can be analyzed) in which high automation can be achieved and constitutes a convenient real-time continuous detector for the different selenocompounds tested. Direct non-chromatographic speciation of inorganic selenium (selenite and selenate in their mixtures) is demonstrated by simple on-off operation of the focused microwaves connected in the flow system.

Validation of this simple on-line FIA system has been carried out by analyzing total Se recovered from spiked tap waters and by speciating mixtures of Se(IV) and Se(VI) spiked to the same samples. The fast conversion of Se compounds into volatile selenium could be considered as a sort of specific “general” detector for Se compounds which can be extremely useful for Se speciation by hybrid chromatographic techniques.     

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

To examine the electron capture dissociation (ECD) behavior of disulfide (S?CS), sulfur?Cselenium (S?CSe), and diselenide (Se?CSe) bonds-containing peptides, a series of free cysteine (Cys) and selenocysteine (Sec) containing peptides were reacted to form interchain S?CS, S?CSe, and Se?CSe bonds, and then studied using ECD with Fourier transform ion cyclotron mass spectrometry (FTICR MS). These results demonstrate that the radical has higher tendency to stay at selenium rather than sulfur after the cleavage of Se?CS bonds by ECD. In addition, ?CSH (?C33), ?CS (?C32), and ?CS + H (?C31) small neutral losses were all observed from the cleavage of C?CS bonds of a disulfide bound peptide. Similar, but minor, fragments were also detected in S?CSe bound peptides. In contrast, the cleavage of C?CSe bonds of the Se?CSe species mainly forms fragments with neutral loss of ?CSe + H (?C78.90868), and the radical tends to stay on the selenium of its corresponding complementary pair. Although the electron affinities of S atom (2.07?eV) and Se atom (2.02?eV) are very close; they have very different reactivity towards electrons. The replacement of sulfur with selenium greatly increases the electron affinities of S?CSe and Se?CSe bonds comparing to S?CS bonds (with an increase of electron affinity by about 0.20?eV by replacing a sulfur with a selenium) (Int J Quantum Chem 110:513-523, 2010), which in turn leads to different ECD fragmentation behavior and mechanisms. Our results are in good agreement with previously published ab initio calculations on Se?CSe compounds by other groups.     

Supramolecular Assemblies of Sulfur- and Selenium- Containing Expanded Porphyrins Mediated Through Noncovalent Interactions

Abstract

Various supramolecular assemblies based on expanded porphyrins building blocks containing sulfur and/or selenium in the core, formed through multiple non-covalent hydrogen bonding interactions are highlighted. Specifically, modified expanded porphyrins such as 22 π sapphyrins, 26 π rubyrins, and 34 π octaphyrins self assemble in solid state through C–H…O, C–H…N, C–H…S, C–H…Se, C–H…π, and C–H…Cl interactions to form dimeric, oligomeric, and three dimensional networks. Furthermore, the supramolecular networks promoted by trapped solvent molecules such as nitrobenzene and bound anions such as chloride or trifluoroacetate through noncovalent interactions will be discussed.     

Catalytic adaptive recognition of thiol (SH) and selenol (SeH) groups toward synthesis of functionalized vinyl monomers

An unprecedented sustainable procedure was developed to produce functionalized vinyl monomers H(2)C═C(R)(FG) starting from a mixture of sulfur and selenium compounds as a functional group donor (FG = S or Se). The reaction serves as a model for efficient utilization of natural resources of sulfur feedstock in oil and technological sources of sulfur/selenium. The catalytic system is reported with amazing ability to recognize SH/SeH groups in the mixture and selectively incorporate them into valuable organic products via wastes-free atom-economic reaction with alkynes (HC≡CR). Formation of catalyst active site and the mechanism of the catalytic reaction were revealed by joint experimental and theoretical study. The difference in reactivity of μ(1)- and μ(2)-type chalcogen atoms attached to the metal was established and was shown to play the key role in the action of palladium catalyst. An approach to solve a challenging problem of dynamically changed reaction mixture was demonstrated using adaptive tuning of the catalyst. The origins of the adaptive tuning effect were investigated at molecular level and were found to be governed by the nature of metal-chalcogen bond.     

Synthesis, Characterization, and Bonding of Two New Heteropolychalcogenides: alpha-CsCu(S(x)()Se(4-x)) and CsCu(S(x)()Se(6-x))

The Cs-Cu-Q (Q = S, Se) system has been investigated using copper metal, cesium chloride, and alkali-metal polychalcogenide salts under mild hydrothermal reaction conditions. Heteropolychalcogenide salts and mixtures of known polysulfide and polyselenide salts have been used as reagents. The reaction products contain the alpha-CsCuQ(4) and CsCuQ(6) structures. The alpha-CsCuQ(4) phase exhibits a smooth transition in lattice parameters from the pure sulfur to the pure selenium phases, based on Vegard's law. The CsCuQ(6) phase has been prepared as the pure sulfur analog and a selenium rich analog. The single-crystal structures of the disordered compounds alpha-CsCuS(2)Se(2) (P2(1)2(1)2(1), Z = 4, a = 5.439(1) ?, b = 8.878(2) ?, c = 13.762(4) ?) and CsCuS(1.6)Se(4.4) (P&onemacr;, Z = 2, a = 11.253(4) ?, b = 11.585(2) ?, c = 7.211(2) ?, alpha = 92.93 degrees, beta = 100.94 degrees, gamma = 74.51 degrees ) have been solved using a correlated-site occupancy model. These disordered structures display a polychalcogenide geometry in which the sulfur atoms prefer positions that are bound to copper. The optical absorption spectra of these materials have been investigated. The optical band gap varies as a function of the sulfur-selenium ratio. Extended Hückel crystal orbital calculations have been performed to investigate the electronic structure and bonding in these compounds in an attempt to explain the site distribution of sulfur and selenium.     

Synthesis and crystal structure of 2‐phosphonoalkyl‐1, 2‐benzisoselenazol‐3(2H)‐ones and their antitumor activities

The title compounds were synthesized in good yields by the condensation reaction of diphenyl α‐aminoalkylphosphonates with 2‐(chloroseleno)‐benzoyl chloride. Their structures were confirmed by spectroscopic methods and microanalyses. The X‐ray analyses showed that the selenium‐containing fused ring has a planar structure and that, by the molecular packing of the unit cells, two adjacent molecules are symmetrically linked to each other through Se(1c) … O=P(1) bonding interactions with an intermolecular Se(1c) … O distance of 2.797 Å. The results of bioassay indicated that some of these compounds possess potent antitumor activities against some human carcinoma cells in vitro. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10:247–254, 1999     

硒对癌症的预防和临床问题的研究现状

从流行病学、实验室和临床等的研究均表明微量元素硒具有化学防癌的潜力，特别是近来富硒酵母应用于肺癌、肠癌、前列腺癌和肝癌等临床干预试验，表现出硒具有强大的保护作用。本文着重论述了硒化合物发展成为化学防癌药物的作用机制问题，这些机制表现在硒具有抗致癌的生物活性。还讨论了对硒的化学预防试验人群的选择，硒的干预试验，临床应用结果和安全范围的指标等问题。     

STEREOCHEMISTRY OF INCIPIENT ELECTROPHILIC AND NUCLEOPHILIC REACTIONS AT DIVALENT SELENIUM CENTER: ELECTROPHILIC-NUCLEOPHILIC PAIRING AND ANISOTROPIC SHAPE OF Se IN Se…Se INTERACTIONS

Abstract

Using the Cambridge Crystallographic Data Base and computer retrieval methods, we have studied the preferred directions of approach of nucleophiles and electrophiles at divalent selenium centers. We find that electrophiles approach Se in a direction nearly normal to the selenide plane Y-Se-Z while nucleophiles approach Se nearly in the selenide plane and along the back side of Y-Se or Z-Se bonds. We also found evidence for the incipient formation of attractive electrophile-nucleophile pairing in Se…Se interactions. Other Se…Se contacts which did not fit the above pattern showed non-spherical effective shape for Se atoms. The preferred directions of approach of electrophiles and nucleophiles for Se and S are very similar and may be understood in terms of frontier orbitals. Such similarity in the shape and orbital environment of S and Se indicate to some extent how Se can play the role of S in some enzymes.     

Synthesis of Phosphanes Bearing 2‐Imino‐1,3‐thiazolidine Ligands – X‐Ray Analyses and NMR Spectroscopy

Pseudo‐ephedrine derived 2‐imino‐1,3‐thiazolidine 1 reacts with tris(diethylamino)phosphane by stepwise replacement of the diethylamino group to give the mono‐, bis‐ and tris(imino)phosphanes 2 , 3 and 4 , respectively, of which 4 could be isolated in pure state. The analogous reaction with diethylamino‐diphenylphosphane affords the imino‐diphenylphosphane 5 . The iminophosphanes react with sulfur or selenium to give the corresponding phosphorus(V) compounds. In contrast, the reaction of the iminophosphanes with oxygen is very slow; anhydrous trimethylamine N‐oxide reacts in the melt with the phosphanes to give the oxides 4(O) and 5(O) . The molecular structures of 4(O) (in mixture with 4 ), 4(Se) , 5(S) and 5(Se) were determined by X‐ray analysis. In all cases the ring‐sulfur and the phosphorus atoms are in cis‐positions at the C=N bonds. The analogous solution structures were determined by ¹H, ¹³C, ¹⁵N, ³¹P and ⁷⁷Se NMR spectroscopy. In the case of the compounds 5 , 5(O) , 5(S) and 5(Se) the isotope‐induced chemical shifts ¹δ^14/15N(³¹P) were determined, using INEPT‐HEED experiments.     

Simultaneous Construction of C−Se And C−S Bonds via the Visible‐Light‐Mediated Multicomponent Cascade Reaction of Diselenides,Alkynes, and SO2

A visible‐light mediated multicomponent cascade reaction of diselenides, alkynes, and sulfur dioxide was developed, in which multiple C?Se and C?S bonds were constructed, and unexpected β‐sulfonylvinylselane compounds were generated with high selectivity for E configuration. β‐Sulfonylvinylselane transformation into 1,4‐oxathiine‐4,4‐dioxide and sulfonylethyne derivates was then investigated. A plausible mechanism involving a selenium radical‐initiated cascade reaction and sulfur dioxide insertion was proposed.     

Speciation of selenium compounds with ion-pair reversed-phase liquid chromatography using inductively coupled plasma mass spectrometry as element-specific detection

For selenium speciation analysis, the hyphenation of chromatographic separation with element-specific detection has proved a useful technique. A powerful separation system, which is capable of resolving several biologically and environmentally important selenium compounds in a single column, is greatly needed. However, that has been difficult to achieve. In this paper eight selenium compounds, namely, selenite [Se(IV)], selenate [Se(VI)], selenocystine (SeCys), selenourea (SeUr), selenomethionine (SeMet), selenoethionine (SeEt), selenocystamine (SeCM) and trimethylselenonium ion (TMSe+), were separated by using mixed ion-pair reagents containing 2.5 mM sodium 1-butanesulfonate and 8 mM tetramethylammonium hydroxide as a mobile phase. The separation of these anionic, cationic and neutral organic selenium compounds on a LiChrosorb RP18 reversed-phase column took only 18 min at a flow-rate of 1.0 ml/min with isocratic elution, and baseline separation among the six organic Se compounds was achieved. Inductively coupled plasma mass spectrometry (ICP-MS) was employed as element-specific detection. A comparison of ICP-MS signal intensity obtained with a Barbington-type nebulizer and with an ultrasonic nebulizer (USN) was made. Different signal enhancement factors were observed for the various selenium compounds when a USN was used. The speciation technique was successfully applied to the study on chemical forms of selenium in a selenium nutritional supplement. Selenomethionine was found to be the predominant constituent of selenium in the supplement.     

Selenium speciation in dill (Anethum graveolens L.) by ion pairing reversed phase and cation exchange HPLC with ICP-MS detection

In the present work, speciation of selenium in dill (Anethum graveolens L.), supplemented with sodium selenite during its growth, was performed using ion pairing reversed phase and cation exchange chromatography. Heptafluorobutyric acid (HFBA) was used as the ion-pairing agent in reversed phase chromatography. In cation exchange chromatography, two different gradient programs were employed for the identification of selenospecies using pyridinium formate as the mobile phase. Low molecular weight selenocompounds were extracted from root, stem and dill leaf with 0.1 M HCl. Enzymatic digestion was used for the extraction of selenospecies related to high molecular weight compounds. The chromatograms obtained from different parts of the plant revealed major differences in the type of selenospecies as well as their concentrations. The major selenospecies found in different parts of the plant is Se-methyl-selenocysteine (MeSeCys). Another major Se species identified is Se-methyl-selenomethionine (MeSeMet), which has the highest relative concentration in the root indicating possible Se volatilization from that part of the plant. Selenomethionine (SeMet) is present in minor quantities in all parts of the plant.     

Non-conventional Hydrogen Bonding and Aromaticity: A Systematic Study on Model Nucleobases and Their Solvated Clusters

The conceptual development of aromaticity is essential to rationalize and understand the structure and behavior of aromatic heterocycles. This work addresses for the first time, the interconnection between aromaticity and sulfur/selenium centered hydrogen bonds (S/SeCHBs) involved in representative heterocycle models of canonical nucleobases (2-Pyridone; 2PY) and its sulfur (2-Thiopyridone; 2TPY) and selenium (2-Selenopyridone; 2SePY) analogs. The nucleus-independent chemical shift (NICS) and gauge induced magnetic current density (GIMIC) values suggested significant reduction of aromaticity upon replacement of exocyclic carbonyl oxygen with sulfur and selenium. However, we observed two-fold (57 %) and three-fold (80 %) enhancement in the aromaticity for 2TPY dimer, and 2SePY dimer, respectively which are connected through S/SeCHBs. Aromaticity enhancement was also noticed in 1 : 1 H-bonded complexes (heterodimers), micro hydrated clusters and for bulk hydration. It is expected that exocyclic S and Se incorporation into heterocycles without compromising aromatic loss would definitely reinforce to design new supramolecular building blocks via S/SeCH-bonded complexes.     

Fluorocarbon-selenium chemistry: Accomplishments and quests

The binary selenium fluorides SeF_n (n = 2, 4, 6) and Se₂F₂ are considered along with corresponding perfluorohalogenoorgano- selenium compounds with selenium in the oxydation states 1, 2, 4 and 6. For lower selenium fluorides, preparation and characterisation are emphasized. Recent results in the chemistry of SeCF₂, CF₃SeCl, CF₃SeSeCF₃, CF₃Se(O)OM and CF₃Se^VI- compounds are presented. A comparison with the corresponding sulfur chemistry is also provided.     

NMR analysis of surfaces and interfaces in 2-nm CdSe

Solid-state NMR analysis on wurtzite 2-nm hexadecylamine-capped CdSe nanocrystals (CdSe-HDA) provides evidence of discrete nanoparticle reconstruction within the Se sublattice of the nanomaterial. The cadmium and selenium atoms are probed with (1)H-(113)Cd and (1)H-(77)Se cross-polarization magic angle spinning (MAS) experiments, which demonstrate five ordered selenium sites in the nanoparticle that can be assigned to contributions arising from different surface sites and a selenium site one layer down from the surface. Intriguingly, in these materials both HDA and thiophenol are observed to selectively bind to specific sites on the nanoparticle surface. 2D heteronuclear chemical shift correlation (HETCOR) experiments provide evidence for thiophenol selectively binding at surface vacancies. Analysis of the NMR provides a model of a 2-nm CdSe-HDA molecular surface.     

Radiation stability of methionine-35S and selenomethionine-75Se

The radiation stability of methionine-³⁵S and selenomethionine⁷⁵Se was investigated using the methods of thin-layer chromatography, gas chromatography and ESR. Radiation decomposition of methionine-³⁵S mainly consists in an oxidation process and in the release of volatile products. The ESR-spectra of irradiated DL-methionine indicated a strong localization of the unpaired electrons on sulfur atoms. Radiation damage to selenomethionine-⁷⁵Se as a function of radiation dose proved an increased stability of this compound, and its radiation decomposition consists in the formation of oxidized products and by direct rupture of the selenium bonds accompanied by the formation of volatile compounds like CH₃SeH and SeH₂. The self-radiolysis of the aqueous solution of selenomethionine-⁷⁵Se during its storage in air leads, however, to a lower decomposition rate which consists in the release of inorganic selenium and in an oxidation process.     

Bicyclische Iodtetrachalcogenatriphosphaheptane

Bicyclic Iodo-tetrachalcogena-triphospha-heptanes The compounds P₃Se_4–nS_nI (n = 0, 1, 2) have been identified by ³¹P nmr spectroscopy. They were prepared by a direct thermal reaction of red phosphorus, sulfur, selenium and iodine in the stoichiometric ratio. These iodides could be converted into corresponding bromides and chlorides by substitution reactions. All molecules show a diselenium bridge in which, in contrast to the other selenium positions, selenium could not be replaced by sulfur. Similar effects were observed for the diselenium bridges of P₂Se₅. Systematic changes of the chemical shifts and coupling constants are caused by alterations in the molecular geometry, either by replacement of selenium by sulfur, or of iodine by more electronegative ligands. An intramolecular exchange reaction is observed for all molecules.     

Novel PI analogues selectively block activation of the pro-survival serine/threonine kinase Akt

The synthesis from l-quebrachitol of a series of 3-deoxygenated ether lipid-type phosphatidylinositol (PI) analogues is reported, that selectively block activation of Akt and downstream substrates without affecting activation of the upstream kinase, PDK-1, or other kinases downstream of ras such as MAPK in H157 and H1703 lung cancer cells that have high levels of constitutively active Akt. The 2-hydroxyl in these compounds was deleted or alkylated with the intent to preclude metabolic degradation of these compounds by PI-specific phospholipase C (PI-PLC). PI analogues with phosphate linkers are more effective than those with carbonate linkers. Specific inhibition of Akt by these compounds validates ligand design targeted to the PH domains of crucial signaling proteins, thus providing a unique class of possible cancer therapeutics.