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PubMed2004

Shallow donors in diamond: chalcogens, pnictogens, and their hydrogen complexes.

The utility of diamond as an electronic material is compromised by the lack of a suitable shallow donor. Here, ab initio theory is used to investigate the donor levels of substitutional pnictogen (N, P, As, and Sb) and chalcogen (S, Se, and Te) impurities and chalcogen-hydrogen defects in diamond. Substitutional S is found to be a deep donor, while As and Sb possess donor levels significantly shallower than P, which so far is the most effective shallow donor found by experiment.

Journal Article↗
PubMed2005

Isosteric replacement of sulfur with other chalcogens in peptides and proteins.

The review addresses the functional and structural properties of the two series of chalcogen analogues of amino acids in peptides and proteins, the methionine and the serine/cysteine series, and discusses the synthesis of the related selenium/tellurium analogues as well as their use in peptide synthesis and protein expression. Advances in synthetic methodologies and recombinant technologies and their combined applications in native and expressed protein ligation allows the isomorphous character of selenium- and tellurium-containing amino acids to be exploited for production of heavy metal mutants of proteins and thus to facilitate the phasing problem in x-ray crystallography. In addition, selenocysteine has been recognized as an ideal tool for the production of selenoenzymes with new catalytic activities. Moreover, the fully isomorphous character of disulfide replacement with diselenide is well suited to increase the robustness of cystine frameworks in cystine-rich peptides and proteins and for the de novo design of even non-native cystine frameworks by exploiting the highly negative redox potential of selenols.

Chalcogens↗
PubMed2004

Insight into the polar reactivity of the onium chalcogen analogues of S-adenosyl-L-methionine.

S-Adenosyl-L-methionine (AdoMet) is one of Nature's most diverse metabolites, used not only in a large number of biological reactions but amenable to several different modes of reactivity. The types of transformations in which it is involved include decarboxylation, electrophilic addition to any of the three carbons bonded to the central sulfur atom, proton removal at carbons adjacent to the sulfonium, and reductive cleavage to generate 5'-deoxyadenosyl 5'-radical intermediates. At physiological pH and temperature, AdoMet is subject to three spontaneous degradation pathways, the first of which is racemization of the chiral sulfonium group, which takes place in a pH-independent manner. The two remaining pathways are pH-dependent and include (1) intramolecular attack of the alpha-carboxylate group onto the gamma-carbon, affording L-homoserine lactone (HSL) and 5'-methylthioadenosine (MTA), and (2) deprotonation at C-5', initiating a cascade that results in formation of adenine and S-ribosylmethionine. Herein, we describe pH-dependent stability studies of AdoMet and its selenium and tellurium analogues, Se-adenosyl-L-selenomethionine and Te-adenosyl-L-telluromethionine (SeAdoMet and TeAdoMet, respectively), at 37 degrees C and constant ionic strength, which we use as a probe of their relative intrinsic reactivities. We find that with AdoMet intramolecular nucleophilic attack to afford HSL and MTA exhibits a pH-rate profile having two titratable groups with apparent pK(a) values of 1.2 +/- 0.4 and 8.2 +/- 0.05 and displaying first-order rate constants of <0.7 x 10(-6) s(-1) at pH values less than 0.5, approximately 3 x 10(-6) s(-1) at pH values between 2 and 7, and approximately 15 x 10(-6) s(-1) at pH values greater than 9. Degradation via deprotonation at C-5' follows a pH-rate profile having one titratable group with an apparent pK(a) value of approximately 11.5. The selenium analogue decays significantly faster via intramolecular nucleophilic attack, also exhibiting a pH-rate profile with two titratable groups with pK(a) values of approximately 0.86 and 8.0 +/- 0.1 with first-order rate constants of <7 x 10(-6) s(-1) at pH values less than 0.9, approximately 32 x 10(-6) s(-1) at pH values between 2 and 7, and approximately 170 x 10(-6) s(-1) at pH values greater than 9. Degradation via deprotonation at C-5' proceeds with one titratable group displaying an apparent pK(a) value of approximately 14.1. Unexpectedly, TeAdoMet did not decay at an observable rate via either of these two pathways. Last, enzymatically synthesized AdoMet was found to racemize at rates that were consistent with earlier studies (Hoffman, J. L. (1986) Biochemistry 25, 4444-4449); however, SeAdoMet and TeAdoMet did not racemize at detectable rates. In the accompanying paper, we use the information obtained in these model studies to probe the mechanism of cyclopropane fatty acid synthase via use of the onium chalcogens of AdoMet as methyl donors.

Alkylating Agents↗
PubMed2006

Synthesis and properties of benzo[a]phenoxazinium chalcogen analogues as novel broad-spectrum antimicrobial photosensitizers.

The goal of this investigation was to develop improved photosensitizers for use as antimicrobial drugs in photodynamic therapy of localized infections. Replacement of the oxygen atom in 5-(ethylamino)-9-diethylaminobenzo[a]phenoxazinium chloride (1) with sulfur and selenium afforded thiazinium and selenazinium analogues 2 and 3, respectively. All three dyes are water soluble, lipophilic, and red light absorbers. The relative photodynamic activities of the chalcogen series were evaluated against a panel of prototypical pathogenic microorganisms: the Gram-positive Enterococcus faecalis, the Gram-negative Escherichia coli, and the fungus Candida albicans. Selenium dye 3 was highly effective as a broad-spectrum antimicrobial photosensitizer with fluences of 4-32 J/cm2 killing 2-5 more logs of all cell types than sulfur dye 2, which was slightly more effective than oxygen analogue 1. These data, taken with the findings of uptake and retention studies, suggest that the superior activity of selenium derivative 3 can be attributed to its much higher triplet quantum yield.

Anti-Infective Agents↗
PubMed2002

Chalcogen rich lanthanide clusters from halide starting materials (II): selenido compounds.

Lanthanides reduce mixtures of I(2) and PhSeSePh in THF, and the resultant heteroligand mixture reacts further with elemental Se in pyridine to give (THF)(6)Ln(4)I(2)(SeSe)(4)(mu(4)-Se).THF (Ln = Tm, Ho, Er, Yb). These selenium rich clusters contain a square array of Ln(III) ions connected through a single (mu(4)-Se) ligand. There are two I(-) ligands coordinating nonadjacent Ln(III) ions on the side of the cluster opposite the (mu(4)-Se), and the edges of the square are bridged by mu(2)-SeSe groups. The electronic spectrum of the Yb compound contains two absorption maxima that can tentatively be assigned as Se(2-) to Yb and SeSe to Yb charge-transfer absorptions, by comparison with the featureless absorption spectra of the Tm, Ho, and Er derivatives. With a 1/1/1/1 Yb/I/Ph(2)S(2)/Se stoichiometry, chalcogen rich compounds are not obtained, but instead, in Yb chemistry, the selenido cluster (THF)(10)Yb(6)Se(6)I(6) can be isolated in 51% yield. The molecular structure of this compound contains a Yb(4)Se(4) cubane fragment, with an additional Yb(2)Se(2) layer capping one face of the cube. Each Yb coordinates a terminal I(-). This intensely colored compound also has an absorption maximum in the visible spectrum. Upon thermolysis, the selenium rich compounds give Ln(2)Se(3) that is free of iodide contamination.

Journal Article↗
PubMed2002

Chalcogen-rich lanthanide clusters with fluorinated thiolate ligands.

Mixtures of Ln(SC(6)F(5))(3) and Ln(EPh)(3) (E = S, Se) react with elemental E to give chalcogen-rich clusters with fluorinated thiolate ancillary ligands. The structures of both (THF)(6)Yb(4)S(SS)(4)(SC(6)F(5))(2) and (THF)(6)Yb(4)Se(SeSe)(4)(SC(6)F(5))(2) have been established by low-temperature single-crystal X-ray diffraction. Both compounds contain a square array of Yb(III) ions connected by a central mu(4)-E(2-) ligand. The edges of the square Yb(4) array are bridged by four mu(2)(EE) ligands, and two terminal SC(6)F(5) are on the same side of the Ln(4) plane that is capped by the mu(4)-E(2-) ion. Redox inactive (THF)(6)Tm(4)Se(SeSe)(4)(SC(6)F(5))(2) was also prepared to establish the extension of this chemistry to the redox inactive Ln. These clusters are soluble in toluene.

Journal Article↗
PubMed2006

Unusual In2N4 cores in complexes containing triazole-based chalcogen-phosphoranyl ligands.

The 4,5-bis(diphenylphosphoranyl)-1,2,3-triazole [4,5-(P(E)Ph2)2tz] derivatives of indium {kappa3-N,N',E-[4,5-(P(E)Ph2)2(mu-tz)]InMe2}2 (E = O2, S3, Se4) were prepared in good yield. In addition, compound 5 (E = O, E' = Se) was obtained from 4 through the replacement of a selenium atom in the P-Se(In) moiety by an oxygen atom, giving the mixed-chalcogen complex. The crystal structures of 2 and 5 exhibit a central C4In2N6O2P4core with an almost planar arrangement (mean deviation = 0.019 and 0.042 A for 2 and 0.100 A for 5), while the C4In2N6S2P4 core in 3 is nonplanar (mean deviation = 0.223 A).

Journal Article↗
PubMed1997

Syntheses and X-ray Structures of a Series of Chalcogen-Containing Manganese Carbonylates [E(2)Mn(3)(CO)(9)](-), [E(8)C(2)Mn(2)(CO)(6)](2-), and [E(2)Mn(4)(CO)(12)](2-) (E = Se, S).

The reaction of SeO(2) with Mn(2)(CO)(10)/KOH in methanol solution forms a trigonal bipyramid cluster anion [Se(2)Mn(3)(CO)(9)](-) (Ia). Cluster Ia can also be synthesized from the treatment of Se powder with Mn(2)(CO)(10)/KOH in 2:1 molar ratio in methanol solution. If Se is reacted with Mn(2)(CO)(10)/KOH in a molar ratio of 8:1, the deoxygenation occurs to produce a novel cluster [Se(8)C(2)Mn(2)(CO)(6)](2)(-) (IIa). Cluster IIa can readily transform into a new octahedral cluster [Se(2)Mn(4)(CO)(12)](2)(-) (IIIa) by further treatment with Mn(2)(CO)(10)/KOH in CH(2)Cl(2)/MeOH solutions, and the use of a molar ratio of 1:1 of Se powder to Mn(2)(CO)(10)/KOH in methanol also can yield complex IIIa. In the sulfur-manganese system, the synthesis of the analogous complexes [S(2)Mn(3)(CO)(9)](-) (Ib), [S(8)C(2)Mn(2)(CO)(6)](2)(-) (IIb), and [S(2)Mn(4)(CO)(12)](2)(-) (IIIb) also can be accomplished from the reactions of S powder with Mn(2)(CO)(10)/KOH in similar fashions. Clusters Ia-IIIa and Ib-IIIb are fully characterized by spectroscopic methods or/and X-ray analyses. The novel clusters [E(8)C(2)Mn(2)(CO)(6)](2)(-) (E = Se, IIa; E = S, IIb) each consist of two separate Mn(CO)(3) groups which are doubly bridged by two CE(4) fragments. Clusters Ia and Ib each display a trigonal bipyramid framework with two chalcogen atoms in the axial positions, while clusters IIIa and IIIb each exhibit an octahedral metal skeleton.

Journal Article↗
PubMed1999

Theoretical Bonding Description of Alkylidene Chalcogen (O, S, Se) Difluorides (H(2)C=XF(2)): Planar versus Bent Conformations.

Theoretical descriptions of alkylidene chalcogen difluorides have been performed on their planar (1-3) and bent (4-6) conformations. The planar T-shaped conformations were the most stable ones, from the calculations performed (Gaussian-G2 and B3LYP/6-311+G). The bonding nature of the T-shaped (C(2)(v)()) structures has also been analyzed by means of the atoms in molecules (AIM) theory and electron localization function analyses.

Journal Article↗
PubMed2004

Tin-chalcogen double-bond compounds, stannanethione and stannaneselone: synthesis, structure, and reactivities.

The first isolation of diarylstannanethione (tin-sulfur double-bond compound) and diarylstannaneselone (tin-selenium double-bond compound), Tbt(Ditp)Sn=X (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl; Ditp = 2,2' '-diisopropyl-m-terphenyl-2'-yl; X = S and Se) was accomplished by dechalcogenation of the corresponding highly hindered tetrachalcogenastannolanes, Tbt(Ditp)SnX(4). The (119)Sn NMR of stannanethione, Tbt(Ditp)Sn=S, and stannaneselone, Tbt(Ditp)Sn=Se, showed only one low-field broad signal at 531 and 440 ppm, respectively, characteristic of a tricoordinated tin, and hence, the stannanethione and stannaneselone display an intrinsic nature of tin-chalcogen double-bond compounds. The X-ray crystallographic analysis of the isolated stannaneselone, Tbt(Ditp)Sn=Se 5a, revealed a completely trigonal geometry around the central tin with a remarkably short Sn-Se bond length, indicative of structural similarity to a ketone.

Journal Article↗
PubMed2005

Electronic structures and molecular properties of chalcogen nitrides Se2N2 and SeSN2.

The electronic structures and molecular properties of S2N2 as well as the currently unknown chalcogen nitrides Se2N2 and SeSN2 have been studied using various ab initio and density functional methods. All molecules share a qualitatively similar electronic structure and can be primarily described as 2pi-electron aromatics having minor singlet diradical character of 6-8% that can be attributed solely to the nitrogen atoms. This diradical character is manifested in the prediction of their molecular properties, in which coupled cluster and multiconfigurational approaches, as well as density functional methods, show the best performance. The conventional ab initio methods RHF and MP2 completely fail to describe these systems. Predictions for the vibrational frequencies, IR intensities, Raman activities, and 14N, 15N, and 77Se chemical shifts, as well as singlet excitation energies of Se2N2 and SeSN2, have been made. The computed high-level spectroscopic data will be of considerable value in future efforts aimed at the preparation of the conducting polymers (SeN)x and (SeNSN)x.

Journal Article↗
PubMed2006

Characterization of ZnE (E = S, Se, or Te) materials synthesized using silylated chalcogen reagents in mesoporous MCM-41.

The binary materials ZnS, ZnSe, and ZnTe have been successfully prepared within the mesoporous framework of MCM-41 at room temperature through the use of silylated chalcogen reagents. Postsynthesis grafting of ethylenediamine facilitates the complexation of anhydrous zinc acetate to the pore surface via a ligand exchange process between monodentate 3,5-lutidine ligands and the anchored chelating moiety. Coordinated zinc acetate readily reacts with E(SiMe(3))(2) (E = S, Se, or Te), thereby encapsulating zinc chalcogenides in the mesoporous channels. ZnE-MCM-41 materials have been characterized by EDX analysis, nitrogen sorption analysis, and Raman, UV-vis, and solid-state NMR spectroscopy. The observed blue shift in the absorption maximum is in agreement with the expected quantum confinement of these materials given the nanometer dimensions of the mesoporous architecture of the silicate host.

Journal Article↗
PubMedDate not supplied

Chalcogen-analogs of amino acids. Their use in X-ray crystallographic and folding studies of peptides and proteins.

Using methionine-auxotrophic Escherichia coli strains quantitative biosynthetic replacement of the methionine residues by seleno- and telluromethionine but not by methoxinine was achieved in various model proteins, clearly indicating a limited tolerance in the editing range of methionyl-tRNA synthetase. For expression of the protein variants the acetyl derivatives of the chalcogen-analogs of methionine, obtained by a new and highly efficient synthetic procedure, proved to be the ideal source in the growth media as they were found to be significantly more stable than the underivatized methionine analogs. The conformational properties in solution, the folding and unfolding parameters as well as X-ray crystallographic data confirmed the highly isomorphous character of the atomic mutants and thus the usefulness of this concept in X-ray analysis of proteins. Quantitative replacement of cysteine residues by selenocysteine has recently been achieved using cysteine-auxotrophic E. coli strains, but a selective replacement of cysteine residues by employing the natural translational machinery of selenocysteine is also conceivable. We have therefore performed a detailed study on synthetic selenocysteine-peptides in order to determine the redox potential of this cysteine analog, and thus the ability of related peptide and protein analogs to undergo the correct oxidative folding. Since the redox potential of selenocysteine was found to be significantly more reducing than that of the parent amino acid, selective formation of a diselenide bridge in presence of additional cysteine residues is highly favored as well documented in the case of the synthetic bis-selenocysteine-endothelin I analog. These results confirm that even cysteine residues may represent an interesting target for the design and expression of isomorphous heteroatomic analogs of proteins.

Amino Acids↗
PubMed2004

Synthesis, properties, and photodynamic properties in vitro of heavy-chalcogen analogues of tetramethylrosamine.

Thio and seleno analogues of tetramethylrosamine were prepared by the directed-metalation/cyclization of the corresponding N,N-diethyl 2-(3-dimethylaminophenylchalcogeno)-4-dimethylaminobenzamide to the 2,7-bis-(N,N-dimethylamino)-9H-chalcogenoxanthen-9-one followed by the addition of phenylmagnesium bromide, dehydration, and ion exchange to the chloride salt. The thio and seleno tetramethylrosamines had longer wavelengths of absorption and higher quantum yields for the generation of singlet oxygen than tetramethylrosamine. Both the thio and selenoanalogues of tetramethylrosamine were efficient photosensitizers against R3230AC rat mammary adenocarcinoma cells in vitro.

Animals↗
PubMed2005

Structure-activity studies of uptake and phototoxicity with heavy-chalcogen analogues of tetramethylrosamine in vitro in chemosensitive and multidrug-resistant cells.

Several thio and seleno analogues of tetramethylrosamine (TMR) were prepared. Thio derivatives of TMR have absorption maxima near 570 nm, while seleno derivatives of TMR have absorption maxima near 580 nm. The 3- or 4-N,N-dimethylaminophenyl substituent in the 9-position greatly increases internal conversion, which lowers quantum yields for fluorescence and the generation of singlet oxygen. Thio and seleno analogues of TMR are effective photosensitizers against chemosensitive AUXB1 cells in vitro and against multidrug-resistant CR1R12 cells in vitro, which have been treated with verapamil. The CR1R12 cells accumulated significantly lower concentrations of the photosensitizers relative to the AUXB1 cells presumably due to the expression of P-glycoprotein (Pgp) in the CR1R12 cells. Following treatment with 5 x 10(-5) M verapamil, the uptake in CR1R12 cells of several fluorescent thio analogues of TMR is comparable to that observed for the chemosensitive AUXB1 cells.

Animals↗
PubMed2005

o-hydroxylmethylphenylchalcogens: synthesis, intramolecular nonbonded chalcogen...OH interactions, and glutathione peroxidase-like activity.

[Structure: see text]. The synthesis and characterization of a series of organochalcogen (Se, Te) compounds derived from benzyl alcohol 13 are described. The synthesis of the key precursor dichalcogenides 15, 22, and 29 was achieved by the ortho-lithiation route. Selenide 18 was obtained by the reaction of the dilithiated derivative 14 with Se(dtc)2. Oxidation of 15 and 22 with H2O2 afforded the corresponding cyclic ester derivatives 17 and 24, respectively. Oxidation of selenide 18 with H2O2 affords the spirocyclic compound 19. The presence of intramolecular interactions in dichalcogenides 15 and 22 has been proven by single-crystal X-ray studies. The cyclic compounds 17 and 19 have also been characterized by single-crystal X-ray studies. GP(X)-like antioxidant activity of selenium compounds has been evaluated by the coupled bioassay method. Density functional theory calculations at the mPW1PW91 level on ditelluride 22 have identified a fairly strong nonbonding interaction between the hydroxy oxygen and tellurium atom. The second-order perturbation energy obtained through NBO analysis conveys the involvement of n(O) --> sigma(Te-Te) orbital overlap in nonbonding interaction. Post wave function analysis with the Atoms in Molecules (AIM) method identified distinct bond critical point in 15 and 22 and also indicated that the nonbonding interaction is predominantly covalent. Comparison between diselenide 15 and ditelluride 22 using the extent of orbital interaction as well as the value of electron density at the bond critical points unequivocally established that a ditelluride could be a better acceptor in nonbonding interaction, when the hydroxy group acts as the donor.

Chalcogens↗
PubMed2006

Hybrid organic semiconductors including chalcogen atoms in pi-conjugated skeletons. Tuning of optical, redox, and vibrational properties by heavy atom conjugation.

In this work, the interactions between heteroatoms (S, Se, and Te) and conjugated skeletons are analyzed. The study is carried out by using electronic absorption and fluorescence spectroscopies, electrochemistry, vibrational Raman spectroscopy, and theoretical calculations in the framework of DFT and TD-DFT theories. Optical spectra are described in terms of one-electron promotions between orbitals around the energy gap. Electrochemistry, in the framework of the Koopman's approach, is also interpreted. The vibrational Raman spectra are assigned to molecular modes and the evolution changing the heteroatom is addressed and an effective tuning of these properties is found. Part of this modulation is associated with local electronic interactions depending on the relative S, Se, and Te electronegativities. Unconventional long-range heteroatom-heteroatom interactions have been proposed which arise from the existence of effective pi-conjugated channels. The molecular level understanding of structure-property relationships in these organic/inorganic semiconductors are of great interest in the interdisciplinary area of material science.

Chalcogens↗
PubMed2001

Electrochemistry of chalcogen compounds: prediction of antioxidant activity.

Synthesis and characterisation of organochalcogens has demonstrated a high correlation between their electrochemical oxidation potential on the glassy carbon electrode, their activity in bioassays and an unprecedented antioxidant activity in neuronal cell culture (EC50 approximately 20 nM) making electrochemical methodology a valuable tool in drug design for Alzheimer's and related diseases.

Animals↗