Substitutional carbon defects in silicon: A quantum mechanical characterization through the infrared and Raman spectra

The infrared (IR) and Raman spectra of eight substitutional carbon defects in silicon are computed at the quantum mechanical level by using a periodic supercell approach based on hybrid functionals, an all electron Gaussian type basis set and the CRYSTAL code. The single substitutional C _s case and its combination with a vacancy (C _sV and C _sSiV) are considered first. The progressive saturation of the four bonds of a Si atom with C is then examined. The last set of defects consists of a chain of adjacent carbon atoms C, with i = 1–3. The simple substitutional case, C _s, is the common first member of the three sets. All these defects show important, very characteristic features in their IR spectrum. One or two C related peaks dominate the spectra: at 596 cm⁻¹ for C _s (and C _sSiV, the second neighbor vacancy is not shifting the C _s peak), at 705 and 716 cm⁻¹ for C _sV, at 537 cm⁻¹ for C and C (with additional peaks at 522, 655 and 689 for the latter only), at 607 and 624 cm⁻¹, 601 and 643 cm⁻¹, and 629 cm⁻¹ for SiC, SiC, and SiC, respectively. Comparison with experiment allows to attribute many observed peaks to one of the C substitutional defects. Observed peaks above 720 cm⁻¹ must be attributed to interstitial C or more complicated defects.     

Optimization of spiroimidazolidinone derivatives synthesis on solid phase using SynPhase™ Lanterns

We report the synthesis of 1,4,8-triazaspiro[4.5]decan-2-one derivatives from N-benzyl-4-piperidone and N-protected amino acid amides on solid support. We have translated the chemistry from solution to solid phase using a backbone amide linker (BAL) mimic. Using a parallel combinatorial approach, we performed the optimization of the synthesis on SynPhase™ Lanterns.     

Titanates de cuivre substitués à structure bixbyite: Les composés Cu1−xTi1−xFe2xO3 (0.15 ≤ x ≤ 0.33)

A new bixbyite family, Cu_1?xTi_1?xFe_2xO₃ (0.15 ≤ x ≤ 0.33) has been synthesized and characterized. The unit cell is cubic: a ～ 9.40Å. The X-ray powder diffraction study shows up an isotypism with the (Fe, Mn)₂O₃ compounds. There is a disordered distribution of Cu^II, Ti^IV, and Fe^III over the two cyrstallographic sites: P_I and P_II. P_II is highly distorted (two long MO distances) by the Jahn-Teller effect of Cu^II. The bixbyite structure is described in terms of polyhedra arrangement, as a particular case of the CM₂O₃ family. The cation packing is discussed in relation with the existence of the bixbyite structure for the Cu_1?xTi_1?xFe_2xO₃ compounds. The electrical properties (σ ～ 10^?5(Ω cm)^?1 for x = 0.286 at room temperature) show an electron conduction with probably a hopping mechanism.     

Radical amination with sulfonyl azides: a powerful method for the formation of C-N bonds

A novel reaction for the introduction of an azide moiety by means of a mild radical process is currently under development. Sulfonyl azides are suitable azidating agents for nucleophilic radicals, such as secondary and tertiary alkyl radicals. More electrophilic radicals, such as enolate radicals, do not react with sulfonyl azides. This feature allowed the development of efficient intra- and intermolecular carboazidations of olefins. Due to the versatility of the azido group, this reaction has an important synthetic potential, as already demonstrated by the preparation of the core of several alkaloids, particularly those containing an amino-substituted quaternary carbon center, such as FR901483.     

Chemo- and Stereoselective Coordination of 5,6-Dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene by d8- and d6-Metal Carbonyls. Diels-Alder reactivity of Dienes Perturbed by Remote Olefin Complexation

The endocyclic double bond C(2), C(3) in 5,6-dimethylidene-7-oxabicyclo[2.2.1]-hept-2-ene ( 1 ) can he coordinated selectively on its exo-face before complexation of the exocyclic s-cis-butadiene moiety. Irradiation of Ru₃(CO)₁₂ or Os₃(CO)₁₂ in the presence of 1 gave tetracarbonyl [(1R,2R, 3S,4S)-2,3-η-(5,6-dimethylidene-7-oxabicyclo[2.2.1]-hept-2-ene)]ruthenium ( 6 ) or -osmium ( 8 ). Similarly, irradiation of Cr(CO)₆ or W(CO)₆ in the presence of 1 gave pentacarbonyl[(1R, 2R, 3S,4S)-2,3-η-(5,6-dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene)]chromium (10) or -tungsten (11) . Irradiation of complexes 6 and 11 in the presence of 1 led to further CO substitution giving bed-tricarbonyl-ae-bis[(1R,2R,3S,4S)-2,3-η-(5,6-dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene)]ruthenium ( 7 ) and trans-tetracarbonyl[(1R,2R,3S,4S)-2,3-η-(5,6-dimethylidene-7-oxabicyclo-[2.2.1]hept-2-ene)]tungsten (12) , respectively. The diosmacyclobutane derivative cis-m?-[(1R,3R,3S,4S)-(5,6-dimethylidene-7-oxabicyclo[2.2.1]hepta-2,3-diyl)]bis(tetracarbonyl-osmium) (Os-Os) (9) wa also obtained. The Diels-Alder reactivity of the exocyclic s-cis-butadiene moiety in complexs 7 and 8 was found to be significantly higher than that of the free triene 1 .     

Use of molecular dynamics in the design and structure determination of a photoinducible beta-hairpin

The study presented here consists of three parts. In the first, the ability of a set of differently substituted diazobenzene-based linkers to act as photoswitchable beta-turn building blocks was assessed. A 12-residue peptide known to form beta-hairpins was taken as the basis for the modeling process. The central (beta-turn) residue pair was successively replaced by six symmetrically ((o,o), (m,m), or (p,p)) substituted (aminomethyl/carboxymethyl or aminoethyl/carboxyethyl) diazobenzene derivatives leading to a set of peptides with a photoswitchable backbone conformation. The folding behavior of each peptide was then investigated by performing molecular dynamics simulations in water (4 ns) and in methanol (10 ns) at room temperature. The simulations suggest that (o,o)- and (m,m)-substituted linkers with a single methylene spacer are significantly better suited to act as photoswitchable beta-turn building blocks than the other linkers examined in this study. The peptide containing the (m,m)-substituted linker was synthesized and characterized by NMR in its cis configuration. In the second part of this study, the structure of this peptide was refined using explicit-solvent simulations and NOE distance restraints, employing a variety of refinement protocols (instantaneous and time-averaged restraining as well as unrestrained simulations). We show that for this type of systems, even short simulations provide a significant improvement in our understanding of their structure if physically meaningful force fields are employed. In the third part, unrestrained explicit-solvent simulations starting from either the NMR model structure (75 ns) or a fully extended structure (25 ns) are shown to converge to a stable beta-hairpin. The resulting ensemble is in good agreement with experimental data, indicating successful structure prediction of the investigated hairpin by classical explicit-solvent molecular dynamics simulations.     

Detection of genetically modified corn (Bt176) in spiked cow blood samples by polymerase chain reaction and immunoassay methods

The fate of DNA and protein transgenic sequences in products derived from animals fed transgenic crops has recently raised public interest. Sensitive molecular tests targeting the Bt176 genetic construct and the transgenic Cry1Ab protein were developed to determine whether plant sequences, especially transgenic sequences, are present in animal products. A protocol for total DNA extraction and purification from cow whole blood samples was first drawn up and assessed by spiking with known amounts of DNA from Bt176 maize. The limit of detection for transgenic sequences (35S promoter and Bt176-specific junction sequence) was determined by both the polymerase chain reaction-enzyme-linked immunosorbent assay (PCR-ELISA) and the 5'-nuclease PCR assay. Four additional PCR systems were built to substantiate the results. The first detects a mono-copy maize-specific sequence (ADH promoter). Two others target multi-copy sequences from plant nucleus (26S rRNA gene) and chloroplast (psaB gene). The last one, used as a positive control, targets a mono-copy animal sequence (alpha(s1)-casein gene). Both methods detected a minimum spiking at 25 copies of Bt176 maize/mL in 10 mL whole blood samples. The sandwich ELISA kit used detected down to 1 ng transgenic Cry1Ab protein/mL spiked whole blood.     

Photoinduced processes within compact dyads based on triphenylpyridinium-functionalized bipyridyl complexes of ruthenium(II)

As an alternative to conventional charge-separation functional molecular models based on long-range ET within redox cascades, a "compact approach" has been examined. To this end, spacer elements usually inserted between main redox-active units within polyad systems have been removed, allowing extended rigidity but at the expense of enhanced intercomponent electronic communication. The molecular assemblies investigated here are of the P-(theta (1))-A type, where the theta (1) twist angle is related to the degree of conjugation between the photosensitizer (P, of {Ru(bpy)(3)}(2+) type) and the electron-acceptor (A). 4-N- and 4-N-,4'-N-(2,4,6-triphenylpyridinio)-2,2'-bipyridine ligands (A(1)-bpy and A(2)-bpy, respectively) have been synthesized to give complexes with Ru(II), 1-bpy and 2-bpy, respectively. Combined solid-state analysis (X-ray crystallography), solution studies ((1)H NMR, cyclic voltammetry) and computational structural optimization allowed verifying that theta (1) angle approaches 90 degrees within 1-bpy and 2-bpy in solution. Also, anticipated existence of strong intercomponent electronic coupling has been confirmed by investigating electronic absorption properties and electrochemical behavior of the compounds. The capability of 1-bpy and 2-bpy to undergo PET process was evaluated by carrying out their photophysical study (steady state emission and time-resolved spectroscopy at both 293 and 77 K). The conformational dependence of photoinduced processes within P-(theta (1))-A systems has been established by comparing the photophysical properties of 1-bpy (and 2-bpy) with those of an affiliated species reported in the literature, 1-phen. A complementary theoretical analysis (DFT) of the change of spin density distribution within model [1-bpy(theta (1))](-) mono-reduced species as a function of theta (1) has been undertaken and the possibility of conformationally switching emission properties of P was derived.     

Does charge carrier dimensionality increase in mixed-valence salts of tetrathiafulvalene-terminated dendrimers?

[structure: see text] In four new dendrimers terminated by 12 electroactive tetrathiafulvalenyl substituents, the tridimensional character of the inter- and intradendrimeric charge and electron transfer, and hence of the electroconductivity, is evidenced by examination of the electronic spectra of their corresponding neutral state and cation radical, dication, and mixed-valence salts, including a closed-shell anion.     

Total synthesis of erythromycin B

We report the details of the first total synthesis of erythromycin B using two different strategies for the end game. The first of these follows a classical approach in which the desosamine and cladinose residues are sequentially appended to a macrocyclic lactone, which was formed by cyclization of a seco acid derivative, to give a bis-glycosylated macrolide intermediate that is converted into erythromycin B. The second strategy features an abiotic approach in which a seco acid bearing a desosamine residue is cyclized to give a monoglycosylated macrocyclic lactone that is then transformed into erythromycin B via a sequence of steps involving refunctionalizations and a glycosylation to introduce the cladinose moiety. Attempts to prepare a bis-glycosylated seco acid by de novo synthesis were unsuccessful. The syntheses of the key seco acid intermediates feature the oxidative transformation of a furan containing C(3)-C(10) to provide a dioxabicyclo[3.3.1]nonenone that served as a template on which to create the stereocenters at C(6) and C(8). A stereoselective aldol reaction was used to establish the C(11)-C(15) segment, and a stereoselective crotylation was implemented to introduce the propionate subunit comprising C(1)-C(2).