Soft Scorpionate Hydridotris(2-mercapto-1-methylimidazolyl) borate) Tungsten-Oxido and -Sulfido Complexes as Acetylene Hydratase Models

A series of W^IV alkyne complexes with the sulfur-rich ligand hydridotris(2-mercapto-1-methylimidazolyl) borate) (Tm^Me) are presented as bio-inspired models to elucidate the mechanism of the tungstoenzyme acetylene hydratase (AH). The mono- and/or bis-alkyne precursors were reacted with NaTm^Me and the resulting complexes [W(CO)(C₂R₂)(Tm^Me)Br] (R=H 1 , Me 2 ) oxidized to the target [WE(C₂R₂)(Tm^Me)Br] (E=O, R=H 4 , Me 5 ; E=S, R=H 6 , Me 7 ) using pyridine-N-oxide and methylthiirane. Halide abstraction with TlOTf in MeCN gave the cationic complexes [WE(C₂R₂)(MeCN)(Tm^Me)](OTf) (E=CO, R=H 10 , Me 11 ; E=O, R=H 12 , Me 13 ; E=S, R=H 14 , Me 15 ). Without MeCN, dinuclear complexes [W₂O(μ-O)(C₂Me₂)₂(Tm^Me)₂](OTf)₂ ( 8 ) and [W₂(μ-S)₂(C₂Me₂)(Tm^Me)₂](OTf)₂ ( 9 ) could be isolated showing distinct differences between the oxido and sulfido system with the latter exhibiting only one molecule of C₂Me₂. This provides evidence that a fine balance of the softness at W is important for acetylene coordination. Upon dissolving complex 8 in acetonitrile complex 13 is reconstituted in contrast to 9 . All complexes exhibit the desired stability toward water and the observed effective coordination of the scorpionate ligand avoids decomposition to disulfide, an often-occurring reaction in sulfur ligand chemistry. Hence, the data presented here point toward a mechanism with a direct coordination of acetylene in the active site and provide the basis for further model chemistry for acetylene hydratase.     

Phosphate desorption kinetics from goethite as induced by arsenate

The kinetics of the arsenate-induced desorption of phosphate from goethite has been studied with a batch reactor system and ATR-FTIR spectroscopy. The effects of arsenate concentration, adsorbed phosphate, pH and temperature between 10 and 45 °C were investigated. Arsenate is able to promote phosphate desorption because both oxoanions compete for the same surface sites of goethite. The desorption occurs in two steps: a fast step that takes place in less than 5 min and a slow step that lasts several hours. In the slow step, arsenate ions exchange adsorbed phosphate ions in a 1:1 stoichiometry. The reaction is first order with respect to arsenate concentration and is independent of adsorbed phosphate under the experimental conditions of this work. The rate law is then r = k_r[As], where r is the desorption rate, k_r is the rate constant and [As] is the arsenate concentration in solution. The values of k_r at pH 7 are 1.87 × 10⁻⁵ L m⁻² min⁻¹ at 25 °C and 7.95 × 10⁻⁵ L m⁻² min⁻¹ at 45 °C. The apparent activation energy of the desorption process is 51 kJ mol⁻¹. Data suggest that the rate-controlling process is intraparticle diffusion of As species, probably As diffusion in pores. ATR-FTIR spectroscopy suggests that adsorbed phosphate species at pH 7 are mainly bidentate inner-sphere surface complexes. The identity of these complexes does not change during desorption, and there is no evidence for the formation of intermediate species during the reaction.     

Inhibition of [FeFe]-hydrogenases by formaldehyde and wider mechanistic implications for biohydrogen activation

Formaldehyde-a rapid and reversible inhibitor of hydrogen evolution by [FeFe]-hydrogenases-binds with a strong potential dependence that is almost complementary to that of CO. Whereas exogenous CO binds tightly to the oxidized state known as H(ox) but very weakly to a state two electrons more reduced, formaldehyde interacts most strongly with the latter. Formaldehyde thus intercepts increasingly reduced states of the catalytic cycle, and density functional theory calculations support the proposal that it reacts with the H-cluster directly, most likely targeting an otherwise elusive and highly reactive Fe-hydrido (Fe-H) intermediate.     

Theoretical investigation of linalool oxidation

This study concerns the autoxidation of one of the most used fragrances in daily life, linalool (3,7-dimethyl-1,6-octadien-3-ol). It reacts with O2 to form hydroperoxides, which are known to be important contact allergens. Pathways for hydroperoxide formation are investigated by means of quantum mechanical electronic structure calculations. Optimized molecular geometries and harmonic vibrational frequencies are determined using density functional theory (DFT). Insight into how the addition of O2 to linalool occurs is obtained by establishing a theoretical framework and systematically investigating three smaller systems: propene, 2-methyl-2-butene, and 2-methyl-2-pentene. 2-Methyl-2-pentene was chosen as a model system and used to compare with linalool. This theoretical study characterizes the linalool-O2 biradical intermediate state, which constitutes a branching point for the further oxidation reactions pathways. Thus, the observed linalool oxidation product spectrum is discussed in terms of a direct reaction path, the ene-type mechanism, and the radical mechanism. The major hydroperoxide found in experiments is 7-hydroperoxy-3,7-dimethyl-octa-1,5-diene-3-ol, and the calculated results support this finding.     

Stereoselective,solid phase-based synthesis of trans 3-alkyl-substituted β-lactams as analogues of cholesterol absorption inhibitors

A straightforward solid phase-based strategy for the rapid generation of two small libraries of trans 3-alkyl-substituted β-lactams is described. For the glycine-derived library, a controlled excess of nonactivated acid chlorides was used to prevent oxazinone formation. The second library involved the attachment of Fmoc-protected p-aminophenol to Wang resin for the preparation of structurally-closed analogues of known cholesterol absorption inhibitors. This strategy allowed us to introduce diversity in the three variable positions of the β-lactam ring.     

Optimization of alkaline transesterification of soybean oil and castor oil for biodiesel production

This article reports experimental data on the production of fatty acid ethyl esters from refined and degummed soybean oil and castor oil using NaOH as catalyst. The variables investigated were temperature (30–70°C), reaction time (1–3 h), catalyst concentration (0.5–1.5 w/wt%), and oil-to-ethanol molar ratio (1:3–1:9). The effects of process variables on the reaction conversion as well as the optimum experimental conditions are presented. The results show that conversions >95% were achieved for all systems investigated. In general, an increase in reaction temperature, reaction time, and in oil-to-ethanol molar ratio led to an enhancement in reaction conversion, whereas an opposite trend was verified with respect to catalyst concentration.     

SNaPshot minisequencing to resolve mitochondrial macro‐haplogroups found in Africa

African mitochondrial DNA (mtDNA) haplogroups are divided into seven macro‐haplogroups (L0′1′2′3′4′5′6), while the rest of the world's lineages are classified as subgroups of macro‐haplogroups M, N and R. The most common approach to characterizing mtDNA variation is the sequencing of hypervariable segments I and II of the non‐coding control region of the molecule. Given the higher mutation rate within the control region compared with the coding regions of the molecule, recurrent mutations in the former can sometimes hide possible phylogenetic structure. The incorporation of haplogroup‐defining coding region mutations has helped in overcoming this limitation. By judiciously selecting 14 coding region SNPs and incorporating them into a multiplex minisequencing assay we were able to resolve mtDNA sequences from some sub‐Saharan African populations into ten macro‐haplogroups (L0–L6, M, N and R). We tested the efficacy of the panel by screening 699 individuals, consisting mostly of Khoe‐San, Bantu speakers and individuals with mixed ancestries (Coloreds) and found no inconsistencies compared with hypervariable segment sequencing results. The panel provided a fast and efficient means of classifying mtDNA into the ten mitochondrial macro‐haplogroups and provided a reliable screening to distinguish African from non‐African‐derived mtDNA lineages.     

Consecutive Photoinduced Electron Transfer (conPET): The Mechanism of the Photocatalyst Rhodamine 6G

The dye rhodamine 6G can act as a photocatalyst through photoinduced electron transfer. After electronic excitation with green light, rhodamine 6G takes an electron from an electron donor, such as N,N-diisopropylethylamine, and forms the rhodamine 6G radical. This radical has a reduction potential of around −0.90 V and can split phenyl iodide into iodine anions and phenyl radicals. Recently, it has been reported that photoexcitation of the radical at 420 nm splits aryl bromides into bromide anions and aryl radicals. This requires an increase in reduction potential, hence the electronically excited rhodamine 6G radical was proposed as the reducing agent. Here, we present a study of the mechanism of the formation and photoreactions of the rhodamine 6G radical by transient absorption spectroscopy in the time range from femtoseconds to minutes in combination with quantum chemical calculations. We conclude that one photon of 540 nm light produces two rhodamine 6G radicals. The lifetime of the photoexcited radicals of around 350 fs is too short to allow diffusion-controlled interaction with a substrate. A fraction of the excited radicals ionize spontaneously, presumably producing solvated electrons. This decay produces hot rhodamine 6G and hot rhodamine 6G radicals, which cool with a time constant of around 10 ps. In the absence of a substrate, the ejected electrons recombine with rhodamine 6G and recover the radical on a timescale of nanoseconds. Photocatalytic reactions occur only upon excitation of the rhodamine 6G radical, and due to its short excited-state lifetime, the electron transfer to the substrate probably takes place through the generation of solvated electrons as an additional step in the proposed photochemical mechanism.