Heterologous Expression of a Hyperthermophilic α-Amylase in Xanthan Gum Producing Xanthomonas campestris Cells

A hyperthermophilic α-amylase encoding gene from Pyrococcus woesei was transferred and expressed in Xanthomonas campestris ATCC 13951. The heterologous α-amylase activity was detected in the intracellular fraction of X. campestris and presented similar thermostability and catalytic properties with the native P. woesei enzyme. The recombinant α-amylase was found to be stable at 90 °C for 4 h and within the same period it retained more than 50% of its initial activity at 110 °C. Furthermore, X. campestris transformants produced similar levels of recombinant α-amylase activity regardless of the carbon source present in the growth medium, whereas the native X. campestris α-amylase production was highly dependent on starch availability and it was suppressed in the presence of glucose or other reducing sugars. On the other hand, xanthan gum yield, which appeared to be similar for both wild type and recombinant X. campestris strains, was enhanced at higher starch or glucose concentrations. Evidence presented in this study supports that X. campestris is a promising cell factory for the co-production of recombinant hyperthermophilic α-amylase and xanthan gum.     

Imidazolium-modification enhances photocatalytic CO2 reduction on ZnSe quantum dots

Colloidal photocatalysts can utilize solar light for the conversion of CO₂ to carbon-based fuels, but controlling the product selectivity for CO₂ reduction remains challenging, in particular in aqueous solution. Here, we present an organic surface modification strategy to tune the product selectivity of colloidal ZnSe quantum dots (QDs) towards photocatalytic CO₂ reduction even in the absence of transition metal co-catalysts. Besides H₂, imidazolium-modified ZnSe QDs evolve up to 2.4 mmol_CO g_ZnSe⁻¹ (TON_QD > 370) after 10 h of visible light irradiation (AM 1.5G, λ > 400 nm) in aqueous ascorbate solution with a CO-selectivity of up to 20%. This represents a four-fold increase in CO-formation yield and 13-fold increase in CO-selectivity compared to non-functionalized ZnSe QDs. The binding of the thiolated imidazolium ligand to the QD surface is characterized quantitatively using ¹H-NMR spectroscopy and isothermal titration calorimetry, revealing that a subset of 12 to 17 ligands interacts strongly with the QDs. Transient absorption spectroscopy reveals an influence of the ligand on the intrinsic charge carrier dynamics through passivating Zn surface sites. Density functional theory calculations indicate that the imidazolium capping ligand plays a key role in stabilizing the surface-bound *CO₂⁻ intermediate, increasing the yield and selectivity toward CO production. Overall, this work unveils a powerful tool of using organic capping ligands to modify the chemical environment on colloids, thus enabling control over the product selectivity within photocatalyzed CO₂ reduction.

A photocatalyst system consisting of ZnSe quantum dots modified with a thiolated imidazolium capping ligand for visible light-driven reduction of aqueous CO₂ to CO is reported without the need for a metal complex co-catalyst.     

Probing the structures of neutral boron clusters using infrared/vacuum ultraviolet two color ionization: B11, B16, and B17

The structures of neutral boron clusters, B(11), B(16), and B(17), have been investigated using vibrational spectroscopy and ab initio calculations. Infrared absorption spectra in the wavelength range of 650 to 1550 cm(-1) are obtained for the three neutral boron clusters from the enhancement of their near-threshold ionization efficiency at a fixed UV wavelength of 157 nm (7.87 eV) after resonant absorption of the tunable infrared photons. All three clusters, B(11), B(16), and B(17), are found to possess planar or quasi-planar structures, similar to their corresponding anionic counterparts (B(n) (-)), whose global minima were found previously to be planar, using photoelectron spectroscopy and theoretical calculations. Only minor structural changes are observed between the neutral and the anionic species for these three boron clusters.     

A five-membered Ru5 ring in a hexagonal La14 cage: the La14Cl20Ru5 structure

The title compound features a five-membered Ru(5) ring embedded in a La(14) hexagonal wheel-like cage, an incommensurate combination of the two building units. A formal electron partition of (La(3+))(14)(Cl(-))(20)(Ru(5))(22-) results in a (Ru(5))(22-) ring isoelectronic to (Cd(5))(2-). However, computational studies show that there is significant electron back-donation from the Ru(5) ring to the La(14) wheel. This interaction strongly stabilizes the Ru(5) ring. The resistivity and magnetic susceptibility of the compound have also been investigated.     

How big is a Cp? Cycloheptatrienyl zirconium complexes with bulky cyclopentadienyl and indenyl ligands

A combination of phase-transfer and traditional alkylation strategies has been employed to synthesise sterically encumbered 1,3-di(cyclohexyl) and 1,3-di(tert-butyl) substituted indenes in multi-gram quantities. These indenyl ligands and sterically demanding alkyl cyclopentadienyl ligands have been used to prepare a series of [(η(7)-C(7)H(7))Zr(η(5)-L)] (L = Cp and Ind) complexes by straightforward salt metathesis between [(η(7)-C(7)H(7))ZrCl(tmeda)] and the corresponding sodium indenide or cyclopentadienide. All of these Zr complexes have been characterized by elemental analysis, NMR spectroscopy and single crystal X-ray diffraction. The structural information derived from these studies was employed to evaluate the steric demand of these ligands in a realistic manner.     

Frustrated Lewis pair addition to conjugated diynes: formation of zwitterionic 1,2,3-butatriene derivatives

The frustrated Lewis pair B(C(6)F(5))(3)/P(o-tolyl)(3) (4a) reacts with 4,6-decadiyne to give the trans-1,2-addition product 5. In contrast, the B(C(6)F(5))(3)/P(t)Bu(3) FLP (4b) reacts with this substrate to give the trans-1,4-adduct trans-6. The cumulene trans-6 undergoes trans-/cis-isomerization upon photolysis to give a ca. 1:1 trans-6/cis-6 mixture. The FLP 4b reacts with 2,6-hexadiyne at r.t. to yield a ca. 4:1 mixture of their trans-1,2- and trans-1,4-addition products (7,8). DFT calculations showed that the zwitterionic 1,4-addition products are favored under thermodynamic control. Thermolysis of the kinetic trans-1,2-addition product (7) (80 °C, bromobenzene) does not lead to the thermodynamically favored 1,4-isomer (8), but instead elimination of isobutylene occurs to the formal trans-1,2-adduct (9) of the B(C(6)F(5))(3)/PH(t)Bu(2) pair. Compounds 5, 6, 7, 8, 9 were analyzed by X-ray diffraction.     

Magnetic properties of the seven-coordinated nanoporous framework material Co(bpy)(1.5)(NO(3))(2) (bpy = 4,4'-bipyridine)

The magnetic properties of the porous metal-organic complex Co(bpy)(1.5)(NO(3))(2) (bpy = 4,4'-bipyridine), investigated by SQUID magnetometry, EPR and heat capacity measurements, are reported. The tongue-and-groove structure of this complex is formed by the assembly of T-shaped building blocks, where each Co is bound to three bpy ligands. Co(ii) is hepta-coordinated by three N atoms from the bpy units, and four O atoms from two nitrate groups. Experimental results showed a large crystal field effect induced anisotropy with a zero field splitting of Δ = 198 K between the ground and excited Kramers doublets, a factor of two larger than previously reported values in Co(ii) hepta-coordinated complexes. EPR revealed orthorhombic crystal field anisotropy, with gyromagnetic principal values of g(1)(*) = 6.1, g(2)(*) = 4.2 and g(3)(*) = 2.2, in an S(*) = 1/2 effective spin on the ground state Kramers doublet. Ab initio simulations allowed us to assign the anisotropy easy axis of magnetization to the binary symmetry axis of the molecule, aligned with the Co-N apical direction of the T-block.     

Pentacyanido(L)ferrate(III) complexes: Crystal structures, electrochemical and DFT studies (L = pyrazine, 4,4′-bipyridine, azide)

The crystal structures of two pentacyanido(L) ferrate(III) complexes, [P(C₆H₅)₄]₂[Fe(CN)₅(prz)]·4H₂O 1, [P(C₆H₅)₄]₂[Fe(CN)₅(4,4′-bipy)]·3H₂O 2, have been solved. Within the two complex anions the iron atoms are hexacoordinated by five cyanido ligands, the sixth position being occupied by the nitrogen atom arising from pyrazine and, respectively, 4,4′-bipyridine. The electrochemical properties of compounds 1, 2 and of the azido derivative, (Ph₄As)₂[Na(H₂O)₄][Fe(CN)₅(N₃)] 3, have been investigated by cyclic voltammetry. A relatively complicated redox behavior of these complexes was found, due especially to the electron transfer involving the central metallic ion that changes reversibly its oxidation state (Fe^III/Fe^II redox site) and also to the coligand (4,4′-bipyridine, pyrazine or azide) which intervenes in a distinct electron transfer. The experimental data have been rationalized through DFT calculations.