Activity of membrane proteins immobilized on surfaces as a function of packing density

A systematic study of the influence of the packing density of proteins on their activity is performed with cytochrome c oxidase (CcO) from R. sphaeroides as an example. The protein was incorporated into a protein-tethered bilayer lipid membrane and CcO was genetically engineered with a histidine-tag, attached to Subunit II, and then tethered by an interaction with functionalized thiol compounds bound to a gold electrode. The packing density was varied by diluting the functionalized thiol with a nonfunctionalized thiol that does not bind to the enzyme. After attaching the CcO to the gold surface, a lipid bilayer was formed to incorporate the tethered proteins. The reconstituted protein-lipid bilayer was characterized by surface enhanced infrared reflection absorption spectroscopy (SEIRAS), electrochemical impedance spectroscopy, surface plasmon resonance, and atomic force microscopy. The activity of the proteins within the reconstituted bilayer was probed by direct electrochemical electron injection and was shown to be very sensitive to the packing density of protein molecules. At low surface density of CcO, the bilayer did not effectively form, and protein aggregates were observed, whereas at very high surface density, very little lipid is able to intrude between the closely packed proteins. In both of these cases, redox activity, measured by the efficiency to accept electrons, is low. Redox activity of the enzyme is preserved in the biomimetic structure but only at a moderate surface coverage in which a continuous lipid bilayer is present and the proteins are not forced to aggregate. Electrostatic and other interaction forces between protein molecules are held responsible for these effects.     

Rational design of iron catalysts for C–X bond activation

We have quantum chemically studied the iron-mediated C X bond activation (X = H, Cl, CH₃) by d⁸-FeL₄ complexes using relativistic density functional theory at ZORA-OPBE/TZ2P. We find that by either modulating the electronic effects of a generic iron-catalyst by a set of ligands, that is, CO, BF, PH₃, BN(CH₃)₂, or by manipulating structural effects through the introduction of bidentate ligands, that is, PH₂(CH₂)_nPH₂ with n = 6–1, one can significantly decrease the reaction barrier for the C X bond activation. The combination of both tuning handles causes a decrease of the C H activation barrier from 10.4 to 4.6 kcal mol⁻¹. Our activation strain and Kohn-Sham molecular orbital analyses reveal that the electronic tuning works via optimizing the catalyst–substrate interaction by introducing a strong second backdonation interaction (i.e., “ligand-assisted” interaction), while the mechanism for structural tuning is mainly caused by the reduction of the required activation strain because of the pre-distortion of the catalyst. In all, we present design principles for iron-based catalysts that mimic the favorable behavior of their well-known palladium analogs in the bond-activation step of cross-coupling reactions.     

Entropy‐Driven Selectivity for Chain Scission: Where Macromolecules Cleave

We show that, all other conditions being equal, bond cleavage in the middle of molecules is entropically much more favored than bond cleavage at the end. Multiple experimental and theoretical approaches have been used to study the selectivity for bond cleavage or dissociation in the middle versus the end of both covalent and supramolecular adducts and the extensive implications for other fields of chemistry including, e.g., chain transfer, polymer degradation, and control agent addition are discussed. The observed effects, which are a consequence of the underlying entropic factors, were predicted on the basis of simple theoretical models and demonstrated via high‐temperature (HT) NMR spectroscopy of self‐assembled supramolecular diblock systems as well as temperature‐dependent size‐exclusion chromatography (TD SEC) of covalently bonded Diels–Alder step‐growth polymers.     

Kinetically Controlled Sequential Growth of Surface‐Grafted Chiral Supramolecular Copolymers

We report a facile strategy to grow supramolecular copolymers on Au surfaces by successively exposing a surface‐anchored monomer to solutions of oppositely charged peptide comonomers. Charge regulation on the active chain end of the polymer sufficiently slows down the kinetics of the self‐assembly process to produce kinetically trapped copolymers at near‐neutral pH. We thereby achieve architectural control at three levels: The β‐sheet sequences direct the polymerization away from the surface, the height of the supramolecular copolymer brushes is well‐controlled by the stepwise nature of the alternating copolymer growth, and 2D spatial resolution is realized by using micropatterned initiating monomers. The programmable nature of the resulting architectures renders this concept attractive for the development of customized biomaterials or chiral interfaces for optoelectronics and sensor applications.     

Synthese von Cadmiumnitrid Cd3N2 durch thermischen Abbau von Cadmiumazid Cd(N3)2 und Kristallstrukturbestimmung aus Röntgen‐Pulverbeugungsdaten

Synthesis of Cadmium Nitride Cd₃N₂ by Thermolysis of Cadmium Azide Cd(N₃)₂ and Crystal Structure Determination from X‐ray Powder Diffraction Data Cadmium nitride Cd₃N₂ was obtained by thermolysis of cadmium azide Cd(N₃)₂ at 2·10^?6 mbar and 210 °C. It was obtained as a black, crystalline powder which becomes brown in air due to formation of cadmium amide Cd(NH₂)₂. The crystal structure of Cd₃N₂ was determined from X‐ray powder diffraction data and refined by the Rietveld method ( , a = 10.829(9) Å, V = 1270(3) ų, Z = 16, R(F²) = 0.1196). Cd₃N₂ crystallizes in the anti‐bixbyite structure type and is isotypic to Ca₃N₂.     

IrIn7GeO8 = [IrIn6](GeO4)(InO4) und Verbindungen der Mischkristallreihe [IrIn6](Ge1+xIn1−4x/3O8) (0 ≤ x ≤ 0.75) – erste Oxide mit [IrIn6]‐Oktaedern

IrIn₇GeO₈ = [IrIn₆](GeO₄)(InO₄) and Compounds of the Solid Solution Series [IrIn₆](Ge_1+xIn_1?4x/3O₈) (0 ≤ x ≤ 0.75): First Oxides containing [IrIn₆] Octahedra The low valent indiumoxides IrIn₇GeO₈ = [IrIn₆](GeO₄)(InO₄) and [IrIn₆](Ge_1+xIn_1?4x/3O₈) (0 x ≤ 0.75) are formed by heating intimate mixtures of Ir, In, In₂O₃ and GeO₂ in corundum crucibles under an atmosphere of argon (1420 K, 70 h). The compounds are black and semiconducting. X‐ray powder diffraction patterns can be indexed on the basis of a face centered cubic unit cell with lattice parameters ranging from a = 1012.3(1) pm (x = 0) to a = 1007.3(1) pm (x = 0.75). Characteristic building units in [IrIn₆](Ge_1+xIn_1?4x/3O₈) are isolated [IrIn₆]⁹⁺ octahedra with short Ir‐In distances of 253.5 pm, which are linked via [GeO₄]^4? and [InO₄]^5? tetrahedra to a three dimensional framework. Starting from IrIn₇GeO₈ = [IrIn₆](GeO₄)(InO₄), the isoelectronic substitution of 4 In³⁺ ions by 3 Ge⁴⁺ ions and one Ge‐vacancy leads to the formation of a solid solution series [IrIn₆](GeO₄)_1+x(O₄)_x/3(InO₄)_1?4x/3, which shows a slight decrease in the cubic lattice parameter with increasing x. According to Rietveld refinements the structure of [IrIn₆](GeO₄)(InO₄) exhibits a statistical distribution of the tetrahedrally coordinated Ge and In atoms ( , R(prof.) = 4.4 %, R(int.) = 2.5 %). The crystal and electronic structures of [IrIn₆](GeO₄)(InO₄) are discussed on the basis of first principles electronic structure calculations.     

Protein elasticity determined by pressure tuning of the tyrosine residue of ubiquitin

We determined the isotropic, isothermal compressibility of ubiquitin by pressure tuning spectral holes burnt into the red edge of the absorption spectrum of the single tyrosine residue. The pressure shift is perfectly linear with burn frequency. From these data, a compressibility of 0.086 GPa(-1) in the local environment of the tyrosine residue could be determined. This value fits nicely into the range known for proteins. Although the elastic behavior at low temperatures does not show any unusual features, the pressure tuning behavior at room temperature is quite surprising: the pressure-induced spectral shift is close to zero, even up to very high pressure levels of 0.88 GPa, well beyond the denaturation point. The reason for this behavior is attributed to equally strong blue as well as red spectral pressure shifts resulting in an average pressure-induced solvent shift that is close to zero.