Tetrahydroxydiboron-Initiated Atom-Transfer Radical Cyclization

In this work, the first diboron reagent initiated atom-transfer radical cyclization was reported, in which the boryl radicals were generated by the homolytic cleavage of a B-B single bond weakened by the coordination of Lewis base. To clarify the role of carbonate and DMF in the cleavage of B-B bond, we calculated the free energy diagram of two pathways by density functional theory (DFT) investigations. The DFT calculation showed that the presence of carbonate facilitates the B-B bond cleavage to form boron radicals which can be further stabilized by DMF. Subsequent atom transfer cyclization initiated by stabilized dihydroxyboron radical are also energetically favored.

Elucidation of a Complete Kinetic Mechanism for a Mammalian Hydroxysteroid Dehydrogenase (HSD) and Identification of All Enzyme Forms on the Reaction Coordinate

Hydroxysteroid dehydrogenases (HSDs) are essential for the biosynthesis and mechanism of action of all steroid hormones. We report the complete kinetic mechanism of a mammalian HSD using rat 3α-HSD of the aldo-keto reductase superfamily (AKR1C9) with the substrate pairs androstane-3,17-dione and NADPH (reduction) and androsterone and NADP+ (oxidation). Steady-state, transient state kinetics, and kinetic isotope effects reconciled the ordered bi-bi mechanism, which contained 9 enzyme forms and permitted the estimation of 16 kinetic constants. In both reactions, loose association of the NADP(H) was followed by two conformational changes, which increased cofactor affinity by >86-fold. For androstane-3,17-dione reduction, the release of NADP+ controlled kcat, whereas the chemical event also contributed to this term. kcat was insensitive to [2H]NADPH, whereas Dkcat/Km and the Dklim (ratio of the maximum rates of single turnover) were 1.06 and 2.06, respectively. Under multiple turnover conditions partial burst kinetics were observed. For androsterone oxidation, the rate of NADPH release dominated kcat, whereas the rates of the chemical event and the release of androstane-3,17-dione were 50-fold greater. Under multiple turnover conditions full burst kinetics were observed. Although the internal equilibrium constant favored oxidation, the overall Keq favored reduction. The kinetic Haldane and free energy diagram confirmed that Keq was governed by ligand binding terms that favored the reduction reactants. Thus, HSDs in the aldo-keto reductase superfamily thermodynamically favor ketosteroid reduction.

Phase evolution in Ni–Nb multilayers upon solid-state reaction

Solid-state amorphization was achieved in the Ni48Nb52 multilayers upon thermal annealing by gradually raising the temperature from 250 to 400 °C and staying at 400 °C for 2 h. More interestingly, before complete amorphization, a sequential disordering of first Ni and then Nb crystalline lattices was observed for the first time, and it was essentially the physical origin of an asymmetric growth of the amorphous interlayer during solid-state reaction reported previously in some binary metal systems. In another two multilayered samples with overall compositions of Ni64Nb36 and Ni70Nb30, thermal annealing under similar conditions resulted in the formation of two metastable crystalline phases with face-centered-cubic and hexagonal-close-packed structures, respectively, although an amorphous phase also appeared and coexisted with one of the metastable crystalline phases in the intermediate states. In the ion mixing experiment, such sequential disordering, as well as formation of metastable phases, was also observed in the respective Ni–Nb multilayers upon room-temperature 200-keV xenon ion irradiation. Comparatively, however, ion irradiation eventually induced complete amorphization in all the multilayers at the respective doses, indicating that ion-induced disordering frequently predominated in the competition between amorphization and the growth of a metastable crystalline phase. A Gibbs free energy diagram, including the free energy curves of the newly formed metastable crystalline phases, of the Ni–Nb system was calculated based on Miedema's model. The constructed free energy diagram can give reasonable explanations of the sequential disordering and the thermodynamic possibility of the formation of either an amorphous or a metastable crystalline phase, of which the free energy difference was quite small. It follows naturally that the phase selection, namely, which phase was more favored to be formed eventually than its competitors, was influenced or even determined by the kinetics involved in the respective processes.

Phase transformation in ball-milled iron-rich Sm–Fe(–C) powders

Two intermetallic phases, R2Fe17 carbide and R2Fe14C, which are promising candidates for permanent magnets, are formed in the iron-rich R–Fe–C ternary alloy system (R = rare earths). Using x-ray diffraction and thermomagnetometry the phase formation, transformation, and thermodynamic relations between the two structures, prepared by high energy ball milling, are studied quantitatively for R = Sm. The results lead to a free energy diagram for the pseudobinary system of Sm2Fe17 and C. A maximum equilibrium carbon content, xc, has been established for the carbide Sm2Fe17Cx and its temperature dependence determined. Beyond the equilibrium concentration, Sm2Fe17Cx transforms into a mixture of Sm2Fe17Cxc, Sm2Fe14C, and α–Fe. Although not thermodynamically stable, Sm2Fe17Cx can still be formed through nonequilibrium processes by being kinetically favored over the stable phase(s). This feature is important for the production of Sm–Fe–C-based permanent magnets.