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SpnF, an enzyme involved in the biosynthesis of spinosyn A, catalyzes a transannular Diels–Alder reaction. Quantum mechanical computations and dynamic simulations now show that this cycloaddition is not well described as either a concerted or stepwise process, and dynamical effects influence the identity and timing of bond formation. The transition state for the reaction is ambimodal and leads directly to both the observed Diels–Alder and an unobserved [6+4] cycloadduct. The potential energy surface bifurcates and the cycloadditions occur by dynamically stepwise modes featuring an "entropic intermediate". A rapid Cope rearrangement converts the [6+4] adduct into the observed [4+2] adduct. Control of nonstatistical dynamical effects may serve as another way by which enzymes control reactions. [ABSTRACT FROM AUTHOR]

Noble metals, especially gold, have been widely used in plasmon resonance applications. Although silver has a larger optical cross section and lower cost than gold, it has attracted much less attention because of its easy corrosion, thereby degrading plasmonic signals and limiting its applications. To circumvent this problem, we report the facile synthesis of superstable AgCu@graphene (ACG) nanoparticles (NPs). The growth of several layers of graphene onto the surface of AgCu alloy NPs effectively protects the Ag surface from contamination, even in the presence of hydrogen peroxide, hydrogen sulfide, and nitric acid. The ACG NPs have been utilized to enhance the unique Raman signals from the graphitic shell, making ACG an ideal candidate for cell labeling, rapid Raman imaging, and SERS detection. ACG is further functionalized with alkyne-polyethylene glycol, which has strong Raman vibrations in the Raman-silent region of the cell, leading to more accurate colocalization inside cells. In sum, this work provides a simple approach to fabricate corrosion-resistant, water-soluble, and graphene-protected AgCu NPs having a strong surface plasmon resonance effect suitable for sensing and imaging. [ABSTRACT FROM AUTHOR]

The mechanism of the Wittig reaction of anisaldehyde with a stabilized ylide was studied by a combination of 13C kinetic isotope effects, conventional calculations, and molecular dynamics calculations in a cluster of 53 THF molecules. The isotope effects support a cycloaddition mechanism involving two sequential transition states associated with separate C—C and P—O bond formations. However, the betaine structure in between the two transition states is bypassed as an equilibrated intermediate in most trajectories. The role of the dynamics of solvent equilibration in the nature of mechanistic intermediates is discussed. [ABSTRACT FROM AUTHOR]

Large-scale cubic InN nanocrystals were synthesized by a combined solution- and vapor-phase method under silica confinement. Nearly monodisperse cubic InN nanocrystals with uniform spherical shape were dispersed stably in various organic solvents after removal of the silica shells. The average size of InN nanocrystals is 5.7 ± 0.6 nm. Powder X-ray diffraction results indicate that the InN nanocrystals are of high crystallinity with a cubic phase. X-ray photoelectron spectroscopy and energy-dispersive spectroscopy confirm that the nanocrystals are composed of In and N elements. The InN nanocrystals exhibit infrared photoluminescence at room temperature, with a peak energy of ∼0.62 eV, which is smaller than that of high-quality wurtzite InN (∼0.65-0.7 eV) and is in agreement with theoretical calculations. The small emission peak energy of InN nanocrystals, as compared to other low-cost solution or vapor methods, reveals the superior crystalline quality of our samples, with low or negligible defect density. This work will significantly promote InN-based applications in IR optoelectronic device and biology. [ABSTRACT FROM AUTHOR]