Browsing Harvested data by Subjects
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Directed evolution of prenylated FMN-dependent Fdc supports efficient in vivo isobutene productionAbstract: Isobutene is a high value gaseous alkene used as fuel additive and a chemical building block. As an alternative to fossil fuel derived isobutene, we here develop a modified mevalonate pathway for the production of isobutene from glucose in vivo. The final step in the pathway consists of the decarboxylation of 3-methylcrotonic acid, catalysed by an evolved ferulic acid decarboxylase (Fdc) enzyme. Fdc belongs to the prFMN-dependent UbiD enzyme family that catalyses reversible decarboxylation of (hetero)aromatic acids or acrylic acids with extended conjugation. Following a screen of an Fdc library for inherent 3-methylcrotonic acid decarboxylase activity, directed evolution yields variants with up to an 80-fold increase in activity. Crystal structures of the evolved variants reveal that changes in the substrate binding pocket are responsible for increased selectivity. Solution and computational studies suggest that isobutene cycloelimination is rate limiting and strictly dependent on presence of the 3-methyl group.
In-situ nanospectroscopic imaging of plasmon-induced two-dimensional [4+4]-cycloaddition polymerization on Au(111)Abstract: Plasmon-induced chemical reactions (PICRs) have recently become promising approaches for highly efficient light-chemical energy conversion. However, an in-depth understanding of their mechanisms at the nanoscale still remains challenging. Here, we present an in-situ investigation by tip-enhanced Raman spectroscopy (TERS) imaging of the plasmon-induced [4+4]-cycloaddition polymerization within anthracene-based monomer monolayers physisorbed on Au(111), and complement the experimental results with density functional theory (DFT) calculations. This two-dimensional (2D) polymerization can be flexibly triggered and manipulated by the hot carriers, and be monitored simultaneously by TERS in real time and space. TERS imaging provides direct evidence for covalent bond formation with ca. 3.7 nm spatial resolution under ambient conditions. Combined with DFT calculations, the TERS results demonstrate that the lateral polymerization on Au(111) occurs by a hot electron tunneling mechanism, and crosslinks form via a self-stimulating growth mechanism. We show that TERS is promising to be plasmon-induced nanolithography for organic 2D materials.
Strain localisation and failure at twin-boundary complexions in nickel-based superalloysAbstract: Twin boundaries (TBs) in Ni-based superalloys are vulnerable sites for failure in demanding environments, and a current lack of mechanistic understanding hampers the reliable lifetime prediction and performance optimisation of these alloys. Here we report the discovery of an unexpected γ″ precipitation mechanism at TBs that takes the responsibility for alloy failure in demanding environments. Using multiscale microstructural and mechanical characterisations (from millimetre down to atomic level) and DFT calculations, we demonstrate that abnormal γ″ precipitation along TBs accounts for the premature dislocation activities and pronounced strain localisation associated with TBs during mechanical loading, which serves as a precursor for crack initiation. We clarify the physical origin of the TBs-related cracking at the atomic level of γ″-strengthened Ni-based superalloys in a hydrogen containing environment, and provide practical methods to mitigate the adverse effect of TBs on the performance of these alloys.