RESUMO
Redox-driven phase transformations in solids determine the performance of lithium-ion batteries, crucial in the technological transition from fossil fuels. Couplings between chemistry and strain define reversibility and fatigue of an electrode. The accurate definition of all phases in the transformation, their energetics, and nanoscale location within a particle produces fundamental understanding of these couplings needed to design materials with ultimate performance. Here we demonstrate that scanning X-ray diffraction microscopy (SXDM) extends our ability to image battery processes in single particles. In LiFePO4 crystals equilibrated after delithiation, SXDM revealed the existence of domains of miscibility between LiFePO4 and Li0.6FePO4. These solid solutions are conventionally thought to be metastable, and were previously undetected by spectromicroscopy. The observation provides experimental verification of predictions that the LiFePO4-FePO4 phase diagram can be altered by coherency strain under certain interfacial orientations. It enriches our understanding of the interaction between diffusion, chemistry, and mechanics in solid state transformations.
RESUMO
The synthesis of glycogen in bacteria and starch in plants is allosterically controlled by the production of ADP-glucose by ADP-glucose pyrophosphorylase. Using computational studies, site-directed mutagenesis, and kinetic characterization, we found a critical region for transmitting the allosteric signal in the Escherichia coli ADP-glucose pyrophosphorylase. Molecular dynamics simulations and structural comparisons with other ADP-glucose pyrophosphorylases provided information to hypothesize that a Pro103-Arg115 loop is part of an activation path. It had strongly correlated movements with regions of the enzyme associated with regulation and ATP binding, and a network analysis showed that the optimal network pathways linking ATP and the activator binding Lys39 mainly involved residues of this loop. This hypothesis was biochemically tested by mutagenesis. We found that several alanine mutants of the Pro103-Arg115 loop had altered activation profiles for fructose-1,6-bisphosphate. Mutants P103A, Q106A, R107A, W113A, Y114A, and R115A had the most altered kinetic profiles, primarily characterized by a lack of response to fructose-1,6-bisphosphate. This loop is a distinct insertional element present only in allosterically regulated sugar nucleotide pyrophosphorylases that could have been acquired to build a triggering mechanism to link proto-allosteric and catalytic sites.