Direct Observation of Insulin Association Dynamics with Time-Resolved X‑ray Scattering

Biological functions frequently require protein–protein interactions that involve secondary and tertiary structural perturbation. Here we study protein–protein dissociation and reassociation dynamics in insulin, a model system for protein oligomerization. Insulin dimer dissociation into monomers was induced by a nanosecond temperature-jump (T-jump) of ∼8 °C in aqueous solution, and the resulting protein and solvent dynamics were tracked by time-resolved X-ray solution scattering (TRXSS) on time scales of 10 ns to 100 ms. The protein scattering signals revealed the formation of five distinguishable transient species during the association process that deviate from simple two-state kinetics. Our results show that the combination of T-jump pump coupled to TRXSS probe allows for direct tracking of structural dynamics in nonphotoactive proteins

Nano-sized Metallic Nickel Clusters Stabilized on Dealuminated beta‑Zeolite: A Highly Active and Stable Ethylene Hydrogenation Catalyst

Supported Ni catalysts were synthesized using the beta-zeolite framework, with and without the framework Al, as a platform for dispersing Ni. The silanol nest sites of dealuminated zeolite beta provide isolated cationic Ni sites that can be reduced under relatively mild conditions to create highly dispersed metal clusters. Compared to the Ni sites present in Ni-[Al]-beta-19, Ni-[DeAl]-beta exhibit a 20-fold increase in the apparent reaction rate for C2H4 hydrogenation and is stable, with little deactivation over 16 h of catalysis. Ni K-edge X-ray absorption spectroscopy (XAS), as well as CO adsorption monitored with Fourier transform infrared spectroscopy, shows that in the oxidized Ni-[DeAl]-beta catalyst Ni reoccupies vacant silanol nests produced from dealumination. After reductive treatment, XAS shows that approximately 50% of Ni is reduced to metallic Ni, forming clusters that are approximately 1 nm in size. Scanning transmission electron microscopy images are consistent with the absence of large (>1 nm) metallic Ni clusters. These results indicate that [DeAl]-beta can be used to synthesize isolated cationic Ni sites as well as stabilize highly dispersed metal clusters that can be used as a highly active and stable C2H4 hydrogenation catalyst

Probing Cytochrome <i>c</i> Folding Transitions upon Phototriggered Environmental Perturbations Using Time-Resolved X‑ray Scattering

Direct tracking of protein structural dynamics during folding–unfolding processes is important for understanding the roles of hierarchic structural factors in the formation of functional proteins. Using cytochrome <i>c</i> (cyt <i>c</i>) as a platform, we investigated its structural dynamics during folding processes triggered by local environmental changes (i.e., pH or heme iron center oxidation/spin/ligation states) with time-resolved X-ray solution scattering measurements. Starting from partially unfolded cyt <i>c</i>, a sudden pH drop initiated by light excitation of a photoacid caused a structural contraction in microseconds, followed by active site restructuring and unfolding in milliseconds. In contrast, the reduction of iron in the heme via photoinduced electron transfer did not affect conformational stability at short timescales (<1 ms), despite active site coordination geometry changes. These results demonstrate how different environmental perturbations can change the nature of interaction between the active site and protein conformation, even within the same metalloprotein, which will subsequently affect the folding structural dynamics

Structure and Site Evolution of Framework Ni Species in MIL-127 MOFs for Propylene Oligomerization Catalysis

A mixed-valence oxotrimer metal–organic framework (MOF), Ni-MIL-127, with a fully coordinated nickel atom and two iron atoms in the inorganic node, generates a missing linker defect upon thermal treatment in helium (>473 K) to engender an open coordination site on nickel which catalyzes propylene oligomerization devoid of any cocatalysts or initiators. This catalyst is stable for ∼20 h on stream at 500 kPa and 473 K, unprecedented for this chemistry. The number of missing linkers on synthesized and activated Ni-MIL-127 MOFs is quantified using temperature-programmed oxidation, 1H nuclear magnetic resonance spectroscopy, and X-ray absorption spectroscopy to be ∼0.7 missing linkers per nickel; thus, a majority of Ni species in the MOF framework catalyze propylene oligomerization. In situ NO titrations under reaction conditions enumerate ∼62% of the nickel atoms as catalytically relevant to validate the defect density upon thermal treatment. Propylene oligomerization rates on Ni-MIL-127 measured at steady state have activation energies of 55–67 kJ mol–1 from 448 to 493 K and are first-order in propylene pressures from 5 to 550 kPa. Density functional theory calculations on cluster models of Ni-MIL-127 are employed to validate the plausibility of the missing linker defect and the Cossee–Arlman mechanism for propylene oligomerization through comparisons between apparent activation energies from steady-state kinetics and computation. This study illustrates how MOF precatalysts engender defective Ni species which exhibit reactivity and stability characteristics that are distinct and can be engineered to improve catalytic activity for olefin oligomerization

Structure and Site Evolution of Framework Ni Species in MIL-127 MOFs for Propylene Oligomerization Catalysis

A mixed-valence oxotrimer metal–organic framework (MOF), Ni-MIL-127, with a fully coordinated nickel atom and two iron atoms in the inorganic node, generates a missing linker defect upon thermal treatment in helium (>473 K) to engender an open coordination site on nickel which catalyzes propylene oligomerization devoid of any cocatalysts or initiators. This catalyst is stable for ∼20 h on stream at 500 kPa and 473 K, unprecedented for this chemistry. The number of missing linkers on synthesized and activated Ni-MIL-127 MOFs is quantified using temperature-programmed oxidation, 1H nuclear magnetic resonance spectroscopy, and X-ray absorption spectroscopy to be ∼0.7 missing linkers per nickel; thus, a majority of Ni species in the MOF framework catalyze propylene oligomerization. In situ NO titrations under reaction conditions enumerate ∼62% of the nickel atoms as catalytically relevant to validate the defect density upon thermal treatment. Propylene oligomerization rates on Ni-MIL-127 measured at steady state have activation energies of 55–67 kJ mol–1 from 448 to 493 K and are first-order in propylene pressures from 5 to 550 kPa. Density functional theory calculations on cluster models of Ni-MIL-127 are employed to validate the plausibility of the missing linker defect and the Cossee–Arlman mechanism for propylene oligomerization through comparisons between apparent activation energies from steady-state kinetics and computation. This study illustrates how MOF precatalysts engender defective Ni species which exhibit reactivity and stability characteristics that are distinct and can be engineered to improve catalytic activity for olefin oligomerization

Effects of Solvent Properties on the Spectroscopy and Dynamics of Alkoxy-Substituted PPV Oligomer Aggregates

Conjugated systems are frequently studied in their nanoaggregate form to probe the effects of solvent and of film formation on their spectral and dynamical properties. This article focuses on the emission spectra and dynamics of nanoaggregates of alkoxy-substituted PPV oligomers with the goal of interpreting the vibronic emission envelopes observed in these systems (<i>J. Phys. Chem. C</i> <b>2009</b>, <i>113</i>, 18851–18862). The aggregates are formed by adding a nonsolvent such as methanol (MeOH) or water to a solution of the oligomers in a good solvent such as methyl tetrahydrofuran (MeTHF) or tetrahydrofuran (THF). The emission spectra of aggregates formed using either of these combinations exhibit a vibronic pattern in which the ratio of the intensity of highest-energy band to that of the lower energy peaks depends strongly on the ratio of good to poor solvent. In aggregates formed from MeTHF:MeOH, this was shown to be due to the presence of both aggregate-like and monomer-like emitters forming a “core” and surrounding “shell”-like structure, respectively, within a single aggregate (<i>J. Phys. Chem. C</i> <b>2011</b>, <i>115</i>, 15607–15616). In support of this model, the monomer-like emission is shown here to be significantly decreased by changing the solvent pair to the more polar THF:water. This suggests that nanoaggregates formed in THF:water contain a much smaller proportion of monomer-like chains than those formed in MeTHF/MeOH, as would be expected from using a more highly polar nonsolvent. Results from bulk steady-state and time-resolved emission measurements as well as fluorescence lifetime imaging microscopy (FLIM) of the aggregates are shown to be consistent with this interpretation

Solution Structures of Highly Active Molecular Ir Water-Oxidation Catalysts from Density Functional Theory Combined with High-Energy X‑ray Scattering and EXAFS Spectroscopy

The solution structures of highly active Ir water-oxidation catalysts are elucidated by combining density functional theory, high-energy X-ray scattering (HEXS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. We find that the catalysts are Ir dimers with mono-μ-O cores and terminal anionic ligands, generated in situ through partial oxidation of a common catalyst precursor. The proposed structures are supported by ¹H and ¹⁷O NMR, EPR, resonance Raman and UV–vis spectra, electrophoresis, etc. Our findings are particularly valuable to understand the mechanism of water oxidation by highly reactive Ir catalysts. Importantly, our DFT-EXAFS-HEXS methodology provides a new in situ technique for characterization of active species in catalytic systems

Cation Incorporation into Copper Oxide Lattice at Highly Oxidizing Potentials

Electrolyte cations can have significant effects on the kinetics and selectivity of electrocatalytic reactions. We show an atypical mechanism through which electrolyte cations can impact electrocatalyst performancedirect incorporation of the cation into the oxide electrocatalyst lattice. We investigate the transformations of copper electrodes in alkaline electrochemistry through operando X-ray absorption spectroscopy in KOH and Ba­(OH)2 electrolytes. In KOH electrolytes, both the near-edge structure and extended fine-structure agree with previous studies; however, the X-ray absorption spectra vary greatly in Ba­(OH)2 electrolytes. Through a combination of electronic structure modeling, near-edge simulation, and postreaction characterization, we propose that Ba2+ cations are directly incorporated into the lattice and form an ordered BaCuO2 phase at potentials more oxidizing than 200 mV vs the normal hydrogen electrode (NHE). BaCuO2 formation is followed by further oxidation to a bulk Cu3+-like BaxCuyOz phase at 900 mV vs NHE. Additionally, during reduction in Ba­(OH)2 electrolyte, we find both Cu–O bonds and Cu–Ba scattering persist at potentials as low as −400 mV vs NHE. To our knowledge, this is the first evidence for direct oxidative incorporation of an electrolyte cation into the bulk lattice to form a mixed oxide electrode. The oxidative incorporation of electrolyte cations to form mixed oxides could open a new route for the in situ formation of active and selective oxidation electrocatalysts