Parametric Representation of 3D Grain Ensembles in Polycrystalline Microstructures

As a straightforward generalization of the well-known Voronoi construction, Laguerre tessellations have long found application in the modelling, analysis and simulation of polycrystalline microstructures. The application of Laguerre tessellations to real (as opposed to computed) microstructures—such as those obtained by modern 3D characterization techniques like X-ray microtomography or focused-ion-beam serial sectioning—is hindered by the mathematical difficulty of determining the correct seed location and weighting factor for each of the grains in the measured volume. In this paper, we propose an alternative to the Laguerre approach, representing grain ensembles with convex cells parametrized by orthogonal regression with respect to 3D image data. Applying our algorithm to artificial microstructures and to microtomographic data sets of an Al-5 wt% Cu alloy, we demonstrate that the new approach represents statistical features of the underlying data—like distributions of grain sizes and coordination numbers—as well as or better than a recently introduced approximation method based on the Laguerre tessellation; furthermore, our method reproduces the local arrangement of grains (i.e., grain shapes and connectivities) much more accurately. The additional computational cost associated with orthogonal regression is marginal.     

Solvent-mediated charge redistribution in photodissociation of IBr(-) and IBr(-)(CO2)

A combined experimental and theoretical investigation of photodissociation dynamics of IBr(-) and IBr(-)(CO(2)) on the B ((2)Σ(1/2)(+)) excited electronic state is presented. Time-resolved photoelectron spectroscopy reveals that in bare IBr(-) prompt dissociation forms exclusively I? + Br(-). Compared to earlier dissociation studies of IBr(-) excited to the A' ((2)Π(1∕2)) state, the signal rise is delayed by 200 ± 20 fs. In the case of IBr(-)(CO(2)), the product distribution shows the existence of a second major (～40%) dissociation pathway, Br? + I(-). In contrast to the primary product channel, the signal rise associated with this pathway shows only a 50 ± 20 fs delay. The altered product branching ratio indicates that the presence of one solvent-like CO(2) molecule dramatically affects the electronic structure of the dissociating IBr(-). We explore the origins of this phenomenon with classical trajectories, quantum wave packet studies, and MR-SO-CISD calculations of the six lowest-energy electronic states of IBr(-) and 36 lowest-energy states of IBr. We find that the CO(2) molecule provides sufficient solvation energy to bring the initially excited state close in energy to a lower-lying state. The splitting between these states and the time at which the crossing takes place depend on the location of the solvating CO(2) molecule.     

Photoelectron spectroscopy of HC4N-

We report the 364-nm photoelectron spectrum of HC(4)N(-). We observe electron photodetachment from the bent X(2)A" state of HC(4)N(-) to both the near-linear X(3)A" and the bent ? (1)A' states of neutral HC(4)N. We observe an extended, unresolved vibrational progression corresponding to X(3)A" ← X(2)A" photodetachment, and we measure the electron affinity (EA) of the X(3)A" state of HC(4)N to be 2.05(8) eV. Photodetachment to the bent ? (1)A' state results in a single intense origin peak at a binding energy of 2.809(4) eV, from which we determine the singlet-triplet splitting (ΔE(ST)) of HC(4)N: 0.76(8) eV. For comparison and to aid in the interpretation of the HC(4)N(-) spectrum, we also report the 364-nm photoelectron spectra of HCCN(-) and DCCN(-). Improved signal-to-noise over the previous HCCN(-) and DCCN(-) photoelectron spectra allows for a more precise determination of the EAs and ΔE(ST)s of HCCN and DCCN. The EAs of HCCN and DCCN are measured to be 2.001(15) eV and 1.998(15) eV, respectively; ΔE(ST)(HCCN) is 0.510(15) eV and ΔE(ST)(DCCN) is 0.508(15) eV. These results are discussed in the context of other organic carbene chains.     

Mutation-induced fold switching among lattice proteins

Recent experiments uncovered a mutational pathway between two proteins, along which a single mutation causes a switch in fold. Searching for such paths between real proteins remains, despite this achievement, a true challenge. Here, we analyze fold switching in the minimalistic hydrophobic/polar model on a square lattice. For this analysis, we generate a comprehensive sequence-structure database for chains of length ≤ 30, which exceeds previous work by five units. Single-mutation-induced fold switching turns out to be quite common in the model. The switches define a fold network, whose topology is roughly similar to what one would expect for a set of randomly connected nodes. In the combinatorially challenging search for fold switches between two proteins, a tempting strategy is to only consider paths containing the minimum number of mutations. Such a restricted search fails to correctly identify 40% of the single-mutation-linked fold pairs that we observe. The thermodynamic stability is correlated with mutational stability and is, on average, markedly reduced at the observed fold switches.     

Copper(I) and Silver(I) Bis(trifluoromethanesulfonyl)imide and Their Interaction with an Arene,Diverse Olefins,and an NTf2−‐Based Ionic Liquid

The chemistry of coinage metal bis(triflyl)imides of technological interest, CuNTf₂ and AgNTf₂, their synthesis and complexes with excess of comparatively weakly coordinating NTf₂^? as well as with ether, olefins, and the arene mesitylene are described. The existence of the solvent‐free pure phase [CuNTf₂]_∞ has not been evidenced so far. Contrary to the literature, in which the preparation of [CuNTf₂]_∞ is supposed to be carried out by reacting mesityl copper, [Cu(Mes)]₅, and HNTf₂, we found that in fact this reaction leads reproducibly to the interesting copper diarene sandwich complex [Cu(η³‐MesH)₂][Cu(NTf₂)₂] ( 1 ) (MesH=1,3,5‐trimethylbenzene). The unexpectedly stable molecular etherate [Cu(Et₂O)(NTf₂)] ( 2 ) turned out to be the best precursor for CuNTf₂ having only an inert and easily exchangeable solvent ligand. The coordination mode of NTf₂^? in 1 and 2 as well as in the hitherto unknown crystalline phase of [AgNTf₂]_∞ ( 3 ) is described. The complex formation, which takes place when dissolving 2 or 3 in the room temperature ionic liquid (RTIL) [emim]NTf₂ ([emim]⁺=1‐ethyl‐3‐methylimidazolium), has been studied. Furthermore, the reaction of 1 – 3 towards the diolefins 1,5‐cyclooctadiene (COD), 2,5‐norbornadiene (NBD) and isoprene (2‐methylbuta‐1,3‐diene) and towards ethylene has been investigated. The products 4 – 13 of these conversions have been isolated and fully characterized by NMR‐ and IR spectroscopies, mass spectrometry, and elemental‐ and XRD analyses. The potential of [Cu(η³‐MesH)₂][Cu(NTf₂)₂] ( 1 ), [Cu(Et₂O)(NTf₂)] ( 2 ) and [AgNTf₂]_∞ ( 3 ) as well as of [emim][M(NTf₂)₂] (M=Cu 4 , Ag 5 ) as chemisorbers for ethylene was studied by NMR spectroscopy.     

Synthesis, structure, and thermochemistry of adduct formation between N-heterocyclic carbenes and isocyanates or mesitylnitrile oxide

Reaction of N-heterocyclic carbenes (NHCs) with isocyanates yields stable zwitterionic imidates/amidates in toluene solution. These compounds were fully characterized and the crystal structures of several species were determined by X-ray crystallography. The thermochemistry of binding of these and related species was studied by solution calorimetry. Comparison is made of the enthalpies of binding of NHC to isocyanates (RNCO) and isomeric nitrile oxides (RCNO) as well as CO₂. DFT calculations were performed to additionally assess the nature of bonding in these compounds.     

Initial flight test results of differential absorption barometric radar for remote sensing of sea surface air pressure

The accuracy of numerical weather model predictions of the intensity and track of tropical storms may be significantly improved by large spatial coverage and frequent sampling of sea surface barometry. The availability of a radar operating at moderate-to-strong O₂ absorption bands in the frequency range 50∼56 GHz to remotely measure surface barometric pressure may provide such capability. At these frequencies, the strength of radar echoes from water surfaces has a strong gradient with frequencies owing to the absorption of atmospheric O₂. Our recent research has developed a technique based on the use of a dual-frequency, O₂-band radar to estimate surface barometric pressure from the measured attenuation due to O₂. The ratio of reflected radar signals at multiple wavelengths is used to minimize the effect of microwave absorption by liquid water and water vapor in the atmosphere, and the influences of sea surface reflection over the frequency of operation. A demonstration instrument has been developed to verify the differential O₂ absorption measurement approach. Recent test flights to evaluate the in-flight performance of the demonstration instrument have been completed. The measured radar return and differential O₂ absorption show good agreement with the modeled results. These flight test results are consistent with our instrumentation goal of ±5 mb uncertainty and indicate that our proposed differential absorption measurement approach may provide a useful measurement of sea surface pressure. Future test flights will provide higher altitude data and assess the precision of the sea surface pressure measurement for the existing demonstration radar.     

The short-lived signaling state of the photoactive yellow protein photoreceptor revealed by combined structural probes

The signaling state of the photoactive yellow protein (PYP) photoreceptor is transiently developed via isomerization of its blue-light-absorbing chromophore. The associated structural rearrangements have large amplitude but, due to its transient nature and chemical exchange reactions that complicate NMR detection, its accurate three-dimensional structure in solution has been elusive. Here we report on direct structural observation of the transient signaling state by combining double electron electron resonance spectroscopy (DEER), NMR, and time-resolved pump-probe X-ray solution scattering (TR-SAXS/WAXS). Measurement of distance distributions for doubly spin-labeled photoreceptor constructs using DEER spectroscopy suggests that the signaling state is well ordered and shows that interspin-label distances change reversibly up to 19 ? upon illumination. The SAXS/WAXS difference signal for the signaling state relative to the ground state indicates the transient formation of an ordered and rearranged conformation, which has an increased radius of gyration, an increased maximum dimension, and a reduced excluded volume. Dynamical annealing calculations using the DEER derived long-range distance restraints in combination with short-range distance information from (1)H-(15)N HSQC perturbation spectroscopy give strong indication for a rearrangement that places part of the N-terminal domain in contact with the exposed chromophore binding cleft while the terminal residues extend away from the core. Time-resolved global structural information from pump-probe TR-SAXS/WAXS data supports this conformation and allows subsequent structural refinement that includes the combined energy terms from DEER, NMR, and SAXS/WAXS together. The resulting ensemble simultaneously satisfies all restraints, and the inclusion of TR-SAXS/WAXS effectively reduces the uncertainty arising from the possible spin-label orientations. The observations are essentially compatible with reduced folding of the I(2)' state (also referred to as the 'pB' state) that is widely reported, but indicates it to be relatively ordered and rearranged. Furthermore, there is direct evidence for the repositioning of the N-terminal region in the I(2)' state, which is structurally modeled by dynamical annealing and refinement calculations.     

Light Dynamics of the Retinal‐Disease‐Relevant G90D Bovine Rhodopsin Mutant

The RHO gene encodes the G‐protein‐coupled receptor (GPCR) rhodopsin. Numerous mutations associated with impaired visual cycle have been reported; the G90D mutation leads to a constitutively active mutant form of rhodopsin that causes CSNB disease. We report on the structural investigation of the retinal configuration and conformation in the binding pocket in the dark and light‐activated state by solution and MAS‐NMR spectroscopy. We found two long‐lived dark states for the G90D mutant with the 11‐cis retinal bound as Schiff base in both populations. The second minor population in the dark state is attributed to a slight shift in conformation of the covalently bound 11‐cis retinal caused by the mutation‐induced distortion on the salt bridge formation in the binding pocket. Time‐resolved UV/Vis spectroscopy was used to monitor the functional dynamics of the G90D mutant rhodopsin for all relevant time scales of the photocycle. The G90D mutant retains its conformational heterogeneity during the photocycle.     

A Distonic Radical-Ion for Detection of Traces of Adventitious Molecular Oxygen (O2) in Collision Gases Used in Tandem Mass Spectrometers

We describe a diagnostic ion that enables rapid semiquantitative evaluation of the degree of oxygen contamination in the collision gases used in tandem mass spectrometers. Upon collision-induced dissociation (CID), the m/z 359 positive ion generated from the analgesic etoricoxib undergoes a facile loss of a methyl sulfone radical [^?SO₂(CH₃); 79-Da] to produce a distonic radical cation of m/z 280. The product-ion spectrum of this m/z 280 ion, recorded under low-energy activation on tandem-in-space QqQ or QqTof mass spectrometers using nitrogen from a generator as the collision gas, or tandem-in-time ion-trap (LCQ, LTQ) mass spectrometers using purified helium as the buffer gas, showed two unexpected peaks at m/z 312 and 295. This enigmatic m/z 312 ion, which bears a mass-to-charge ratio higher than that of the precursor ion, represented an addition of molecular oxygen (O₂) to the precursor ion. The exceptional affinity of the m/z 280 radical cation towards oxygen was deployed to develop a method to determine the oxygen content in collision gases. Figure

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