Terminal Hydride Species in [FeFe]‐Hydrogenases Are Vibrationally Coupled to the Active Site Environment

A combination of nuclear resonance vibrational spectroscopy (NRVS), FTIR spectroscopy, and DFT calculations was used to observe and characterize Fe?H/D bending modes in CrHydA1 [FeFe]‐hydrogenase Cys‐to‐Ser variant C169S. Mutagenesis of cysteine to serine at position 169 changes the functional group adjacent to the H‐cluster from a ‐SH to ‐OH, thus altering the proton transfer pathway. The catalytic activity of C169S is significantly reduced compared to that of native CrHydA1, presumably owing to less efficient proton transfer to the H‐cluster. This mutation enabled effective capture of a hydride/deuteride intermediate and facilitated direct detection of the Fe?H/D normal modes. We observed a significant shift to higher frequency in an Fe?H bending mode of the C169S variant, as compared to previous findings with reconstituted native and oxadithiolate (ODT)‐substituted CrHydA1. On the basis of DFT calculations, we propose that this shift is caused by the stronger interaction of the ‐OH group of C169S with the bridgehead ‐NH‐ moiety of the active site, as compared to that of the ‐SH group of C169 in the native enzyme.     

Analysis of Cu segregation to oxide–metal interface of Ni‐based alloy in a metal‐dusting environment

Hard X‐ray photoelectron spectroscopy (HX‐PES) has been realized using high‐brilliance synchrotron radiation. High‐energy photon excitation enables us to probe photoelectrons with larger escape depth compared to conventional PES. This allows us to conduct, without destruction, a study of the embedded interface of materials as the oxide‐ metal interface. We apply HX‐PES to investigate for Cu segregation in the oxide–metal interface during metal‐dusting corrosion. The effective concentration of Cu in the segregation was estimated a few times higher than the bulk concentration. These results on the interface layer can explain the variation in the corrosion resistance. Copyright © 2008 John Wiley & Sons, Ltd.     

Fine Pore-Structure Engineering by Ligand Conformational Control of Naphthalene Diimide-Based Semiconducting Porous Coordination Polymers for Efficient Chemiresistive Gas Sensing

Exploring new porous coordination polymers (PCPs) that have tunable structure and conductivity is attractive but remains challenging. Herein, fine pore structure engineering by ligand conformation control of naphthalene diimide (NDI)-based semiconducting PCPs with π stacking-dependent conductivity tunability is achieved. The π stacking distances and ligand conformation in these isoreticular PCPs were modulated by employing metal centers with different coordination geometries. As a result, three conjugated PCPs (Co−pyNDI, Ni−pyNDI, and Zn−pyNDI) with varying pore structure and conductivity were obtained. Their crystal structures were determined by three-dimensional electron diffraction. The through-space charge transfer and tunable pore structure in these PCPs result in modulated selectivity and sensitivity in gas sensing. Zn−pyNDI can serve as a room-temperature operable chemiresistive sensor selective to acetone.     

Synthesizing Interpenetrated Triazine-based Covalent Organic Frameworks from CO2

Crystalline triazine-based covalent organic frameworks (COFs) are aromatic nitrogen-rich porous materials. COFs typically show high thermal/chemical stability, and are promising for energy applications, but often require harsh synthesis conditions and suffer from low crystallinity. In this work, we propose an environmentally friendly route for the synthesis of crystalline COFs from CO₂ molecules as a precursor. The mass ratio of CO₂ conversion into COFs formula unit reaches 46.3 %. The synthesis consists of two steps; preparation of 1,4-piperazinedicarboxaldehyde from CO₂ and piperazine, and condensation of the dicarboxaldehyde and melamine to construct the framework. The CO₂-derived COF has a 3-fold interpenetrated structure of 2D layers determined by powder X-ray diffraction, high-resolution transmission electron microscopy, and select-area electron diffraction. The structure shows a high Brunauer–Emmett–Teller surface area of 945 m² g⁻¹ and high stability against strong acid (6 M HCl), base (6 M NaOH), and boiling water over 24 hours. Post-modification of the framework with oxone has been demonstrated to modulate hydrophilicity, and it exhibits proton conductivity of 2.5×10⁻² S cm⁻¹ at 85 °C, 95 % of relative humidity.     

Population study of sizes and components of self-reproducing giant multilamellar vesicles

Population analysis of a system of self-reproducing giant multilamellar vesicles (GMVs) was carried out by means of flow cytometry. The multidimensional distribution of forward light scattering (FS), side light scattering (SS), and fluorescence (FL) intensities originating from each GMV provided information about changes in a population composed of 104 vesicles. FS-FL dot plots indicated that, after the addition of the membrane precursor, the size distribution of the newly generated vesicles was nearly the same as that of the original, but the catalyst content was reduced. This result can be interpreted as evidence for the occurrence of the self-reproduction of GMVs. Moreover, the new GMVs recovered the amount of catalyst to the initial value, keeping their size distribution constant, when a solution of the catalyst was added to the new GMVs. These results are the first experimental evidence for a novel phenomenon on GMV size distribution during their self-reproducing cycle.     

New layered rare-earth hydroxides with anion-exchange properties

We report the synthesis of a new series of layered hydroxides based on rare-earth elements with a composition of RE(OH)2.5Cl(0.5).0.8 H2O (RE: Eu, Tb, etc.) through the homogeneous precipitation of RECl3.x H2O with hexamethylenetetramine (HMT). Rietveld analysis combined with direct methods revealed an orthorhombic layered structure comprising a positively charged layer of [RE(OH)2.5-(H2O)0.8]0.5+ and interlayer Cl- ions. The Cl- ions were readily exchangeable for various anions (NO3-, SO4(2-), dodecylsulfonate, etc.) at ambient temperature. Photoluminescence studies showed that the compounds display typical RE3+ emission. With rare-earth-based host layers and tunable interlayer guests, the new compounds may be of interest for optoelectronic, magnetic, catalytic, and biomedical materials.     

Structural disorder and diffusional pathway of oxide ions in a doped Pr2NiO4-based mixed conductor

MEM nuclear density analysis from neutron diffraction data measured in situ at 1015.6 degrees C has indicated the two-dimensional network of curved O2-O3-O2 oxide-ion diffusion paths on the (Pr,La)-O layer in a K2NiF4-type structured oxide-ionic and electronic mixed conductor (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+delta.     

Progress in 13C and 1H solid-state nuclear magnetic resonance for paramagnetic systems under very fast magic angle spinning

High-resolution solid-state NMR (SSNMR) of paramagnetic systems has been largely unexplored because of various technical difficulties due to large hyperfine shifts, which have limited the success of previous studies through depressed sensitivity/resolution and lack of suitable assignment methods. Our group recently introduced an approach using "very fast" magic angle spinning (VFMAS) for SSNMR of paramagnetic systems, which opened an avenue toward routine analyses of small paramagnetic systems by (13)C and (1)H SSNMR [Y. Ishii et al., J. Am. Chem. Soc. 125, 3438 (2003); N. P. Wickramasinghe et al., ibid. 127, 5796 (2005)]. In this review, we discuss our recent progress in establishing this approach, which offers solutions to a series of problems associated with large hyperfine shifts. First, we demonstrate that MAS at a spinning speed of 20 kHz or higher greatly improves sensitivity and resolution in both (1)H and (13)C SSNMR for paramagnetic systems such as Cu(II)(DL-alanine)(2)H(2)O (Cu(DL-Ala)(2)) and Mn(acac)(3), for which the spectral dispersions due to (1)H hyperfine shifts reach 200 and 700 ppm, respectively. Then, we introduce polarization transfer methods from (1)H spins to (13)C spins with high-power cross polarization and dipolar insensitive nuclei enhanced by polarization transfer (INEPT) in order to attain further sensitivity enhancement and to correlate (1)H and (13)C spins in two-dimensional (2D) SSNMR for the paramagnetic systems. Comparison of (13)C VFMAS SSNMR spectra with (13)C solution NMR spectra revealed superior sensitivity in SSNMR for Cu(DL-Ala)(2), Cu(Gly)(2), and V(acac)(3). We discuss signal assignment methods using one-dimensional (1D) (13)C SSNMR (13)C-(1)H rotational echo double resonance (REDOR) and dipolar INEPT methods and 2D (13)C(1)H correlation SSNMR under VFMAS, which yield reliable assignments of (1)H and (13)C resonances for Cu(Ala-Thr). Based on the excellent sensitivity/resolution and signal assignments attained in the VFMAS approach, we discuss methods of elucidating multiple distance constraints in unlabeled paramagnetic systems by combing simple measurements of (13)C T(1) values and anisotropic hyperfine shifts. Comparison of experimental (13)C hyperfine shifts and ab initio calculated shifts for alpha- and beta-forms of Cu(8-quinolinol)(2) demonstrates that (13)C hyperfine shifts are parameters exceptionally sensitive to small structural difference between the two polymorphs. Finally, we discuss sensitivity enhancement with paramagnetic ion doping in (13)C SSNMR of nonparamagnetic proteins in microcrystals. Fast recycling with exceptionally short recycle delays matched to short (1)H T(1) of approximately 60 ms in the presence of Cu(II) doping accelerated 1D (13)C SSNMR for ubiquitin and lysozyme by a factor of 7.3-8.4 under fast MAS at a spinning speed of 40 kHz. It is likely that the VFMAS approach and use of paramagnetic interactions are applicable to a variety of paramagnetic systems and nonparamagnetic biomolecules.     

Glass Formation of a Coordination Polymer Crystal for Enhanced Proton Conductivity and Material Flexibility

The glassy state of a two‐dimensional (2D) Cd²⁺ coordination polymer crystal was prepared by a solvent‐free mechanical milling process. The glassy state retains the 2D structure of the crystalline material, albeit with significant distortion, as characterized by synchrotron X‐ray analyses and solid‐state multinuclear NMR spectroscopy. It transforms to its original crystal structure upon heating. Thus, reversible crystal‐to‐glass transformation is possible using our new processes. The glass state displays superior properties compared to the crystalline state; specifically, it shows anhydrous proton conductivity and a dielectric constant two orders of magnitude greater than the crystalline material. It also shows material flexibility and transparency.