Melilite LaSrGa_3−<i>x</i>Al_<i>x</i>O₇ Series: A Combined Solid-State NMR and Neutron Diffraction Study

Oxides characterized by a layered melilite structure, with general formula ABT¹₍₁₎T²₍₂₎O₇, find applications in many different technological fields due to their relevant magnetic, optical, and electrical properties. These functional properties are, in turn, related to local features such as structural defects and cation substitutions. Therefore, a complete structural characterization of these complex anisotropic compounds is mandatory, and the combined use of long-range (X-ray and neutron diffraction) and short-range (solid state NMR) techniques is a key approach to this aim. In this work, we present the full structural characterization of the series LaSrGa_3–<i>x</i>Al_<i>x</i>O₇ (<i>x</i> = 0, 1, 1.5, 2, and 3), which was obtained for the first time by means of a new sol–gel approach. Analysis of neutron diffraction data revealed that the distribution of La/Sr and Ga/Al on the respective sites is random. ²⁷Al and ⁷¹Ga solid state NMR enabled us to rationalize the local structure of the T sites in terms of nearest and next-nearest neighbors. This study provides a deep structural insight that can be helpful for the understanding of the functional properties and is a powerful strategy for the analysis of complex oxide systems

Proton-Based Structural Analysis of a Heptahelical Transmembrane Protein in Lipid Bilayers

The structures and properties of membrane proteins in lipid bilayers are expected to closely resemble those in native cell-membrane environments, although they have been difficult to elucidate. By performing solid-state NMR measurements at very fast (100 kHz) magic-angle spinning rates and at high (23.5 T) magnetic field, severe sensitivity and resolution challenges are overcome, enabling the atomic-level characterization of membrane proteins in lipid environments. This is demonstrated by extensive ¹H-based resonance assignments of the fully protonated heptahelical membrane protein proteorhodopsin, and the efficient identification of numerous ¹H–¹H dipolar interactions, which provide distance constraints, inter-residue proximities, relative orientations of secondary structural elements, and protein–cofactor interactions in the hydrophobic transmembrane regions. These results establish a general approach for high-resolution structural studies of membrane proteins in lipid environments via solid-state NMR

Local Environments of Dilute Activator Ions in the Solid-State Lighting Phosphor Y_3–<i>x</i>Ce_<i>x</i>Al₅O₁₂

The oxide garnet Y₃Al₅O₁₂ (YAG), when substituted with a few percent of the activator ion Ce³⁺ to replace Y³⁺, is a luminescent material that is nearly ideal for phosphor-converted solid-state white lighting. The local environments of the small number of substituted Ce³⁺ ions are known to critically influence the optical properties of the phosphor. Using a combination of powerful experimental methods, the nature of these local environments is determined and is correlated with the macroscopic luminescent properties of Ce-substituted YAG. The rigidity of the garnet structure is established and is shown to play a key role in the high quantum yield and in the resistance toward thermal quenching of luminescence. Local structural probes reveal compression of the Ce³⁺ local environments by the rigid YAG structure, which gives rise to the unusually large crystal-field splitting, and hence yellow emission. Effective design rules for finding new phosphor materials inferred from the results establish that efficient phosphors require rigid, highly three-dimensionally connected host structures with simple compositions that manifest a low number of phonon modes, and low activator ion concentrations to avoid quenching

Straightforward Access to Stable, 16-Valence-Electron Phosphine-Stabilized Fe⁰ Olefin Complexes and Their Reactivity

The use of the dialkene divinyltetramethyldisiloxane (dvtms) allows easy access to the reactive 16-valence-electron complexes [Fe⁰(L-L)­(dvtms)] (L-L = dppe (1,2-bis­(diphenylphosphino)­ethane; <b>1</b>), dppp (1,2-bis­(diphenylphosphino)­propane; <b>2</b>), pyNMeP­(ⁱPr)₂ (<i>N</i>-(diisopropylphosphino)-<i>N</i>-methylpyridin-2-amine; <b>4</b>), dipe (1,2-bis­(diisopropylphosphino)­ethane; <b>5</b>)) and [Fe⁰(L)₂(dvtms)] (L = PMe₃; <b>3</b>) by a mild reductive route using AlEt₂(OEt) as reducing agent. In contrast, by the same methodology, the 18-valence-electron complexes [Fe⁰(L-L)₂(ethylene)] (L-L = dppm (1,2-bis­(diphenylphosphino)­methane; <b>6</b>), dppa (1,2-bis­(diphenylphosphino)­amine; <b>7</b>), dppe (<b>8</b>)) were obtained, which do not contain dvtms. In addition, a combined DFT and solid-state paramagnetic NMR methodology is introduced for the structure determination of <b>5</b>. A comparative study of the reactivity of <b>1</b>, <b>2</b>, <b>4</b>–<b>6</b>, and <b>8</b> with 3-hexyne highlights emerging mechanistic implications for C–C coupling reactions using these complexes as catalysts

Structure and Mechanism of the Influenza A M2_18–60 Dimer of Dimers

We report a magic angle spinning (MAS) NMR structure of the drug-resistant S31N mutation of M2_18–60 from Influenza A. The protein was dispersed in diphytanoyl-<i>sn</i>-glycero-3-phosphocholine lipid bilayers, and the spectra and an extensive set of constraints indicate that M2_18–60 consists of a dimer of dimers. In particular, ∼280 structural constraints were obtained using dipole recoupling experiments that yielded well-resolved ¹³C–¹⁵N, ¹³C–¹³C, and ¹H–¹⁵N 2D, 3D, and 4D MAS spectra, all of which show cross-peak doubling. Interhelical distances were measured using mixed ¹⁵N/¹³C labeling and with deuterated protein, MAS at ω_r/2π = 60 kHz, ω_0H/2π = 1000 MHz, and ¹H detection of methyl–methyl contacts. The experiments reveal a compact structure consisting of a tetramer composed of four transmembrane helices, in which two opposing helices are displaced and rotated in the direction of the membrane normal relative to a four-fold symmetric arrangement, yielding a two-fold symmetric structure. Side chain conformations of the important gating and pH-sensing residues W41 and H37 are found to differ markedly from four-fold symmetry. The rmsd of the structure is 0.7 Å for backbone heavy atoms and 1.1 Å for all heavy atoms. This two-fold symmetric structure is different from all of the previous structures of M2, many of which were determined in detergent and/or with shorter constructs that are not fully active. The structure has implications for the mechanism of H⁺ transport since the distance between His and Trp residues on different helices is found to be short. The structure also exhibits two-fold symmetry in the vicinity of the binding site of adamantyl inhibitors, and steric constraints may explain the mechanism of the drug-resistant S31N mutation

Identifying the Critical Role of Li Substitution in P2–Na_<i>x</i>[Li_<i>y</i>Ni_<i>z</i>Mn_{1–<i>y</i>–<i>z</i>}]O₂ (0 < <i>x</i>, <i>y</i>, <i>z</i> < 1) Intercalation Cathode Materials for High-Energy Na-Ion Batteries

Li-substituted layered P2–Na_0.80[Li_0.12Ni_0.22Mn_0.66]­O₂ is investigated as an advanced cathode material for Na-ion batteries. Both neutron diffraction and nuclear magnetic resonance (NMR) spectroscopy are used to elucidate the local structure, and they reveal that most of the Li ions are located in transition metal (TM) sites, preferably surrounded by Mn ions. To characterize structural changes occurring upon electrochemical cycling, in situ synchrotron X-ray diffraction is conducted. It is clearly demonstrated that no significant phase transformation is observed up to 4.4 V charge for this material, unlike Li-free P2-type Na cathodes. The presence of monovalent Li ions in the TM layers allows more Na ions to reside in the prismatic sites, stabilizing the overall charge balance of the compound. Consequently, more Na ions remain in the compound upon charge, the P2 structure is retained in the high voltage region, and the phase transformation is delayed. Ex situ NMR is conducted on samples at different states of charge/discharge to track Li-ion site occupation changes. Surprisingly, Li is found to be mobile, some Li ions migrate from the TM layer to the Na layer at high voltage, and yet this process is highly reversible. Novel design principles for Na cathode materials are proposed on the basis of an atomistic level understanding of the underlying electrochemical processes. These principles enable us to devise an optimized, high capacity, and structurally stable compound as a potential cathode material for high-energy Na-ion batteries

InSane 2016-2

Very fast magic-angle spinning (MAS > 80 kHz) NMR combined with high-field magnets has enabled the acquisition of proton-detected spectra in fully protonated solid samples with sufficient resolution and sensitivity. One of the primary challenges in structure determination of protein is observing long-range ¹H–¹H contacts. Here we use band-selective spin-lock pulses to obtain selective ¹H–¹H contacts (e.g., H^N–H^N) on the order of 5–6 Å in fully protonated proteins at 111 kHz MAS. This approach is a major advancement in structural characterization of proteins given that magnetization can be selectively transferred between protons that are 5–6 Å apart despite the presence of other protons at shorter distance. The observed contacts are similar to those previously observed only in perdeuterated proteins with selective protonation. Simulations and experiments show the proposed method has performance that is superior to that of the currently used methods. The method is demonstrated on GB1 and a β-barrel membrane protein, AlkL