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Chen, Xi

Publications and source records attributed to Chen, Xi.

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Tandem sialoglycan-binding modules in a Streptococcus sanguinis serine-rich repeat adhesin create target dependent avidity effects

Sialic acid–binding immunoglobulin-like lectins (Siglec)–like domains of streptococcal serine-rich repeat (SRR) adhesins recognize sialylated glycans on human salivary, platelet, and plasma glycoproteins via a YTRY sequence motif. The SRR adhesin from Streptococcus sanguinis strain SK1 has tandem sialoglycan-binding domains and has previously been shown to bind sialoglycans with high affinity. However, both domains contain substitutions within the canonical YTRY motif, making it unclear how they interact with host receptors. To identify how the S. sanguinis strain SK1 SRR adhesin affects interactions with sialylated glycans and glycoproteins, we determined high-resolution crystal structures of the binding domains alone and with purified trisaccharides. These structural studies determined that the ligands still bind at the noncanonical binding motif, but with fewer hydrogen-bonding interactions to the protein than is observed in structures of other Siglec-like adhesins. Complementary biochemical studies identified that each of the two binding domains has a different selectivity profile. Interestingly, the binding of SK1 to platelets and plasma glycoproteins identified that the interaction to some host targets is dominated by the contribution of one binding domain, whereas the binding to other host receptors is mediated by both binding domains. These results provide insight into outstanding questions concerning the roles of tandem domains in targeting host receptors and suggest mechanisms for how pathogens can adapt to the availability of a range of related but nonidentical host receptors. They further suggest that the definition of the YTRY motif should be changed to ΦTRX, a more rigorous description of this sialic acid–recognition motif given recent findings.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Creation and sustainment of wide pedestal quiescent H-mode with zero net neutral beam torque

Recent experiments on DIII-D have shown it is possible to create and sustain wide pedestal quiescent H-mode (QH-mode) plasmas with zero net torque from neutral beam injection (NBI) for the full discharge duration. Wide pedestal QH-mode has many of the features of the previously investigated QH-mode while having the advantage of increased edge pedestal pressure and excellent energy confinement time. Both QH-mode variants operate without edge localized modes. Accordingly, these discharges demonstrate that significant input torque is not essential to the exploitation of wide pedestal QH-mode in future devices that are expected to have small or nonexistent NBI torque. In conclusion, developing operating conditions that allowed net zero torque access to wide pedestal QH-mode required implementing several techniques to avoid locked modes including minimizing intrinsic error fields, avoiding large sawteeth, and driving toroidal rotation via neoclassical toroidal viscosity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

The role of 9-O-acetylated glycan receptor moieties in the typhoid toxin binding and intoxication

Typhoid toxin is an A2B5 toxin secreted from Salmonella Typhi-infected cells during human infection and is suggested to contribute to typhoid disease progression and the establishment of chronic infection. To deliver the enzymatic ‘A’ subunits of the toxin to the site of action in host cells, the receptor-binding ‘B’ subunit PltB binds to the trisaccharide glycan receptor moieties terminated in N-acetylneuraminic acid (Neu5Ac) that is α2–3 or α2–6 linked to the underlying disaccharide, galactose (Gal) and N-acetylglucosamine (GlcNAc). Neu5Ac is present in both unmodified and modified forms, with 9-O-acetylated Neu5Ac being the most common modification in humans. Here we show that host cells associated with typhoid toxin-mediated clinical signs express both unmodified and 9-O-acetylated glycan receptor moieties. We found that PltB binds to 9-O-acetylated α2–3 glycan receptor moieties with a markedly increased affinity, while the binding affinity to 9-O-acetylated α2–6 glycans is only slightly higher, as compared to the affinities of PltB to the unmodified counterparts, respectively. We also present X-ray co-crystal structures of PltB bound to related glycan moieties, which supports the different effects of 9-O-acetylated α2–3 and α2–6 glycan receptor moieties on the toxin binding. Lastly, we demonstrate that the cells exclusively expressing unmodified glycan receptor moieties are less susceptible to typhoid toxin than the cells expressing 9-O-acetylated counterparts, although typhoid toxin intoxicates both cells. These results reveal a fine-tuning mechanism of a bacterial toxin that exploits specific chemical modifications of its glycan receptor moieties for virulence and provide useful insights into the development of therapeutics against typhoid fever.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Tuning Zr 12 O 22 Node Defects as Catalytic Sites in the Metal–Organic Framework hcp UiO-66

Defects in metal–organic frameworks (MOFs) play important roles in MOF reactivity and catalysis. Now, we report evidence of the reactivity and the quantitative characterization of the missing linker defects on the Zr 12 O 22 nodes in the MOF hcp UiO-66 (these are paired Zr 6 O 8 nodes bridged by OH groups) and those on the Zr 6 O 8 nodes of the MOF UiO-66. The defect sites catalyze the ring-opening reactions of epoxides with alcohols, and new sites formed by removal of bridging OH groups on the Zr 12 O 22 nodes also participate in the catalysis. The hcp UiO-66 was synthesized from UiO-66 and from molecular precursors, and, under various synthesis conditions, the nodes incorporated acetate ligands, where linkers were missing, and the number of these ligands was controlled by the synthesis conditions. These ligands are inhibitors of the catalytic reactions, and their removal by reaction with, for example, methanol (to form, for example, methyl acetate) preceded catalysis on the defect sites. The former MOF incorporated more defect sites than the latter, correspondingly being a more active catalyst. The defect sites on the Zr 12 O 22 nodes are 2–6 times more active per site than those on the isolated Zr 6 O 8 nodes, with the node-bridging OH groups increasing the catalytic activity of the neighboring node defect sites because new sites are formed by their removal. In conclusion, the results help point the way to the design and control of catalytic sites on metal oxide-like MOF nodes by tuning of the number and reactivity of the defect sites.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Revealing the Intrinsic Electronic Structure of 3D Half-Heusler Thermoelectric Materials by Angle-Resolved Photoemission Spectroscopy

Accurate determination of the intrinsic electronic structure of thermoelectric materials is a prerequisite for utilizing an electronic band engineering strategy to improve their thermoelectric performance. Herein, with high-resolution angle-resolved photoemission spectroscopy (ARPES), the intrinsic electronic structure of the 3D half-Heusler thermoelectric material ZrNiSn is revealed. An unexpectedly large intrinsic bandgap is directly observed by ARPES and is further confirmed by electrical and optical measurements and firstprinciples calculations. Moreover, a large anisotropic conduction band with an anisotropic factor of 6 is identified by ARPES and attributed to be one of the most important reasons leading to the high thermoelectric performance of ZrNiSn. These successful findings rely on the grown high-quality single crystals, which have fewer Ni interstitial defects and negligible in-gap states on the electronic structure. This work demonstrates a realistic paradigm to investigate the electronic structure of 3D solid materials by using ARPES and provides new insights into the intrinsic electronic structure of the half-Heusler system benefiting further optimization of thermoelectric performance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Enterococcus faecalis α1–2‐mannosidase (EfMan‐I): an efficient catalyst for glycoprotein N‐glycan modification

While multiple α 1–2‐mannosidases are necessary for glycoprotein N‐glycan maturation in vertebrates, a single bacterial α1–2‐mannosidase can be sufficient to cleave all α1–2‐linked mannose residues in host glycoprotein N‐glycans. We report here the characterization and crystal structure of a new α1–2‐mannosidase (EfMan‐I) from Enterococcus faecalis , a Gram‐positive opportunistic human pathogen. EfMan‐I catalyzes the cleavage of α1–2‐mannose from not only oligomannoses but also high‐mannose‐type N‐glycans on glycoproteins. Its 2.15 Å resolution crystal structure reveals a two‐domain enzyme fold similar to other CAZy GH92 mannosidases. An unexpected potassium ion was observed bridging two domains near the active site. These findings support EfMan‐I as an effective catalyst for in vitro N‐glycan modification of glycoproteins with high‐mannose‐type N‐glycans.

Li, Yanhong↗
DOE OSTI2019

Evaporation-driven engines

Evaporation-driven engines are disclosed herein. An example engine can include a water source having a high humidity zone proximate the surface of the water source, a supporting structure, and a hygroscopic material disposed on the supporting structure and configured to generate mechanical force in response to a changing relative humidity. The hygroscopic material can be repeatedly exposed to the high humidity zone and removed from the high humidity zone thereby causing the hygroscopic material to generate mechanical force.

Chen, Xi↗
DOE OSTI2019

Water Contributes to Higher Energy Density and Cycling Stability of Prussian Blue Analogue Cathodes for Aqueous Sodium-Ion Batteries

Here in this work, we performed a comprehensive study of Prussian blue and its analogues (PBAs), one of the most promising cathode materials for aqueous sodium-ion batteries for large-scale energy-storage systems, using first-principles calculations. It is confirmed that dry PBAs generally undergo a phase transition from a rhombohedral Na 2 PR(CN) 6 (where P and R are transition metals) to a tetragonal/cubic PR(CN) 6 during Na extraction, in agreement with experimental observations. Using a grand potential phase diagram construction, we show that water and Na co-intercalation result in fundamentally different phase transition behavior and, hence, electrochemical voltage profiles in wet versus dry electrolytes. Lattice water increases the average voltage and reduces the volume change during electrochemical cycling, resulting in both higher energy density and better cycling stability. Finally, we identified four new PBA compositions, Na 2 CoMn(CN) 6 , Na 2 NiMn(CN) 6 , Na 2 CuMn(CN) 6 , and Na 2 ZnMn(CN) 6 , that show great promise as cathodes for aqueous rechargeable Na-ion batteries.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Determination of Gas-Phase Ion Structures of Locally Polar Homopolymers Through High-Resolution Ion Mobility Spectrometry–Mass Spectrometry

The strong synergy arising from coupling two orthogonal analytical techniques such as ion mobility and mass spectrometry can be used to separate complex mixtures and determine structural information of analytes in the gas phase. A tandem study is performed using two systems with different gases and pressures to ascertain gas-phase conformations of homopolymer ions. Aside from spherical and stretched configurations, intermediate configurations formed by a multiply charged globule and a “bead-on-a-string” appendix are confirmed for polyethylene-glycol (PEG), polycaprolactone (PCL), and polydimethylsiloxane (PDMS). These intermediate configurations are shown to be ubiquitous for all charge states and masses present. For each charge state, configurations evolve in two distinctive patterns: an inverse evolution which occurs as an elementary charge attached to the polymer leaves the larger globule and incorporates itself into the appendage, and a forward evolution which reduces the globule without relinquishing a charge while leaving the appendix relatively constant. Forward evolutions are confirmed to form self-similar family shapes that transcend charge states for all polymers. Identical structural changes occur at the same mass over charge regardless of the system, gas or pressure strongly suggesting that conformations are only contingent on number of charges and chain length, and start arranging once the ion is at least partially ejected from the droplet, supporting a charge extrusion mechanism. Configurational changes are smoother for PDMS which is attributed to the larger steric hindrance caused by protruding pendant groups. This study has implications in the study of the configurational space of more complex homopolymers and heteropolymers. .

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Standing Waves Induced by Valley-Mismatched Domains in Ferroelectric SnTe Monolayers

Two-dimensional (2D) quasiparticle standing waves originate from the interference of coherent quantum states and are usually created by the scattering off edges, atomic steps, or adatoms that induce large potential barriers. We report standing waves close to the valence band maximum (E V ), confined by electrically neutral domain walls of newly discovered ferroelectric SnTe monolayers, as revealed by spatially resolved scanning tunneling spectroscopy. Ab initio calculations show that this novel confinement arises from the polarization lifted hole valley degeneracy and a ~90° rotation of the Brillouin zones that render holes’ momentum mismatched across neighboring domains. These results show a potential for polarization-tuned valleytronics in 2D ferroelectrics.

2-dimensional systems↗
DOE OSTI2019

From an atomic layer to the bulk: Low-temperature atomistic structure and ferroelectric and electronic properties of SnTe films

SnTe hosts ferroelectricity that competes with its weak nontrivial band topology: in the high-symmetry rocksalt structure—in which its intrinsic electric dipole is quenched—this material develops metallic surface bands, but in its rhombic ground-state configuration—which hosts a nonzero spontaneous electric dipole—the crystalline symmetry is lowered, and the presence of surface electronic bands is not guaranteed. In this work, the type of ferroelectric coupling and the atomistic and electronic structure of SnTe films ranging from 2 to 40 atomic layers (ALs) are examined on freestanding samples, to which atomic layers were gradually added. Four-AL SnTe films are antiferroelectrically coupled, while thicker freestanding SnTe films are ferroelectrically coupled. The electronic band gap reduces its magnitude in going from 2 to 40 ALs, but it does not close due to the rhombic nature of the structure. Lastly, these results bridge the structure of SnTe films from the monolayer to the bulk.

36 MATERIALS SCIENCE↗