In-situ nanoscale imaging of charge transfer of BiNiO3 under high pressure

Over last decades, both synchrotron radiation techniques and high pressure research have made great progress. Advanced synchrotron capabilities with high spatial resolution, high flux, and high energy resolution provides us many new avenues to conduct advanced high pressure researches. In this talk, we will focus on the new developments of the nanoscale imaging techniques on the pressure induced phase separation in three dimensions. BiNiO3 under goes a charge transfer induced phase transition under high pressure or temperature, which shows excellent colossal negative thermal expansion effect [1]. Co-exist of both high density and low density phases over a wide range pressure or temperature plays the key roles on the negative thermal expansion behavior. We utilized a newly developed X-ray absorption near edge spectroscopy tomography method, and successfully resolved the mixture of high/low pressure phases as a function of pressure at tens of nanometer resolution. By choosing incident x-ray energy near Ni absorption edge, the pressure induced valence transition can be mapped at tens of nanometer scale in 3d, which provides crucial information on the HP-LP phase boundary [2]. As temperature driven grain growth upon heating, we can draw fundamental information on the pressure-induced phase growth mechanism.

Helicobacter pylori Csd4 is a peptidoglycan metallocarboxypeptidase

The bacterial cell wall is a polymeric structure that determines the overall shape of the cell and undergoes constant remodelling during cell growth, requiring enzymes that cleave the existing peptidoglycan structure. Csd4 is an enzyme important for cell shape as deleting it in Helicobacter pylori causes the helical-shaped cells to become rod-like. Csd4 is a zinc carboxypeptidase that can cleave the tripeptide moiety found in peptidoglycan (i.e. L-Ala-γ-D-Glu-m-DAP) to release meso-diaminopimelic acid (mDAP). Structures of Csd4 were solved by X-ray crystallography up to 1.75 Å resolution in space group P212121 with zinc and substrate/product bound and contain the same unit cell dimensions. Csd4 is a monomeric enzyme with three domains: an N-terminal M14-family carboxypeptidase domain followed by two smaller domains likely important in protein-protein or protein-peptidoglycan interactions. Key interactions are observed between the protein and substrate in the active site, supporting specific substrate recognition by Csd4. A water or hydroxide molecule, which is required for catalytic activity, is also observed bound to the zinc and is poised to interact with the substrate molecule upon activation.

Perovskites in low dimensional multi-layer structure types

This study introduces examples of structure property relationships within the multi-layered Sillen-Aurivillius family (shown in Figure) and aims to investigate the effect of chemical doping and lattice matching effects. The first example involves doping 1/3 of the n = 3 ferroelectric perovskite layers with magnetic transition metal cations in Bi₅PbTi₃O₁₄Cl [1] with charge balancing by removing Pb²⁺ for Bi³⁺. A statistical 1:2 distribution of M³⁺ and Ti⁴⁺ across all three perovskite layers was found in Bi₆Ti₂MO₁₄Cl, M = Cr³⁺, Mn³⁺, Fe³⁺, resulting in highly strained structures (enhancing the ferroelectricity compared to Bi₅PbTi₃O₁₄Cl) and pronounced spin-glass behavior below T_irr(0) = 4.46 K. Ferroelectric transitions were observed at high temperature for each of the new compounds. Ferroelectric properties were also measured on Bi₆Ti₂FeO₁₄Cl using piezoresponse force microscopy showing hysteretic phase behavior. A new n = 2 Sillen-Aurivillius compound Bi₃Sr₂Nb₂O₁₁Br, based on Bi₃Pb₂Nb₂O₁₁Cl [2], was synthesized by simultaneously replacing Pb²⁺ with Sr²⁺ and Cl^- with Br^-. Inter-layer mismatch prevented the formation of Bi₃Sr₂Nb₂O₁₁Cl and Bi₃Pb₂Nb₂O₁₁Br. Sr²⁺ doping reduces the impact of the stereochemically active 6s² lone pair found on Pb²⁺ and Bi³⁺, resulting in a stacking contraction in the lattice parameters by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br^-. X-ray Absorption Near Edge Structure spectra analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi₃Sr₂Nb₂O₁₁Br compared to Bi₃Pb₂Nb₂O₁₁Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.

Histone mimicry and hijacking of host proteins by influenza virus protein NS1

Pathogens can interfere with vital biological processes of their host by mimicking host proteins. The NS1 protein of the influenza A H3N2 subtype possesses a histone H3K4-like sequence at its carboxyl terminus and has been reported to use this mimic to hijack host proteins. However, this mimic lacks a free N-terminus that is essential for binding to many known H3K4 readers. Here we show that the double chromodomains of CHD1adopt an 'open pocket' to interact with the free N-terminal amine of H3K4, and the open pocket permits the NS1 mimic to bind in a distinct conformation. We also explored the possibility that NS1 hijacks other cellular proteins and found that the NS1 mimic has access to only a subset of chromatin-associated factors, such as WDR5. Moreover, methylation of the NS1 mimic can not be reversed by the H3K4 demethylase LSD1. Overall, we thus conclude that the NS1 mimic is an imperfect histone mimic.