Genetic Encoding of 7-Aza-l-tryptophan: Isoelectronic Substitution of a Single CH-Group in a Protein for a Nitrogen Atom for Site-Selective Isotope Labeling

Genetic encoding of a noncanonical amino acid (ncAA) in an in vivo expression system requires an aminoacyl-tRNA synthetase that specifically recognizes the ncAA, while the ncAA must not be recognized by the canonical protein expression machinery. We succeeded in genetically encoding 7-aza-tryptophan (7AW), which is isoelectronic with tryptophan. The system is fully orthogonal to protein expression in Escherichia coli, enabling high-yielding site-selective isotope labeling in vivo. 7AW is readily synthesized from serine and 7-aza-indole using a tryptophan synthetase β-subunit (TrpB) mutant, affording easy access to isotope-labeled 7AW. Using labeled 7AW produced from 15N/13C-labeled serine, we produced 7AW mutants of the 25 kDa Zika virus NS2B-NS3 protease. 15N-HSQC spectra display single cross-peaks at chemical shifts near those observed for the wild-type protein labeled with 15N/13C-tryptophan, confirming the structural integrity of the protein and yielding straightforward NMR resonance assignments for site-specific probing.

3-D molecular stars with covalent axial bonding

In designing three-dimensional (3-D) molecular stars, it is very difficult to enhance the molecular rigidity through forming the covalent bonds between the axial and equatorial groups because corresponding axial groups will generally break the delocalized π bond over equatorial frameworks and thus break their star-like arrangement. In this work, exemplified by designing the 3-D stars Be₂ ©Be₅ E₅ ⁺ (E = Au, Cl, Br, I) with three delocalized σ bonds and delocalized π bond over the central Be₂ ©Be₅ moiety, we propose that the desired covalent bonding can be achieved by forming the delocalized σ bond(s) and delocalized π bond(s) simultaneously between the axial groups and equatorial framework. The covalency and rigidity of axial bonding can be demonstrated by the total Wiberg bond indices of 1.46-1.65 for axial Be atoms and ultrashort Be-Be distances of 1.834-1.841 Å, respectively. Beneficial also from the σ and π double aromaticity, these mono-cationic 3-D molecular stars are dynamically viable global energy minima with well-defined electronic structures, as reflected by wide HOMO-LUMO gaps (4.68-5.06 eV) and low electron affinities (4.70-4.82 eV), so they are the promising targets in the gas phase generation, mass-separation, and spectroscopic characterization.

Screening and Design of Novel 2D Ferromagnetic Materials with High Curie Temperature above Room Temperature

Two-dimensional (2D) intrinsic ferromagnets with high Curie temperature ( T_C) are desirable for spintronic applications. Using systematic first-principles calculations, we investigate the electronic and magnetic properties of 22 monolayer 2D materials with layered bulk phases. From these candidates, we screen out five ferromagnetic monolayer materials belonging to three types of structures: type i (ScCl, YCl, LaCl), type ii (LaBr₂), and type iii (CrSBr). Type i is a kind of metallic ferromagnetic material, whereas LaBr₂ and CrSBr of type ii and iii are small-bandgap ferromagnetic semiconductors with T_C near room temperature. Moreover, the ferromagnetic CrSBr monolayer possesses a large magnetic moment of ∼3 μ_B per Cr atom, originating from its distorted octahedron coordination. The robust ferromagnetism of the CrSBr monolayer is ascribed to the halogen-mediated (Cr-Br-Cr) and chalcogen-mediated (Cr-S-Cr) superexchange interactions; then, an isoelectronic substitution strategy is proposed to tailor the magnetic coupling strength. Hence, monolayer structures of CrSI, CrSCl, and CrSeBr with notably enhanced Curie temperature up to 500 K as well as favorable formation energy are designed. The moderate interlayer binding energy and high T_C make these monolayer ferromagnetic materials feasible for experimental synthesis and attractive as 2D spintronic devices.