▲ 作者：THUY AN BUI, DAVID LAMPRECHT, JACOB MADSEN, MARCIN KURPAS, PETER KOTRUSZ, ALEXANDER MARKEVICH, CLEMENS MANGLER, JANI KOTAKOSKI, LADO FILIPOVIC , AND KIMMO MUSTONEN
According to the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory,学网 the transition from a solid to liquid in two dimensions proceeds through an orientationally ordered liquid-like hexatic phase. However, alternative mixed melting scenarios, in which melting proceeds through the hexatic phase with both continuous and discontinuous transitions, have also been observed in some two-dimensional systems. In this study, we imaged silver iodide embedded in multilayer graphene using time- and temperature-resolved in situ atomic-resolution scanning transmission electron microscopy and nanobeam electron diffraction. We observed the hexatic phase and provide evidence supporting a mixed melting scenario.
Constraints on lepton number violation with the 2 tonne · year CUORE dataset
We live in a world with too much matter (as opposed to antimatter). One of the theories that can account for this imbalance posits that the lepton number is not conserved. A process that illustrates lepton number violation is the so-called neutrinoless double beta decay, in which two neutrons inside a nucleus turn into two protons and release two electrons but no neutrinos. The Cryogenic Underground Observatory for Rare Events (CUORE) collaboration searched for this decay in an isotope of tellurium using a system of 988 cryogenic calorimeters housed in an underground facility. Several years’ worth of data yielded no statistically significant evidence of the decay, but Bucci et al. succeeded in improving the constraints on the half-life for this process.
化学Chemistry
Biocatalytic, asymmetric radical hydrogenation of unactivated alkenes
非活化烯烃的生物催化不对称自由基氢化
▲ 作者：JAICY VALLAPURACKAL, RAJIB MANDAL, JUSTIN BOSSENBROEK, ARIS V. RUBIO, ETHAN POLADIAN, JAMES D. COLLINGS, CESAR TORRES, MATTHEW HENDRICKSON, JULIAN MORALES, AND SOUMITRA V. ATHAVALE
Alkene hydrogenation is a cornerstone of chemical synthesis, yet enzymatic strategies remain limited to electron-deficient substrates by means of hydride transfer. Using heme enzymes, we unlock a hydrogenation pathway for the asymmetric reduction of unactivated olefins. A silane-promoted heme-cysteine redox cycle in the active site catalyzes sequential hydrogen atom transfer to challenging scaffolds, including 1,1-disubstituted as well as tri- and tetrasubstituted alkenes. The evolved enzymes are promiscuous and oxygen tolerant, use Earth-abundant iron, and can operate on the gram scale under ambient conditions. Orthogonal hydrogen atom sources enable site-divergent asymmetric isotope labeling. Mechanistic and computational studies support a stepwise radical process. Our work introduces a biochemical approach for stereoselective olefin reduction and provides a platform for next-generation biocatalytic hydrogenation.
A stoichiometrically conserved homologous series with infinite structural diversity
化学计量守恒且结构多样性无限的同系物系列
▲ 作者：HENGDI ZHAO, XIUQUAN ZHOU, ZILIANG WANG, PATRICIA E. MEZA, YIHAO WANG, DENIS T. KEANE, STEVEN J. WEIGAND, SAUL H. LAPIDUS, DUCK-YOUNG CHUNG, AND MERCOURI G. KANATZIDIS
A series of BaSbQ? crystals (where Q is sulfur or tellurium) has been synthesized, and its structural diversity is directly driven by systematically varying the S/Te ratio. Zhao et al. synthesized at least 10 of these “stoichiomorphs” composed of rocksalt slab fragments stacked together with polytelluride zigzag chains. Each member differed only in the size and assembly of these blocks, and the crystal phase that resulted depended on differences in anion electron affinity and sizes.
Plant cells are wrapped in a semirigid wall that reconfigures as cells expand. The chemical and material properties of pectin, a key component of cell walls, can influence cell division. Zhu et al. found different pectin properties in new versus mature cell walls, which are controlled by pectin-modifying enzymes such as PME5. The authors established that PME5 RNA is sequestered in the nucleus by RNA-binding proteins. The PME5 messenger RNA (mRNA) is released into the cytoplasm during cytokinesis when the nuclear envelope breaks down. By spatially controlling PME5 mRNA, modification of pectin can be precisely timed to coincide with the formation of new cell division plates.
High-fidelity human chromosome transfer and elimination
高保真人类染色体转移与消除
▲ 作者：GIANLUCA PETRIS, SIMONA GRAZIOLI, LINDA VAN BIJSTERVELDT, PIERRE MURAT, KIM C. LIU, JAKOB BIRNBAUM, JULIAN E. SALE, AND JASON W. CHIN
The synthesis of human genomes and other gigabase-scale genomes will require new strategies. Here, we realized key steps in our pipeline for building synthetic human chromosomes. We established: (i) the facile transfer of human chromosomes from human cells to mouse embryonic stem cells (assembly cells), where they are haploid, are nonessential, and may be operated on; (ii) the transfer of these human chromosomes from monochromosomal hybrids back into human cells to generate defined, synthetic aneuploidies; and (iii) the elimination of the corresponding endogenous human chromosomes to regenerate diploid cells containing a transferred chromosome. All steps were performed in nontransformed cells without chromothripsis and generated minimal structural variants, insertions, deletions, or single-nucleotide variants.
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