分类: 物理学 >> 核物理学 提交时间: 2023-05-30
摘要: Thorium-229 possesses the lowest first nuclear excited state, with an energy of approximately 8 eV. The extremely narrow linewidth of the first nuclear excited state, with an uncertainty of 53 THz, prevents direct laser excitation and realization of the nuclear clock. We present a proposal using the Coulomb crystal of a linear chain formed by 229Th3+ ions, where the nuclei of 229Th3+ ions in the ion trap are excited by the electronic bridge (EB) process. The 7 P1∕2 state of the thorium-229 nuclear ground state is chosen for EB excitation. Using the two-level optical Bloch equation under experimental conditions, we calculate that 2 out of 36 prepared thorium ions in the Coulomb crystal can be excited to the first nuclear excited state, and it takes approximately 2h to scan over an uncertainty of 0.22 eV. Taking advantage of the transition enhancement of EB and the long stability of the Coulomb crystal, the energy uncertainty of the first excited state can be limited to the order of 1 GHz
分类: 生物学 >> 生物物理学 提交时间: 2016-05-18
摘要: Autotransporters deliver virulence factors to the bacterial surface by translocating an effector passenger domain through a membrane-anchored barrel structure. Although passenger domains are diverse, those found in enteric bacteria autotransporters, including AIDA-I in diffusely adhering Escherichia coli (DAEC) and TibA in enterotoxigenic E. coli, are commonly glycosylated. We show that AIDA-I is heptosylated within the bacterial cytoplasm by autotransporter adhesin heptosyltransferase (AAH) and its paralogue AAH2. AIDA-I heptosylation determines DAEC adhesion to host cells. AAH/AAH2 define a bacterial autotransporter heptosyltransferase (BAHT) family that contains ferric ion and adopts a dodecamer assembly. Structural analyses of the heptosylated TibA passenger domain reveal 35 heptose conjugates forming patterned and solenoid-like arrays on the surface of a beta helix. Additionally, CARC, the AIDA-like autotransporter from Citrobacter rodentium, is essential for colonization in mice and requires heptosylation by its cognate BAHT. Our study establishes a bacterial glycosylation system that regulates virulence and is essential for pathogenesis.