A Stone-Wales ͑SW͒ defect is a dipole of 5-7 ring pair in a hexagonal network, which is one of the most important defective structures in carbon nanotubes ͑CNTs͒ that will affect mechanical, chemical, and electronic properties of CNTs. Using the extended Hückel method, we calculated the formation energy of SW defects in carbon nanotubes. The formation energy of SW defects was then fitted to a simple formula as a function of the tube radius and the orientation of a SW defect in the tube. This result provides a convenient tool for the study of thermodynamics and kinetics of SW defects, as well as the interaction of SW defects with other types of defects in CNTs. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1599961͔ A carbon nanotube ͑CNT͒ was first thought of as a perfect graphene sheet wrapped up into a cylinder. However, as more experimental results became available and theoretical investigations went deeper, CNT was found to be not as perfect as it seems. Defects such as the 5-7 rings, kinks, junctions, and impurities may be presented in as-prepared CNTs. These defects can significantly change the electrical, chemical, and mechanical properties of CNTs.1-3 Therefore, it is highly desirable to gain an understanding on the energetic condition for the formation and thermodynamic behavior of defects in CNT for applications such as nanoelectronic devices, composite reinforcement, and energy storage. Unlike bulk materials, the structure of CNT has two degrees of freedom: one is the radius (r) of the tube and the other is the chiral angle ͑͒. The (r,) notation of CNT can be easily translated from the normal (n,m) notation as follows:where a is the C-C bond length, and is limited to being 0рр/6 due to the geometrical symmetry of the hexagon network. Such two degrees of freedom introduce a complexity in the description of the formation energy of a defect: with the change of radius and chiral angle of a CNT, the formation energy of a defect may also change. In addition, the orientation of the defect itself in relation to the CNT may cause variations in formation energy. The Stone-Wales ͑SW͒ defect is one of most important defective structures in CNTs. It is formed by rotating a C-C bond in the hexagonal network by 90°͑the so-called StoneWales transformation͒, 5 resulting in the creation of a dipole of a 5-7 ring pair ͓see Fig. 1͑a͔͒. Murry and co-workers 6 examined the kinetics of the SW transformation as an essential part of fullerene annealing and fragmentation. Beyond a critical level of tension, CNT releases its excessive strain via a spontaneous formation of topological defects. It was proposed that at high temperatures, a plastic response could occur due to the separation and gliding of SW defects, whereas at lower temperatures the result could be fractures. 7 The formation energy of the SW defect is sensitive to the applied strain along the axial direction of CNT.8,9 Samsonidze et al. 10 presented an analytic expression of the formation energy (E sw ) for SW defects under an applied strain . There are som...

No abstract

D-Tagatose 3-epimerase family enzymes can efficiently catalyze the epimerization of free keto-sugars, which could be used for D-psicose production from D-fructose. In previous studies, all optimum pH values of these enzymes were found to be alkaline. In this study, a D-psicose 3-epimerase (DPEase) with neutral pH optimum from Clostridium bolteae (ATCC BAA-613) was identified and characterized. The gene encoding the recombinant DPEase was cloned and expressed in Escherichia coli. In order to characterize the catalytic properties, the recombinant DPEase was purified to electrophoretic homogeneity using nickel-affinity chromatography. Ethylenediaminetetraacetic acid was shown to inhibit the enzyme activity completely; therefore, the enzyme was identified as a metalloprotein that exhibited the highest activity in the presence of Co²⁺. Although the DPEase demonstrated the most activity at a pH ranging from 6.5 to 7.5, it exhibited optimal activity at pH 7.0. The optimal temperature for the recombinant DPEase was 55 °C, and the half-life was 156 min at 55 °C. Using D-psicose as the substrate, the apparent K(m), k(cat), and catalytic efficiency (k(cat)/K(m)) were 27.4 mM, 49 s⁻¹, and 1.78 s⁻¹ mM⁻¹, respectively. Under the optimal conditions, the equilibrium ratio of D-fructose to D-psicose was 69:31. For high production of D-psicose, 216 g/L D-psicose could be produced with 28.8 % turnover yield at pH 6.5 and 55 °C. The recombinant DPEase exhibited weak-acid stability and thermostability and had a high affinity and turnover for the substrate D-fructose, indicating that the enzyme was a potential D-psicose producer for industrial production.

The glucose transporter GLUT1, a plasma membrane protein that mediates glucose homeostasis in mammalian cells, is responsible for constitutive uptake of glucose into many tissues and organs. Many studies have focused on its vital physiological functions and close relationship with diseases. However, the molecular mechanisms of its activation and transport are not clear, and its detailed distribution pattern on cell membranes also remains unknown. To address these, we first investigated the distribution and assembly of GLUT1 at a nanometer resolution by super-resolution imaging. On HeLa cell membranes, the transporter formed clusters with an average diameter of ∼250 nm, the majority of which were regulated by lipid rafts, as well as being restricted in size by both the cytoskeleton and glycosylation. More importantly, we found that the activation of GLUT1 by azide or MβCD did not increase its membrane expression but induced the decrease of the large clusters. The results suggested that sporadic distribution of GLUT1 may facilitate the transport of glucose, implying a potential association between the distribution and activation. Collectively, our work characterized the clustering distribution of GLUT1 and linked its spatial structural organization to the functions, which would provide insights into the activation mechanism of the transporter.

The fact that the lifetime of photoluminescence is often difficult to access because of the weakness of the emission signals, seriously limits the possibility to gain local bioimaging information in time‐resolved luminescence probing. We aim to provide a solution to this problem by creating a general photophysical strategy based on the use of molecular probes designed for single‐luminophore dual thermally activated delayed fluorescence (TADF). The structural and conformational design makes the dual TADF strong in both diluted solution and in an aggregated state, thereby reducing sensitivity to oxygen quenching and enabling a unique dual‐channel time‐resolved imaging capability. As the two TADF signals show mutual complementarity during probing, a dual‐channel means that lifetime mapping is established to reduce the time‐resolved imaging distortion by 30–40 %. Consequently, the leading intracellular local imaging information is serialized and integrated, which allows comparison to any single time‐resolved signal, and leads to a significant improvement of the probing capacity.

No abstract

β-coronaviruses reshape host cell endomembranes to form double-membrane vesicles (DMVs) for genome replication and transcription. Ectopically expressed viral nonstructural proteins nsp3 and nsp4 interact to zipper and bend the ER for DMV biogenesis. Genome-wide screens revealed the autophagy proteins VMP1 and TMEM41B as important host factors for SARS-CoV-2 infection. Here, we demonstrated that DMV biogenesis, induced by virus infection or expression of nsp3/4, is impaired in the VMP1 KO or TMEM41B KO cells. In VMP1 KO cells, the nsp3/4 complex forms normally, but the zippered ER fails to close into DMVs. In TMEM41B KO cells, the nsp3–nsp4 interaction is reduced and DMV formation is suppressed. Thus, VMP1 and TMEM41B function at different steps during DMV formation. VMP1 was shown to regulate cross-membrane phosphatidylserine (PS) distribution. Inhibiting PS synthesis partially rescues the DMV defects in VMP1 KO cells, suggesting that PS participates in DMV formation. We provide molecular insights into the collaboration of host factors with viral proteins to remodel host organelles.

No abstract