Bipullbacks of fractions and the snail lemma

We establish conditions giving the existence of bipullbacks in bicategories of fractions. We apply our results to construct a ${\pi }_0$-${\pi }_1$ exact sequence associated with a fractor between groupoids internal to a pointed exact category.     

Terminal C(sp3)H alkylation of internal alkenes via Ni/H-catalyzed isomerization

An efficient nickel-catalyzed reductive relay cross-coupling of internal alkenes with alkyl (or aryl) halides has been developed. This method has demonstrated broad substrate scope, mild reaction conditions and excellent terminal-selectivity. Moreover, this efficient strategy could be applied to the terminal-selective alkylation of isomeric mixtures of internal alkenes.     

Improved DNA fragment length estimation in capillary electrophoresis

In CE the charged DNA strands are fractionated according to fragment lengths as they migrate through the capillary, since shorter DNA fragments pass through the sieving matrix faster. Multiplexed internal size standards are used to estimate the size of unknown DNA fragments. In the literature there are statements about migration abnormalities for the 250 and 340 bp fragments in the GeneScan-500 (GS500) internal size standards. Such anomalous migration of size standards could obviously introduce errors in the estimation of unknown fragments. Therefore, a number of analysis programs simply exclude some of these fragments. In the present work we first evaluate the effect of excluding each of the fragments in the internal size standards used in CE. Next, a method which is based on estimating the true values of the anomalous fragments is presented. The results obtained by the new method indicate a significant improvement compared to results obtained when one (or both) of the anomalous fragments in GS500 is (are) excluded or included when estimating the size of unknown DNA fragments. In the higher-molecular-weight region, the average error is reduced from 1.91 bp in ABI GeneMapper (excluding 250 bp) to 0.15 bp in the new method (using the estimated values for 250 and 340 bp). In the lower-molecular-weight region, excluding both fragments will improve the results by an average of 0.74 bp compared to ABI GeneMapper.     

Internal and external eigenvalue problems of Hermitian operators and their use in electronic structure theory

Within the fragment resolution of molecular systems the conceptual and interpretative advantages of using the separate eigenvalue problems for the internal and external part of the Hermitian matrix representing a physical quantity in quantum mechanics are examined. By definition, these two parts accordingly combine only the diagonal and off-diagonal subsystem-resolved blocks of matrix elements. These two partial eigenvalue problems bring about the matrix internal or external decouplings, respectively, which have recently been used in several interpretations of the molecular electronic structure. A character and structure of the external eigensolutions is examined in some detail and their recent applications in the Charge Sensitivity Analysis—to extract the most important electron-transfer effects between constituent atoms of model chemisorption systems, and in the Molecular-Orbital theory—to precisely identify the inter-orbital flows of electrons, are summarized and commented upon. The grouping relation, for combining the external/internal eigensolutions into those for the whole matrix, is derived in the context of the complementary “rotations” of the basis set vectors.     

Enhancing Built-in Electric Fields for Efficient Photocatalytic Hydrogen Evolution by Encapsulating C60 Fullerene into Zirconium-Based Metal-Organic Frameworks

High-efficiency photocatalysts based on metal-organic frameworks (MOFs) are often limited by poor charge separation and slow charge-transfer kinetics. Herein, a novel MOF photocatalyst is successfully constructed by encapsulating C₆₀ into a nano-sized zirconium-based MOF, NU-901. By virtue of host-guest interactions and uneven charge distribution, a substantial electrostatic potential difference is set-up in C₆₀@NU-901. The direct consequence is a robust built-in electric field, which tends to be 10.7 times higher in C₆₀@NU-901 than that found in NU-901. In the catalyst, photogenerated charge carriers are efficiently separated and transported to the surface. For example, photocatalytic hydrogen evolution reaches 22.3 mmol g⁻¹ h⁻¹ for C₆₀@NU-901, which is among the highest values for MOFs. Our concept of enhancing charge separation by harnessing host-guest interactions constitutes a promising strategy to design photocatalysts for efficient solar-to-chemical energy conversion.     

Rhodium-Catalyzed Regio- and Diastereoselective [3+2] Cycloaddition of gem-Difluorinated Cyclopropanes with Internal Olefins

Direct synthesis of gem-difluorinated carbocyclic molecules represents a longstanding challenge in organic chemistry. Herein, a Rh-catalyzed [3+2] cycloaddition reaction between readily available gem-difluorinated cyclopropanes (gem-DFCPs) and internal olefins has been developed, enabling the efficient synthesis of gem-difluorinated cyclopentanes with good functional group compatibility, excellent regioselectivity and good diastereoselectivity. The resulting gem-difluorinated products can undergo downstream transformations to access various mono-fluorinated cyclopentenes and cyclopentanes. This reaction demonstrates the use of gem-DFCPs as a type of “CF₂” C3 synthon for cycloaddition under transition metal catalysis, which provides potential strategy for synthesizing other gem-difluorinated carbocyclic molecules.     

Analysis of ultrafast relaxation in photoexcited DNA base pairs of adenine and thymine

The photoinduced dynamics in base pairs of adenine and thymine were analyzed by femtosecond pump-probe spectroscopy. On the short-time scale up to a few picoseconds, the characteristic time constants for the dimers are quite similar to the corresponding values of the monomers. This leads to the conclusion that ultrafast intramolecular relaxation proceeds via ππ^* and nπ^* states of one component within the dimer. On the long-time scale, we obtained a novel time constant of roughly 40 ps for the thymine dimer and the adenine–thymine base pair. This time constant was never observed in the monomers and is tentatively assigned to an intermolecular relaxation process, possibly via a hydrogen transfer state.     

细胞体系中内噪声对弱信号检测的积极作用

在小尺度的生物化学反应体系中内噪声是普遍存在的,本文采用化学Langevin方程研究了细胞体系中内噪声对弱信号检测的作用. 研究发现, 在Hopf分岔点附近, 当弱信号非常小以至于不能独立地激发细胞内的钙尖峰振荡时, 内噪声能够帮助细胞内的钙振荡信号越过一个域值, 并且在一个最佳的内噪声强度下, 内噪声、内噪声诱导的钙振荡和弱信号之间会产生共振现象, 由此极大地增强了细胞体系对弱信号的检测能力. 由于内噪声是通过细胞尺度变化而改变的, 因此这种现象也说明了在对弱信号检测行为中最佳细胞尺度的存在. 研究还发现最佳的细胞尺度与真正的细胞体积是相吻合的, 并且基本不随外界刺激的改变而改变, 这具有重要的生物学意义.     

二阶关联成像系统宽光谱吸收膜研制

为满足关联成像系统抑制背景的需求,选用Al、Cr和SiO_2作为镀膜材料,依据薄膜吸收理论,结合膜系设计软件设计了宽光谱吸收膜,并采用真空沉积技术获得了该薄膜样品.通过真空阶梯式退火,减小了膜层内应力,解决了薄膜牢固度问题;采用交互式分析对测试结果逆向反演,通过优化工艺参量,使膜系中敏感薄层厚度得以精准控制,并减小了膜厚控制误差.制备的吸收膜在400~1 100nm波段平均吸收率达到99.1%,满足系统使用要求.     

Cylindrospermopsin determination using 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES) as the internal standard

Cylindrospermopsin (CYN) was determined by liquid chromatography/electrospray ionization-mass spectrometry (LC/ESI-MS) using 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES) as the internal standard. In the selected ion monitoring of LC/ESI-MS, m/z 414 for CYN and 237 for HEPES were monitored using the negative mode; the retention times of CYN and HEPES were 12.41 and 14.21 min, respectively. CYN was determined from peak area ratios of m/z 414/237. By the treatment of an anion exchange cartridge using a buffer at pH 10.5, CYN was isolated and condensed. No interfering peak was observed. Linearity of this method was observed at the range of 0.10-31.12 ng. Total coefficients of variation were 5.1 and 2.9% at 104 and 1038 μg CYN L⁻¹. The quantitative limit at a signal-to-noise (S/N) ratio of 10 was 0.16 ng.CYN concentration in natural waters is low. CYN in waters should be condensed for determination. This method including the treatment for isolation and condensation of CYN is useful for determination of CYN in environmental and/or drinking waters.