Size-dependent phase stability of a molecular nanocrystal: a proxy for investigating the early stages of crystallization

We make the link between the size-dependent phase stability of a nanocrystal and the phase-transition behavior of emerging crystallites during the earliest stages of crystallization, by using the former as a proxy for the latter. We outline an extension of the classical nucleation theory to describe crystal nucleation and subsequent transformations of competing polymorphic phases that characterize Ostwald's rule of stages. The theoretical framework reveals that the relative stability of the competing phases is a function of cluster size, which in turn varies with time, and therefore explains the complex transformation behavior observed for some systems. We investigated the stability of a nanocrystal of dl-norleucine by means of molecular simulation as a proxy for post-nucleation phase-transformation behavior in emerging crystallites. The simulations reveal that, for nanocrystals, the surface energy of the transition state of a transformation can dominate the barrier to phase change, thus causing metastable phases to be stabilized, not because they are thermodynamically stable, but rather due to kinetic hindering. Therefore, in the context of the earliest stages of crystal growth, not only does phase stability vary as a function of cluster size, and hence time, but thermodynamically feasible transformations are also prone to kinetic hindering.     

蒂巴茵衍生物的合成研究

The reaction of 7α-acetyl-6,14-endoethano-6,7,8,14-tetrahydrothebaine with 2-（thien-2-yl）ethylmagnesium bromide was investigated. The tertiary alcohol derivative 7α-[R-l-hydroxyl-l-methyl-3-（thien-2-yl）propyl]）-6,14- endoethano-6,7,8,14-tetrahydrothebaine （3） and a by-product 4 were isolated. The structure of 4 was elucidated by X-ray analysis. The Grignard reaction shows high degree of stereoselectivity according with Cram rule. The crystal structure of 4 indicates that dihydrofuran ring was opened to form a phenolic hydroxyl group and a three-membered ring structure. It maintains the main rigid structure of morphine and contains a C（6）-C（14） enthano bridge. The 1-hydroxyl-1-methyl-3-（thien-2-yl）propyl group at C（7） position adopted S-configuration.     

氧化数教学争议的化解

从氧化数的定义和氧化数概念的发展过程入手,分析了氧化数教学中存在的争议,提出确定氧化数的规则应加上元素的正氧化数小于或等于原子的外围电子总数,解决了结构比较复杂和结构未知的化合物中元素氧化数的计算问题。     

Application of the pKa rule to synthesize salts of bezafibrate

ABSTRACT

Co-crystallization frequently employs forces such as hydrogen bonds, halogen bonds, and π-π stacking to assemble molecules in a multi-component crystal. In an effort to increase the strength of the intermolecular interaction between the anti-cholesterol drug bezafibrate (BEZA), a wastewater contaminant, and hydrogen-bond-acceptor molecules, we modified the pK_a values of the acceptors. Here, we describe the first series of salts incorporating BEZA and achieve a variety of supramolecular architectures including discrete assemblies, 1D chains, tapes, and 2D sheets. We discuss exceptions to the pK_a rule, and demonstrate that the presence of hydrogen-bond-donor atoms on the acceptor molecule supports salt formation.     

Integrating Time-Resolved Imaging Information by Single-Luminophore Dual Thermally Activated Delayed Fluorescence

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.     

Single-Round DNA Aptamer Selection by Combined Use of Capillary Electrophoresis and Next Generation Sequencing: An Aptaomics Approach for Identifying Unique Functional Protein-Binding DNA Aptamers

In DNA aptamer selection, existing methods do not discriminate aptamer sequences based on their binding affinity and function and the reproducibility of the selection is often poor, even for the selection of well-known aptamers like those that bind the commonly used model protein thrombin. In the present study, a novel single-round selection method (SR-CE selection) was developed by combining capillary electrophoresis (CE) with next generation sequencing. Using SR-CE selection, a successful semi-quantitative and semi-comprehensive aptamer selection for thrombin was demonstrated with high reproducibility for the first time. Selection rules based on dissociation equilibria and kinetics were devised to obtain families of analogous sequences. Selected sequences of the same family were shown to bind thrombin with high affinity. Furthermore, data acquired from SR-CE selection was mined by creating sub-libraries that were categorized by the functionality of the aptamers (e. g., pre-organized aptamers versus structure-induced aptamers). Using this approach, a novel fluorescent molecular recognition sensor for thrombin with nanomolar detection limits was discovered. Thus, in this proof-of-concept report, we have demonstrated the potential of a “DNA Aptaomics” approach to systematically design functional aptamers as well as to obtain high affinity aptamers.     

Heat capacities of concentrated multicomponent aqueous electrolyte solutions at various temperatures

The specific heat capacities of the aqueous multicomponent system NaCl +KCl+MgCl₂+CaCl₂ with ionic strength between 8.3 and 9.6 (resembling Dead Sea waters) were measured between 15°C and 45°C. The obtained data were fitted to an empirical equation as a function of concentration and temperature. The thermodynamic functions of the studied multicomponent system were found to be strongly influenced by changes in MgCl₂ concentrations. The application of Young's rule to such concentrated systems was checked at 25°C. The calculated (by Young's rule) specific heat capacitiesC _p and apparent molar heat capacities C_p, of these multicomponent electrolyte solutions were in reasonable agreement with the measured values (–0.008 J-g^–1-K^–1 and –2.6 J-mol^–1-K^–1, respectively).     

Fullerenes分子的键构规则

Fullerenes分子是科学家在研究碳原子簇过程中发现的一类崭新的有机分子,简单情形是C_(60)。Fullerenes及其衍生物为碳化学打开了—个广阔天地,同时也为固体物理学提供了一个新的研究领域。它还将为材料科学带来新的机会。尽管化学家和物理学家为这类新奇的原子簇勾画了许多理论图象,但关于其键构的—般性规则的研究仍然太少。本文将在     

Fluorine-Stabilized Sulfur–Carbon Multiple Bonds

Alkylidene and alkylidyne sulfur fluorides contain sulfur–carbon multiple bonds. In contrast to the sulfur ylides, alkylidene sulfur fluorides fulfill all the criteria for double bonds, i.e. they have short bond lengths, strong anisotropic distribution of electron density, and rotation about the C? S bond is restricted. Alkylidyne sulfur fluorides have especially short bond distances and, due to a high amplitude bending motion, appear to be more or less linear, depending on the physical state. The advantage of the C? S multiple bond systems in contrast with numerous others, e.g. those of phosphorus and silicon, is that they exist without steric stabilization. Moreover, the limits of the triple-bond principle are outlined: the prognosis for triple bonds between two elements of higher periods is poor, because carbene-like or fully bridged structures win in terms of stability.