液相色谱-质谱法研究附子生物碱的煎煮减毒机理

建立了基于液相色谱-质谱分析的附子中生物碱指纹分析方法,并将其应用于附子煎煮减毒的机理研究。基于质谱特征鉴定了生物碱标准品在加热回流过程中产生的6种降解产物,确定了双酯型生物碱的煎煮条件降解规律。在此基础上研究了附子饮片煎煮不同时间条件下生物碱的指纹变化,结果表明,长时间煎煮可明显降低双酯型生物碱和生物脂碱的浓度,为附子的安全用药提供了科学依据。     

Electrolytic nucleation of silver on a glassy carbon electrode: Part I. Mechanism of critical nucleus formation

The initial stage of electrodeposition of silver on a glassy carbon electrode is investigated using a double-pulse potentiostatic method. Data are obtained for the concentration dependence of the steady-state nucleation rate I and conclusions are drawn for the probable mechanism of critical nucleus formation.     

Experimental study of homogeneous nucleation from the bismuth supersaturated vapor: evaluation of the surface tension of critical nucleus

The homogeneous nucleation of bismuth supersaturated vapor is studied in a laminar flow quartz tube nucleation chamber. The concentration, size, and morphology of outcoming aerosol particles are analyzed by a transmission electron microscope (TEM) and an automatic diffusion battery (ADB). The wall deposit morphology is studied by scanning electron microscopy. The rate of wall deposition is measured by the light absorption technique and direct weighting of the wall deposits. The confines of the nucleation region are determined in the "supersaturation cut-off" measurements inserting a metal grid into the nucleation zone and monitoring the outlet aerosol concentration response. Using the above experimental techniques, the nucleation rate, supersaturation, and nucleation temperature are measured. The surface tension of the critical nucleus and the radius of the surface of tension are determined from the measured nucleation parameters. To this aim an analytical formula for the nucleation rate is used, derived from author's previous papers based on the Gibbs formula for the work of formation of critical nucleus and the translation-rotation correction. A more accurate approach is also applied to determine the surface tension of critical drop from the experimentally measured bismuth mass flow, temperature profiles, ADB, and TEM data solving an inverse problem by numerical simulation. The simulation of the vapor to particles conversion is carried out in the framework of the explicit finite difference scheme accounting the nucleation, vapor to particles and vapor to wall deposition, and particle to wall deposition, coagulation. The nucleation rate is determined from simulations to be in the range of 10(9)-10(11) cm(-3) s(-1) for the supersaturation of Bi(2) dimers being 10(17)-10(7) and the nucleation temperature 330-570 K, respectively. The surface tension σ(S) of the bismuth critical nucleus is found to be in the range of 455-487 mN/m for the radius of the surface of tension from 0.36 to 0.48 nm. The function σ(S) changes weakly with the radius of critical nucleus. The value of σ(S) is from 14% to 24% higher than the surface tension of a flat surface.     

Binary nucleation of water and sodium chloride

Nucleation processes in the binary water-sodium chloride system are investigated in the sense of the classical nucleation theory (CNT). The CNT is modified to be able to handle the electrolytic nature of the system and is employed to investigate the acceleration of the nucleation process due to the presence of sodium chloride in the steam. This phenomenon, frequently observed in the Wilson zone of steam turbines, is called early condensation. Therefore, the nucleation rates of the water-sodium chloride mixture are of key importance in the power cycle industry.     

Volumes of critical bubbles from the nucleation theorem

A corollary of the nucleation theorem due to Kashchiev [Nucleation: Basic Theory with Applications (Butterworth-Heinemann, Oxford, 2000)] allows the volume V(*) of a critical bubble to be determined from nucleation rate measurements. The original derivation was limited to one-component, ideal gas bubbles with a vapor density much smaller than that of the ambient liquid. Here, an exact result is found for multicomponent, nonideal gas bubbles. Provided a weak density inequality holds, this result reduces to Kashchiev's simple form which thus has a much broader range of applicability than originally expected. Limited applications to droplets are also mentioned, and the utility of the p(T,x) form of the nucleation theorem as a sum rule is noted.     

Deoxygenation of sugar trifluoromethanesulfonates by sodium metal in liquid ammonia and by photolysis

Several trifluoromethanesulfonyl(TFMS) derivatives of sugars were prepared and treated with sodium in liquid ammonia or subjected to ultraviolet irradiation. Three 3-$\text{O}$-TFMS derivatives gave the corresponding 3-deoxy compounds, but a 2-$\text{O}$-TFMS derivative gave a branched-chain sugar.     

聚苯胺的成核及生长机理

本文通过恒电位阶跃法研究了聚苯胺在不同介质中的成核与膜的生长过程动力学。结果表明, 在硫酸介质中, 成核过程为扩散控制下的三维连续成核, 得到疏松、多孔的膜; 而在高氯酸介质中, 成核则是电化学动力学控制下的二维成核过程。在高电位时(E>1.02V, vs, SCE)为二维连续成核过程, 而在较低的电位时, 主要表现为二维瞬时成核, 膜层呈网状且致密。     

Molecular mechanism of crystal nucleation from solution

Nucleation from solution is fundamental to many natural and industrial processes. The understanding of molecular mechanism of nucleation from solution is conducive to predict crystal structure, control polymorph and design desired crystal materials. In this review, the nucleation theories, including classical nucleation theory(CNT), nonclassical nucleation theory, as well as other new proposed theories, were reprised, and the molecular mechanism of these theories was compared. Then, the molecular process of nucleation, including the current study techniques, the effect of molecular self-assembly in solutions, desolvation process, as well as the properties of solvent and crystal structure on nucleation from solution were summarized. Furthermore, the relationship of molecular conformation in solution and in crystal, and the effect of solute molecular flexibility on nucleation were discussed.Finally, the current challenges and future scopes of crystal nucleation from solution were discussed.     

Computer simulation of nucleation in a gas-saturated liquid

Molecular dynamics methods have been used to investigate the kinetics of the liquid-gas phase transition in a two-component Lennard-Jones system at negative pressures and elastic stretches of the liquid to values close to spinodal ones. The molecular dynamics system consists of 2048 interacting particles with parameters of the Lennard-Jones potential for argon and neon. Density dependences of pressure and internal energy have been calculated for stable and metastable states of the mixture at a temperature T* approximately 0.7+/-0.01 and three values of the concentration. The location of mechanical and the diffusion spinodals has been determined. It has been established that a gas-saturated mixture retains its stability against finite variations of state variables up to stretches close to the values near the diffusion spinodal. The statistic laws of the process of destruction of the metastable state have been investigated. The lifetimes of the metastable phase have been determined. It is shown that owing to the small height of the potential barrier that separates the microheterogeneous from the homogeneous state a system of finite size has a possibility to make the reverse transition from the microheterogeneous into the homogeneous state. The lifetimes of the system in the microheterogeneous state, as well as the expectation times of the occurrence of a critical nucleus, are described by Poissonian distributions.     

Model for the nucleation mechanism of protein folding

A nucleation-like pathway of protein folding involves the formation of a cluster containing native residues that grows by including residues from the unfolded part of the protein. This pathway is examined by using a heteropolymer as a protein model. The model heteropolymer consists of hydrophobic and hydrophilic beads with fixed bond lengths and bond angles. The total energy of the heteropolymer is determined by the pairwise repulsive/attractive interactions between nonlinked beads and by the contribution from the dihedral angles involved. The parameters of these interactions can be rigorously defined, unlike the ill-defined surface tension of a cluster of protein residues that constitutes the basis of a previous nucleation model. The main idea underlying the new model consists of averaging the dihedral potential of a selected residue over all possible configurations of all neighboring residues along the protein chain. The resulting average dihedral potential depends on the distance between the selected residue and the cluster center. Its combination with the average pairwise potential of the selected residue and with a confining potential caused by the bonds between the residues leads to an overall potential around the cluster that has a double-well shape. Residues in the inner (closer to the cluster) well are considered as belonging to the folded cluster, whereas those in the outer well are treated as belonging to the unfolded part of the protein. Transitions of residues from the inner well into the outer one and vice versa are considered as elementary emission and absorption events, respectively. The double-well character of the potential well around the cluster allows one to determine the rates of both emission and absorption of residues by the cluster using a first passage time analysis. Once these rates are found as functions of the cluster size, one can develop a self-consistent kinetic theory for the nucleation mechanism of folding of a protein. The model allows one to evaluate the size of the nucleus and the protein folding time. The latter is evaluated as the sum of the times necessary for the first nucleation event to occur and for the nucleus to grow to the maximum size (of the folded protein). Depending on the diffusion coefficients of the native residues in the range from 10(-6) to 10(-8) cm2/s, numerical calculations for a protein of 2500 residues suggest that the folding time ranges from several seconds to several hundreds of seconds.     

Duration of nucleation process in supercooled halide melts

We present a model allowing to estimate the so-called time lag of nucleating halide melts using electrical conductivity measurements. Due to the complex-forming nature of molten halide salts we suppose two basic types of charge carriers in the melt: complexes (playing the role of monomers-building units) and clusters of a newly forming solid phase. Within context of the nonstationary nucleation theory we determined a formula expressing the time dependency of electrical conductivity of such a system and compared this result with the experimental data obtained for the melts of PbBr2, PbCl2, and KPb2Cl5. In terms of this formula the time lag of nucleation may be estimated. This important quantity characterizing the moment from which the nucleated clusters only grow to the macroscopic sizes has been found to be approximately 75% of the total duration of the nucleation process itself.     

Steady-state nucleation rate and flux of composite nucleus at saddle point

The steady-state nucleation rate and flux of composite nucleus at the saddle point is studied by extending the theory of binary nucleation. The Fokker-Planck equation that describes the nucleation flux is derived using the Master equation for the growth of the composite nucleus, which consists of the core of the final stable phase surrounded by a wetting layer of the intermediate metastable phase nucleated from a metastable parent phase recently evaluated by Iwamatsu [J. Chem. Phys. 134, 164508 (2011)]. The Fokker-Planck equation is similar to that used in the theory of binary nucleation, but the non-diagonal elements exist in the reaction rate matrix. First, the general solution for the steady-state nucleation rate and the direction of nucleation flux is derived. Next, this information is then used to study the nucleation of composite nucleus at the saddle point. The dependence of steady-state nucleation rate as well as the direction of nucleation flux on the reaction rate in addition to the free-energy surface is studied using a model free-energy surface. The direction of nucleation current deviates from the steepest-descent direction of the free-energy surface. The results show the importance of two reaction rate constants: one from the metastable environment to the intermediate metastable phase and the other from the metastable intermediate phase to the stable new phase. On the other hand, the gradient of the potential Φ or the Kramers crossover function (the commitment or splitting probability) is relatively insensitive to reaction rates or free-energy surface.     

Temperature dependence of ice critical nucleus size

We present a molecular dynamics study of ice growth from supercooled water. By performing a series of simulations with different initial conditions, we have quantitative established the relationship existing between the critical nucleus size and the temperature. The results show that ice embryos containing hundreds or thousands of molecules are needed for the system to crystallize macroscopically, even at high degrees of supercooling. Our findings explain the difficulty in observing spontaneous ice nucleation in atomistic simulations and the relative ease with which water droplets can be supercooled under controlled experiments.     

Fractal approach of nucleation in liquid and gas phases

Summary The authors present a generalised fractal treatment of the nucleation from the liquid solution or gas-phase reactant.     

Direct observation of nucleus structure and nucleation pathways in apoferritin crystallization.

Using atomic force microscopy (AFM) in situ during the crystallization of the protein apoferritin from its solution, we imaged the arrangement of the molecules in near-critical clusters, larger or smaller than the crystal nucleus, that are representative of the nucleus structure. At supersaturations Delta mu/k(B)T of 1.1 -- 1.6 -- 2.3, the nuclei contain about 50 -- 20 -- 10 molecules. The molecular arrangement within the nuclei is similar to that in the crystal bulk. Contrary to the general belief, the observed nuclei are not compact molecular clusters, but are planar arrays of several rods of 4--7 molecules set in one or two monomolecular layers. Similarly unexpected nuclei structures might be common, especially for anisotropic molecules. Hence, the nucleus structure should be considered as a variable by advanced theoretical treatments.     

Study of nucleation and growth mechanism of the metallic nanodumbbells

We propose a general nucleation and growth model that can explain the mechanism of the formation of CoPt(3)/Au, FePt/Au, and Pt/Au nanodumbbells. Thus, we found that the nucleation event occurs as a result of reduction of Au(+) ions by partially oxidized surface Pt atoms. In cases when Au(3+) is used as a gold precursor, the surface of seeds should be terminated by ions (e.g., Co(2+), Pb(2+)) that can reduce Au(3+) to Au(+) ions, which can further participate in the nucleation of gold domain. Further growth of gold domain is a result of reduction of both Au(3+) and Au(+) by HDA at the surface of gold nuclei. We explain the different ability of CoPt(3), Pt, and FePt seeds to serve as a nucleation center for the reduction of gold and further growth of dumbbells. We report that the efficiency and reproducibility of the formation of CoPt(3)/Au, FePt/Au, and Pt/Au dumbbells can be optimized by the concentration and oxidation states of the surface ions on metallic nanocrystals used as seeds as well as by the type of the gold precursor.