基于粗粒化模型对有机溶剂的分子动力学模拟

在基于Boltzmann分布对四种基本构象进行Monte Carlo取样后, 通过与全原子模型的范德华势比较得到了Gay-Berne (GB)参数. 又在对用量化计算得到的分子体系的电势进行电荷、偶极矩和四极矩的拟合后, 得到了电多极展开势(EMP)参数. 利用得到的粗粒化参数, 基于粗粒化模型, 对CHCl₃及四氢呋喃(THF)两种有机溶剂进行了分子动力学模拟(MDS), 并将结果同全原子模拟进行了比较. 计算结果表明用粗粒化模型从整体上能重复全原子模型的模拟结果, 但在某些细节的计算与全原子模型有偏差, 其原因可能是目前工作仅考虑了单位点情况, 为此今后在对具有复杂结构的分子进行粗粒化模拟时还应考虑合理放置及增加相互作用位点.     

桂枝的引药作用机制及其对环丙沙星在小鼠体内分布的影响

研究了桂枝和桂皮醛对环丙沙星在小鼠体内分布的影响,并基于此探讨了桂枝作为引药使用的作用机制.实验中将小鼠分为三组,分别灌胃生理盐水、桂枝(50gkg-1)和桂皮醛(0.25mLkg-1),然后对三组小鼠尾静脉注射环丙沙星(20mgkg-1),并以荧光法测定不同时间点小鼠体内各组织中环丙沙星的浓度.结果显示桂枝、桂皮醛均促进了环丙沙星在四肢的分布,桂枝与桂皮醛组相对于生理盐水组可将环丙沙星在四肢的含量提高约40%~200%,证实了桂枝引药到达肢节的作用.桂枝与桂皮醛组对环丙沙星分布情况的影响没有显著性差异,说明桂皮醛很可能是发挥桂枝引药作用的主要物质.     

Structure of MgSO_4 in Concentrated Aqueous Solutions by X-Ray Diffraction

Detailed time-and-space-averaged structure of MgSO4 in the concentrated aqueous solutions was investigated via X-ray diffraction with an X’pert Pro θ-θ diffractometer at 298 K, yielding structural function and radial distribution function(RDF). The developed KURVLR program was employed for the theoretical investigation in consideration of the ionic hydration and ion association. Multi-peaks Gaussian fitting method was applied to deconvolving the overlapping bands of Differential radial distribution function...     

iMatch: a retention index tool for analysis of gas chromatography-mass spectrometry data

A method was developed to employ National Institute of Standards and Technology (NIST) 2008 retention index database information for molecular retention matching via constructing a set of empirical distribution functions (DFs) of the absolute retention index deviation to its mean value. The effects of different experimental parameters on the molecules' retention indices were first assessed. The column class, the column type, and the data type have significant effects on the retention index values acquired on capillary columns. However, the normal alkane retention index (I(norm)) with the ramp condition is similar to the linear retention index (I(T)), while the I(norm) with the isothermal condition is similar to the Kováts retention index (I). As for the I(norm) with the complex condition, these data should be treated as an additional group, because the mean I(norm) value of the polar column is significantly different from the I(T). Based on this analysis, nine DFs were generated from the grouped retention index data. The DF information was further implemented into a software program called iMatch. The performance of iMatch was evaluated using experimental data of a mixture of standards and metabolite extract of rat plasma with spiked-in standards. About 19% of the molecules identified by ChromaTOF were filtered out by iMatch from the identification list of electron ionization (EI) mass spectral matching, while all of the spiked-in standards were preserved. The analysis results demonstrate that using the retention index values, via constructing a set of DFs, can improve the spectral matching-based identifications by reducing a significant portion of false-positives.     

Separation of poly-3-hydroxyvalerate-co-3-hydroxybutyrate through gradient polymer elution chromatography

Polyhydroxyalkanoates are biodegradable polyesters produced by bacteria that can have a wide distribution in molecular weight, composition of monomers, and functionalities. This large distribution often leads to unpredictable physical properties making commercial applications challenging. To improve polymer homogeneity and obtain samples with a clear set of physical characteristics, poly-3-hydroxyvalerate-co-3-hydroxybutyrate copolymers were fractionated using gradient polymer elution chromatography (GPEC) as opposed to extensively used bulk fractionation. Separation was achieved using a reversed-phase column with chloroform and ethanol as the solvent and non-solvent, respectively. A separation was also conducted on a normal-phase column to compare elution patterns between columns of varied polarity. The fractions were analyzed using Size Exclusion Chromatography (SEC) and NMR to determine the percentage of 3-hydroxyvalerate in the copolymer as well as its molecular weight. It was found that as the percentage of "good" solvent was increased in the mobile phase, the polymers eluted with decreasing percentage of 3-hydroxyvalerate and increasing molecular weight which indicates the importance of precipitation/redissolution in the separation. The elution pattern of the polymer remained unchanged when using both a normal- and reversed-phase column which also illustrates the dominance of precipitation/redissolution in GPEC of polyhydroxyalkanoates. As such, GPEC is shown to be an excellent choice to provide polyhydroxyalkanoate samples with a narrower distribution in composition than the original bulk copolymer sample.     

Computation of distribution of minimum resolution for log-normal distribution of chromatographic peak heights

General equations are derived for the distribution of minimum resolution between two chromatographic peaks, when peak heights in a multi-component chromatogram follow a continuous statistical distribution. The derivation draws on published theory by relating the area under the distribution of minimum resolution to the area under the distribution of the ratio of peak heights, which in turn is derived from the peak-height distribution. Two procedures are proposed for the equations' numerical solution. The procedures are applied to the log-normal distribution, which recently was reported to describe the distribution of component concentrations in three complex natural mixtures. For published statistical parameters of these mixtures, the distribution of minimum resolution is similar to that for the commonly assumed exponential distribution of peak heights used in statistical-overlap theory. However, these two distributions of minimum resolution can differ markedly, depending on the scale parameter of the log-normal distribution. Theory for the computation of the distribution of minimum resolution is extended to other cases of interest. With the log-normal distribution of peak heights as an example, the distribution of minimum resolution is computed when small peaks are lost due to noise or detection limits, and when the height of at least one peak is less than an upper limit. The distribution of minimum resolution shifts slightly to lower resolution values in the first case and to markedly larger resolution values in the second one. The theory and numerical procedure are confirmed by Monte Carlo simulation.     

Distribution and Comparison of Steranes From Blue-Green Algae Under Different Simulated Conditions

Two remarkably opposite distribution patterns of sterane series are revealed from living blue-green alga (or cyanobacterium) under different simulated conditions. The organic matter derived from hydrous-pressure pyrolysis of the blue-green alga is characterised by the predominance of C_(27) sterane over C_(29) sterane; this is coincident with traditional thought. On the contrary, the organic matter derived from artificially silicified microfossils of the blue-green alga is characterised by the predominance of C_(29) sterane over C_(27) sterane; this appears to be an unusual distribution pattern. Such kind of unusual steranedistribution also has been foundin the samples of Precambrian cherts with abundant well-preserved microfossil algae. All these imply that the special source of organisms and the medium, procedure or conditions of simulated pyrolysis may result in the different distribution patterns of sterane biomarkers.     

Size exclusion chromatographic characterization of sodium hyaluronate fractions prepared by high energetic sonication

Summary A high molecular weight sodium hyaluronate isolated from rooster combs was degraded by ultrasonication. High-performance size exclusion chromatography allowed rapid and accurate determination of molecular weight parameters (M_z, M_w, M_n) and distributions. The time dependence of hyaluronan ultrasonication to the molecular characteristics of the polymer was investigated. A non-random nature of the degradation process was demonstrated and the reciprocal M_n value was found to be linearly proportional to the time of ultrasonication.     

湖州表层土壤稀土元素含量及分布特征

为了研究湖州表层土壤样品中稀土元素的分布特征,利用ICP-MS测定了土壤中稀土元素的含量。结果表明湖州表层土壤中稀土元素含量的大小顺序为：Ce＞La＞Nd＞Pr＞Sm＞Gd＞Dy＞Yb＞Er＞Eu＞Ho＞Tb＞Tm＞Lu,遵循Oddo-Harkins法则。湖州表层土壤中稀土元素的分量均值和总量均值皆高于全国水平,稀土元素存在显著的分馏现象,轻稀土元素明显富集。表层土壤中Ce和Eu都有部分亏损。     

Effect of particle agglomeration and interphase on the glass transition temperature of polymer nanocomposites

In this article, we utilize finite element modeling to investigate the effect of nanoparticle agglomeration on the glass transition temperature of polymer nanocomposites. The case of an attractive interaction between polymer and nanofiller is considered for which an interphase domain of gradient properties is developed. This model utilizes representative volume elements that are created and analyzed with varying degrees of nanoparticle clustering and length scale of interphase domain. The viscoelastic properties of the composites are studied using a statistical approach to account for variations due to the random nature of the microstructure. Results show that a monotonic increase in nanofiller clustering not only results in the loss of interphase volume but also obstructs the formation of a percolating interphase network in the nanocomposite. The combined impacts lead to a remarkable decrease of T_g enhancement of clustering nanofillers in comparison with a well‐dispersed configuration. Our simulation results provide qualitative support for experimental observations that clustering observed at high nanofiller concentrations negatively impacts the effects of the nanofiller on overall properties. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011