Systematic search in conformational analysis

The coupling of conformation to activity and reactivity is a widely accepted concept, and as such has driven the development of tools which execute conformational searches in rapid and robust fashion [T.F. Havel, Prog. Biophys. Molec. Biol., 56 (1991) 43–78; A.R. Leach, In Rev. Comput. Chem.; K.B. Lipkowitz and D.B. Boyd, Ed.; VCH Publishers, Inc.: New York, N.Y., 1991, Vol. II, pp. 1–55]. Among the aims of these methods are the determination of a complete set of local minima from which the global energy minimum can be identified, or the generation of conformations consistent with constraints derived from SAR or structural studies. Most methods fall into two broad categories: those which are random or stochastic, and those which are systematic. Yet another group consists of those which are based on heuristics and artificial intelligence [A.R. Leach, K. Prout, D.P. Dolata, J. Comput. Chem. 11 (1990) 680–693]. The first category is typified by molecular dynamics [W.F. van Gunsteren and H.J.C. Berendsen, Angew. Chem. Int. Ed. Eng., 29 (1990) 992–1023], Monte Carlo [M.P. Alien and D.J. Tildesley, Computer Simulation of Liquids, Oxford Science Publications, 1989], distance geometry [J.M. Blaney and J.S. Dixon, in K.B. Lipkowitz and D.B. Boyd (Eds.), Reviews in Computational Chemistry, VCH, New York, Vol. 5, pp. 299–335, 1994], and other approaches [M. Saunders, J. Comput. Chem., 10 (1989) 203–208] in which the path by which conformational space is examined is ideally completely random, but bounded by the geometries of covalent bond lengths and angles. In traditional systematic searches, the variable to be examined, e.g. torsion angles, is divided into a regular grid. Each and every grid point is evaluated in a systematic fashion to determine its validity. The path through the grid points is regular and defined. In principle, systematic search can, within the resolution of the grid, identify all sterically allowed conformations of a molecule. Consequently, systematic search is an ideal tool for conformational analysis because it is not path dependent and cannot become entrapped in local minima. In this article we review some of the basics of systematic search, algorithmic improvements that have enhanced its speed, and new developments that have increased its accuracy by moving away from the limitations of a fixed torsional grid.     

Clusters and Aggregates of 1,1′-Bis(diphenylphosphino)ferrocene (dppf)

The emerging interest of ferrocenyl diphosphines in metal complexes has led to some significant developments recently in the clusters, polymetallic aggregates and oligomers of 1,1-bis(diphenylphosphino)ferrocene (dppf). In this review, we shall focus on the synthetic strategies and the chemical, structural and bonding characteristics of these materials.     

水溶性β-环糊精聚合物和疏水改性聚丙烯酰胺的主客体相互作用

主体环糊精聚合物(β-CDE)与客体疏水改性丙烯酰胺共聚物P(AM/POEA)构成超分子结构的高分子识别体系. 这种客体聚合物是含有少量疏水体(x_POEA＜0.01)的水溶性聚合物, NMR测定结果表明β-CDE和P(AM/POEA)的主客体相互作用是通过环糊精空腔和疏水体POEA形成包结络合物进行的. 在P(AM/POEA)聚合物水溶液中加入β-CDE, 由于主客体聚合物相互作用出现粘度的大幅上升, 增粘的幅度可通过改变聚合物浓度和疏水体含量来调节, 同时对盐浓度和温度的影响也进行了研究. 通过透射电镜直观观察的结果表明, 此类缔合聚合物体系的主客体相互作用生成实心球状多分子聚集体.     

Synthesis and application of novel fluoroalkyl end-capped cooligomers having adamantane as a pendant group

Fluoroalkyl end-capped cooligomers having adamantane as a pendant group [R_F-(Ad-HAc) _x –(Co-M) _y -R_F] were prepared by the reactions of fluoroalkanoyl peroxide with 3-hydroxy-1-adamantylacrylate (Ad-HAc) and comonomers (Co-M) such as acrylic acid (ACA), N,N-dimethylacrylamide (DMAA), and acryloylmorpholine (ACMO) under mild conditions. Thermogravimetric analyses (TGA) showed that thermal stability of R_F-(Ad-HAc) _x –(Co-M) _y -R_F was superior to that of the corresponding fluoroalkyl end-capped cooligomers having adamantane in the main chains [R_F-(Ad) _x –(Co-M) _y -R_F] and the fluoroalkyl end-capped homooligomers possessing no adamantyl segments [R_F-(M) _n -R_F]. It is interesting to note that fluoroalkyl end-capped Ad-Hac–DMAA cooligomer [R_F-(Ad-HAc) _x –(DMAA) _y -R_F] was found to form the nanometer size-controlled cooligomeric aggregates which consist of around 16 fluorinated cooligomeric molecules in methanol/water mixed solvents. Furthermore, these fluorinated cooligomeric aggregate could occupy around 320 ADMDD [5-(2-adamantylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione] molecules as guest molecules per aggregate core, although ADMDD could not be encapsulated into the R_F-(Ad) _x –(Co-M) _y -R_F cooligomeric and R_F-(M) _n -R_F homooligomeric aggregate cores under similar conditions.     

贵阳污灌区菜地土壤团聚体中有机碳和重金属的含量特征及相关性分析

以贵阳某污灌区菜地土壤为研究对象,分别采用微波消解-电感耦合等离子体质谱(ICP-MS)法和水合热重铬酸钾氧化分光光度法分析不同粒径土壤团聚体中重金属和有机碳的含量特征,并对有机碳和重金属的相关性进行分析。结果表明,以2mm粒径团聚体的含量为最高,约占75%。Cu、Zn、Cd和Pb在0.25～0.5mm粒径团聚体中含量最高,Cr在5～8mm粒径团聚体中含量最高,As在不同粒径团聚体中的含量变化不大,重金属含量随土层深度增大而减小。土壤重金属富集因子表现为CuPbCdZnCrAs,Cu、Zn、Cd和Pb在0.25～0.5mm粒径团聚体中分布因子最高,而在5～8mm粒径团聚体中重金属的质量负载因子最大。土壤有机碳含量随团聚体粒径的增大表现为先增大后减小,不同粒径团聚体中Cu、Cd、Pb和As含量与有机碳含量均呈显著正相关(p0.05)。污灌区菜地土壤Cr、As在5～8mm粒径团聚体中富集现象显著,Cu、Zn、Cd、Pb在0.25～0.5mm粒径团聚体中均表现出显著的富集特征。6种重金属在5～8mm粒径团聚体中的质量负载因子均为最高,表明6种重金属在5～8mm粒径团聚体中的贡献最大。     

NMR spectrocopy of organolithium compounds,part XXXIV: Cyclopropyllithium and lithium bromide (1:1) in diethylether/tetrahydrofuran—Identification of a fluxional mixed tetramer

Cyclopropyllithium, C₃H₅Li ( 1 ), was studied in the presence of one equivalent lithium bromide (LiBr) in diethylether (DEE)/tetrahydrofuran (THF) mixtures and in THF as solvents. Increasing the THF concentration in DEE/THF leads in the ⁶Li NMR spectrum to a main signal (S1) at δ0.85 (rel. to ext. LiBr/THF) and a second resonance (S2) at δ0.26 aside from a minor component at δ0.07. In pure THF, the ratio of these signals was 66: 28:6. ⁶Li and ¹³C NMR allowed to identify the main signal as belonging to a mixed dimer, 1 •LiBr, and the signal at 0.26 ppm to a fluxional mixed tetramer, 1 ₂•(LiBr)₂. ¹J(¹³C,⁶Li) coupling constants of 11.0 and 9.8 Hz were measured at 168 K for S1 and S2, respectively, and chemical exchange between both signals was detected by 2D ⁶Li,⁶Li exchange spectroscopy and analyzed by temperature-dependent 1D ⁶Li line-shape calculations. These yielded the equilibrium constants K_eq for the chemical exchange Li₄(C₃H₅)₂Br₂ ⇌ 2 Li₂C₃H₅Br. Their temperature dependence leads to van't Hoff parameters of ΔH° = 4.6 kJ/mol, ΔS° = 41.4 J/mol K, and ΔG°₂₉₈ = −7.8 kJ/mol. From the rate constants k, Eyring parameters of ΔH^* = 42.0 kJ/mol, ΔS^* = 33.0 J/mol K, and ΔG^*₂₉₈ = 32.2 kJ/mol were calculated for the forward reaction Li₄(C₃H₅)₂Br₂ → 2 Li₂C₃H₅Br and ΔH^* = 37.5 kJ/mol, ΔS^* = −8.4 J/mol K, and ΔG^*₂₃₈ = 40.0 kJ/mol for the reverse reaction 2Li₂C₃H₅Br → Li₄(C₃H₅)₂Br₂.     

砂浆专用改性剂WD对湿拌砂浆的影响

研究了砂浆专用改性剂WD与细骨料对湿拌砂浆稠度保持、保水率、收缩率、抗压强度、粘结强度等影响。结果表明：掺入专用改性剂WD的湿拌砂浆的粘结强度、稠度保持、保水率等性能明显优于未改性空白砂浆,可保持施工性能24 h以上;细骨料细度模数增大,比表面积变小,胶凝材料更好地包裹细骨料;细颗粒砂改善了砂浆保水性,但细度模数过小会导致砂浆收缩率增加并降低抗压强度;细骨料细度模数2.5时湿拌砂浆综合性能较佳。     

SERS microRaman spectral probing of adsorbate‐containing,liquid‐overlayed nanosponge Ag aggregates assembled from fractal aggregates

Adsorbate‐containing, nanosponge Ag aggregates overlayed by a thin (~1.5 mm) liquid layer are reported as a new type of sample for Surface‐enhanced Raman scattering (SERS) microRaman spectral measurements and adsorbate (analyte) detection. Macroscopic Ag aggregates (of about 1.5 × 1.0 × 0.025 mm size) with the nanosponge internal morphology (revealed by Scanning electron microscopy (SEM)) were prepared by 3D assembling of fused fractal aggregates (D = 1.84 ± 0.04) formed in Ag nanoparticle hydrosol/HCl/adsorbate systems with 2,2’‐bipyridine (bpy) and/or a cationic free‐base tetrakis(2‐methyl‐4‐pyridiniumyl) porphine (H₂TMPyP) as the testing adsorbates. For SERS microRaman measurements, the macroscopic aggregate was overlayed by a thin (~1.5 mm) layer of the residual liquid. Preparation procedure, nanoscale imaging, and SERS spectral probing including the determination of the detection limits of the adsorbates revealed the following advantages of the adsorbate‐containing, liquid‐overlayed 3D nanosponge aggregate as a sample for SERS microRaman spectral measurements: (1) localization of adsorbate (analyte) into hot spots and, simultaneously, prevention of the analyte decomposition during the spectral measurement (carried out without an immersion objective), (2) fast and simple sample preparation, and (3) minimization of sample volume and an efficient concentration of hot spots into the focus of the laser beam. The advantages of the nanosponge Ag aggregates are further demonstrated by the 40 fmol limit of detection of bpy as Ag(0)‐bpy surface complex, as well as by preservation of the native structure of the cationic free‐base porphyrin H₂TMPyP. Copyright © 2015 John Wiley & Sons, Ltd.     

表面活性素单分子膜在空气/水界面的迟滞现象

表面活性素是一类具有较强表面活性的微生物脂肽类化合物,能在空气/水界面形成不溶性单分子膜.利用Langmuir膜天平测定了表面活性素单分子膜的压缩-扩张循环曲线,发现单分子膜在经历了“平台区”后出现较大的迟滞环,迟滞环的形状与亚相pH有关.将“平台区”的单分子膜转移到云母表面后,用原子力显微镜(AFM)和扫描电子显微镜(SEM)均观察到高度达几十至数百纳米的表面聚集体,说明表面活性素在单分子膜的“平台区”伴随着自聚集.研究结果表明,表面活性素单分子膜在空气/水界面的迟滞现象是分子浸入亚相和形成三维表面聚集体共同作用的结果.     

Effect of surfactants on the aggregation of pyronin B and pyronin Y in aqueous solution

Molecular dynamics of pyronin B (PyB) and pyronin Y (PyY) in aqueous solution containing different surfactants were investigated by using absorption and fluorescence spectroscopy techniques. First, the interactions of PyB and PyY with the negatively charged surfactant sodium dodecyl sulphate (SDS) were investigated in the below and above critical micelle concentration (cmc). The H-aggregate formation of the dye compounds was observed for below the cmc of SDS surfactant. The absorbance of H-aggregate absorption band of PyB and PyY decreased according to the aggregate-monomer equilibrium by increasing SDS surfactant concentration towards the cmc. Therefore, equilibrium constants of the aggregate formation and oscillator strengths of monomer and aggregate of the dye compounds were calculated from spectral studies. Moreover, aggregate formation dynamics was discussed in terms of thermodynamic functions by using temperature studies. The interactions of PyB and PyY with the positively charged hexadecyltrimethylammonium bromide (CTAB) and neutral Triton X-100 (TX-100) were also studied and it was observed that there was no aggregate formation on the absorption and fluorescence spectra for below and above the cmc.