Mapping potential energy surfaces

A recently proposed dynamical method [A. Laio and M. Parrinello, Proc. Natl. Acad. Sci. U.S.A. 99, 12562 (2002)] allows us to globally sample the free energy surface. This approach uses a coarse-grained non-Markovian dynamics to bias microscopic atomic trajectories. After a sufficiently long simulation time, the global free energy surface can be reconstructed from the non-Markovian dynamics. Here we apply this scheme to study the T=0 free energy surface, i.e., the potential energy surface in coarse-grained space. We show that the accuracy of the reconstructed potential energy surface can be dramatically improved by a simple postprocessing procedure with only minor computational overhead. We illustrate this approach by conducting conformational analysis on a small organic molecule, demonstrating its superiority over traditional unbiased approaches in sampling potential energy surfaces in coarse-grained space.     

Improvement in enzyme activity and stability by addition of low molecular weight polyethylene glycol to sodium bis(2-ethyl-L-hexyl)sulfosuccinate/isooctane reverse micellar system

The activity and stability of Chromobacterium viscosum lipase (glycerolester hydrolase, EC 3.1.1.3)-catalyzed olive oil hydrolysis in sodium bis (2-ethyl-1-hexyl)sulfosuccinate (AOT)/isooctane reverse micelles is increased appreciably when low molecular weight polyethylene glycol (PEG 400) is added to the reverse micelles. To understand the effect of PEG 400 on the phase behavior of the reverse micellar system, the phase diagram of AOT/PEG 400/water/isooctane system was studied. The influences of relevant parameters on the catalytic activity in AOT/PEG 400 reverse micelles were investigated and compared with the results in the simple AOT reverse micelles. In the presence of PEG 400, the linear decreasing trend of the lipase activity with AOT concentration, which is observed in the simple AOT reverse micelles, disappeared. Enzyme entrapped in AOT/PEG reverse micelles was very stable, retaining>75% of its initial activity after 60 d, whereas the half-life in simple AOT reverse micelles was 38 d. The kinetics parameter maximum velocity (V _max)exhibiting the temperature dependence and the activation energy obtained by Arrhenius plot was suppressed significantly by the addition of PEG 400.     

Ionization potential of excited states of Be‐like sequence in the concept of iso‐spectrum‐level series

With the introduction of the concept of the iso‐spectrum‐level series, a linear relationship is found between the first differences of the ionization potential of excited states and nuclear charge Z along an iso‐spectrum‐level series, and the ionization potential of excited states of Be‐like sequence are studied systematically on the basis of the weakest bound electron potential model theory. The expression of nonrelativistic ionization potential is derived from the weakest bound electron potential model theory, and relativistic effects are included by using a fourth‐order polynomial in Z. As a demonstration, the ionization potentials of [He]2s2p ³P, [He]2s3s ¹S₀, [He]2s3p ¹P, [He]2s3d ¹D₂, and [He]2s4d ¹D₂ series for a range of Be‐like sequence from Z = 4–23 are calculated. The results are compared with the experimental data and the recent sophisticated ab initio results. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 344–350, 2003     

Constituents from Rubia Ustulata Diels and R. Yunnanensis Diels and Their Antiplatelet Aggregation Activity

A new anthraquinone glycoside, rubiayannone‐A ( 1 ), and a new coumarin, rubilatin‐A ( 2 ), together with twenty‐two known compounds were isolated and characterized from the roots of Rubia ustulata. A new anthraquinone, 2‐carbomethoxyanthraquinone ( 3 ), and rubiayannone‐A, 2‐formylanthraquinone were obtained from the roots of R. yunnanensis. The structures of those compounds were elucidated by spectroscopic methods. The antiplatelet aggregation activity of the isolated compounds 1, 4～6 were also discussed.     

Novel approach to synthesizing hydrophobically associating copolymer using template copolymerization: the synthesis and behaviors of acrylamide and 4-(omega-propenoyloxyethoxy) benzoic acid copolymer

Hydrophobically associating copolymers of acrylamide (AM) with a small amount of 4-(omega-propenoyloxyethoxy) benzoic acid (PEBA, <2.5%) were synthesized by template copolymerization in the presence of poly(allylammonium chloride) (PAAC) as a template in an aqueous medium. These template copolymers exhibited remarkable thickening properties due to the effective hydrophobic association, which were similar to those copolymers with a multiblock structure obtained by the micellar process. The pH of the reaction medium and the molecular weight of the template strongly influenced the thickening properties of the products. In the experimental range, the higher the PEBA content, the larger the thickening capacity of these hydrophobic copolymers. The aggregate behaviors of these copolymers were studied by fluorescence, transmission electron microscopy (TEM), and light scattering techniques. The apparent critical interpolymer aggregate concentration (cac) of the copolymer solution was about 0.5 g/dL. As the concentration of the copolymer became higher than the cac, the aggregates changed their morphology from small hollow spheres to big flower-shaped aggregates. All the above results indicated that the template copolymerization gave access to a very simple and powerful means for the preparation of hydrophobically associating copolymers and other functional polymer materials.     

New pavine N-oxide alkaloids from the stem bark of Cryptocarya chinensis Hemsl

Three new pavine N-oxide alkaloids, (-)-isocaryachine-N-oxide B, (+)-caryachine-N-oxide, (-)-caryachine-N-oxide, and a new isoquinoline alkaloid, 6,7-methylenedioxy-N-methylisoquinoline together with 11 known alkaloids were isolated and characterized from the stem bark of Cryptocarya chinensis. The structures of the isolated compounds were determined by spectral methods. The stereochemistry of pavine-N-oxide alkaloids is also discussed.     

Activated carbon oxygen content influence on water and surfactant adsorption

This research investigates the adsorption properties of three activated carbons (AC) derived from coconut, coal, and wood origin. Each carbon demonstrates different levels of resistance to 2 M NaOH treatment. The coconut AC offers the greatest and wood AC the least resistance. The influence of base treatment is mapped in terms of its effects on specific surface area, micropore volume, water adsorption, and dodecanoic acid adsorption from both water and 2 M NaOH solution. A linear relationship exists between the number of water molecules adsorbed at the B-point of the water adsorption isotherm and the oxygen content determined from elemental analysis. Surfactant adsorption isotherms from water and 2 M NaOH indicate that the AC oxygen content effects a greater dependence on affinity for surfactant than specific surface area and micropore volume. We show a linear relationship between the plateau amount of surfactant adsorbed and the AC oxygen content in both water and NaOH phases. The higher the AC oxygen content, the lower the amount of surfactant adsorbed. In contrast, no obvious relationship could be drawn between the surfactant amount adsorbed and the surface area.     

Synthesis and chiroptical analysis of optically active chiral shell dendrons

We have prepared a series of chiral dendrons (1-4) in which chiral subunits are placed in individual generational shells at varying distances from the focal point. The optical activity of these chiral dendritic structures is successfully modeled using structurally similar low-molecular weight model compounds. In dendrons 1a and 1b a chiral subunit is directly adjacent to the focal point, whereas in dendrons 2, 3, and 4a,b the chiral subunits are incorporated in the interior of the dendron. A marked difference in optical activity between the former 1a and 1b) and latter (2, 3, 4a,b) dendrons is mirrored in the optical activities of model compounds 12a, 12b, 19a, and 19b. These model compounds directly mimic the surrounding constitution of the chiral subunits in the dendrons. This successful analysis of the chiroptical data using low-molecular weight model compounds suggests that these dendrons do not possess conformational order in solution.     

Reactions of the Dirhenium(II) Complexes Re2 X 4(μ-dppm)2 (X=Cl, Br; dppm=Ph2PCH2PPh2) with Isocyanides. Part XV: A Fifth Structural Isomer of the [Re2Cl3(μ-dppm)2(CO)(CNXyl)2]+ Cation (Xyl = 2, 6-Dimethylphenyl)

The reaction of the unsymmetrical, coordinatively unsaturated dirhenium(II) complex [(XylNC)(OC)CIRe(μ-dppm)₂ReCl₂]O₃SCF₃ (dppm = Ph₂PCH₂PPh₂) with one equivalent of XylNC in CH₂Cl₂ affords a fifth structural isomer of the [Re₂Cl₃(μ-dppm)₂(CO)(CNXyl)₂] ⁺ cation; this is believed to have a CO-bridged structure of the type [(XylNC)ClRe(μ-Cl)(μ-CO)(μ-dppm)₂ReCl(CNXyl)]⁺. The latter complex reacts with a further equivalent of XylNC in the presence of Tl⁺ to form the [Re₂Cl₂(μ-dppm)₂(CO)(CNXyl)₃]²⁺ cation, which has been shown by IR spectroscopy, and by the X-ray crystallographic characterization of its neutral congener Re₂Cl₂(μ-dppm)₂(CO)(CNXyl)₃, to contain a very weak and unsymmetrical CO bridge.     

羟乙基皂仁胶水溶胶和凝胶体系流变性的研究——不含交联剂溶胶体系的粘度

研究了羟乙基皂仁胶水溶胶体系的粘度对浓度、温度和切变速率的依赖性。溶胶的表观粘度进行了非牛顿校正,其表观粘度的切变速率依赖性可用幂律模型描述。采用Spencer-Dillon方程求得的体系零切粘度与浓度的关系可用Onogi公式描述。对于溶剂和较稀溶液,温度对粘度的影响可用Andrade公式描述,但对较浓溶液,此式不再适用。