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81.
82.
Sando L Henriques ST Foley F Simonsen SM Daly NL Hall KN Gustafson KR Aguilar MI Craik DJ 《Chembiochem : a European journal of chemical biology》2011,12(16):2456-2462
Featuring a circular, knotted structure and diverse bioactivities, cyclotides are a fascinating family of peptides that have inspired applications in drug design. Most likely evolved to protect plants against pests and herbivores, cyclotides also exhibit anti-cancer, anti-HIV, and hemolytic activities. In all of these activities, cell membranes appear to play an important role. However, the question of whether the activity of cyclotides depends on the recognition of chiral receptors or is primarily modulated by the lipid-bilayer environment has remained unknown. To determine the importance of lipid membranes on the activity of the prototypic cyclotide, kalata B1, we synthesized its all-D enantiomer and assessed its bioactivities. After the all-D enantiomer had been confirmed by (1)H NMR to be the structural mirror image of the native kalata B1, it was tested for anti-HIV activity, cytotoxicity, and hemolytic properties. The all-D peptide is active in these assays, albeit with less efficiency; this reveals that kalata B1 does not require chiral recognition to be active. The lower activity than the native peptide correlates with a lower affinity for phospholipid bilayers in model membranes. These results exclude a chiral receptor mechanism and support the idea that interaction with phospholipid membranes plays a role in the activity of kalata B1. In addition, studies with mixtures of L and D enantiomers of kalata B1 suggested that biological activity depends on peptide oligomerization at the membrane surface, which determines affinity for membranes by modulating the association-dissociation equilibrium. 相似文献
83.
Kristopher White Nikki Lorenz Tom Potts W. Roy Penney Robert Babcock Amber Hardison Elizabeth A. Canuel Jamie A. Hestekin 《Fuel》2011,90(11):3193-3199
Tall oil fatty acids are a byproduct of the paper industry and consist predominantly of free-fatty acids (FFAs). Although this feedstock is ideal for biodiesel production, there has been relatively little study of its conversion to biodiesel. Thus, the purpose of this study was to investigate the high temperature reaction of methanol with tall oil at subcritical and supercritical pressures to produce fatty acid methyl esters. This study investigates the effects of mixing, pressure, temperature, and methanol to oil molecular ratio in order to determine the potential use of tall oil as a biodiesel feedstock. In this work, tall oil fatty acids were successfully reacted with supercritical and subcritical methanol in a continuous tubular reactor, resulting in a reaction that is primarily temperature dependent. Conversions at subcritical pressures of 4.2 MPa and 6.6 MPa were 81% and 75%, respectively. Pressure seemed to have little correlation to conversion in both regimes, and conversions were comparable between the two. Additionally, it was found that tall oil fatty acids react well with methanol to give comparable conversions at the relatively low molecular flow ratio of 5:1 methanol to tall oil. Both of these observations suggest that hydrolyzed triglycerides or free fatty acid feedstocks would make the primary high temperature biodiesel reaction and the subsequent separation and purification operations less expensive than was previously believed. 相似文献
84.
Christopher C. M. Neumann Eduardo Laborda Kristina Tschulik Kristopher R. Ward Richard G. Compton 《Nano Research》2013,6(7):511-524
The electrocatalytic activity for oxygen reduction reaction (ORR) at neutral pH of citrate-capped silver nanoparticles (diameter = 18 nm) supported on glassy carbon (GC) is investigated voltammetrically. Novelly, the modification of the substrate by nanoparticles sticking to form a random nanoparticle array and the voltammetric experiments are carried out simultaneously by immersion of the GC electrode in an air-saturated 0.1 M NaClO4 solution (pH = 5.8) containing chemically-synthesized nanoparticles. The experimental voltammograms of the resulting nanoparticle array are simulated with homemade programs according to the two-proton, two-electron reduction of oxygen to hydrogen peroxide where the first electron transfer is rate determining. In the case of silver electrodes, the hydrogen peroxide generated is partially further reduced to water via heterogeneous decomposition. Comparison of the results obtained on a silver macroelectrode and silver nanoparticles indicates that, for the silver nanoparticles and particle coverages (0.035%–0.457%) employed in this study, the ORR electrode kinetics is slower and the production of hydrogen peroxide larger on the glassy carbon-supported nanoparticles than on bulk silver. 相似文献
85.
86.
An optical sensor was developed with the capability of detecting liquid water in the flow channels of a proton exchange membrane fuel cell (PEMFC) as well as simultaneously measuring temperature. This work is an extension of previous research in which an optical temperature sensor was developed for measuring the in situ temperature of PEM fuel cells based on the principles of phosphor thermometry. The optical sensor was installed in the cathode flow channel of a 5 cm2 proton exchange membrane fuel cell. The fuel cell was tested under both dry and humid conditions. Liquid water formation in the flow channels was quantitatively measured from the experimental data. An observed time fraction value was estimated for characterizing flow channel flooding. The observed time fraction of liquid water in the flow channel was found to be closely related to the relative humidity of reactants and the operating current of the fuel cell. 相似文献
87.
88.
Bauer Daniel J.; Preacher Kristopher J.; Gil Karen M. 《Canadian Metallurgical Quarterly》2006,11(2):142
The authors propose new procedures for evaluating direct, indirect, and total effects in multilevel models when all relevant variables are measured at Level 1 and all effects are random. Formulas are provided for the mean and variance of the indirect and total effects and for the sampling variances of the average indirect and total effects. Simulations show that the estimates are unbiased under most conditions. Confidence intervals based on a normal approximation or a simulated sampling distribution perform well when the random effects are normally distributed but less so when they are nonnormally distributed. These methods are further developed to address hypotheses of moderated mediation in the multilevel context. An example demonstrates the feasibility and usefulness of the proposed methods. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
89.
A unified theory of the Laplace-transform analytic-element method (LT-AEM) for solving transient porous-media flow problems
is presented. LT-AEM applies the analytic-element method (AEM) to the modified Helmholtz equation, the Laplace-transformed
diffusion equation. LT-AEM uses superposition and boundary collocation with Laplace-space convolution to compute flexible
semi-analytic solutions from a small collection of fundamental elements. The elements discussed are derived using eigenfunction
expansions of element shapes in their natural coordinates. A new formulation for a constant-strength line source is presented
in terms of elliptical coordinates and complex-parameter Mathieu functions. Examples are given illustrating how leaky and
damped-wave hydrologic problems can be solved with little modification using existing LT-AEM techniques. 相似文献
90.
Zhen Xiang Geb W. Thomas Kristopher M. Thornburg Nathalie Cabrol Edmond Grin Robert C. Anderson 《Journal of Intelligent and Robotic Systems》2009,54(4):595-612
When remotely navigating a mobile robot, operators must estimate the slope of local terrain in order to avoid areas that are
too steep to climb or that slope so steeply downward that the operator would lose control of the rover. Although many rovers
are equipped with sensor systems to aid the operator in this task, it is sometimes necessary to estimate slopes from two-dimensional
images, either when planning operations or when the operator wishes to monitor the results of a sensor system. This experiment
compares the operator’s estimates of the slope in Martian terrain with the actual slope determined from three-dimensional
data. The ten participants overestimated the slope of the indicated regions by an average of 19° (SD 16°). An analytic model
of the error, based on psychophysical analysis, accurately predicts the average magnitude of the errors. Implementation of
this model eliminates an average amount of participant error. However, the large estimate variance within and between participants
and images still poses a challenge for accurate slope estimation. 相似文献