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291.
Bispo MS Veloso MC Pinheiro HL De Oliveira RF Reis JO De Andrade JB 《Journal of chromatographic science》2002,40(1):45-48
This work relates the development of an analytical methodology to simultaneously determine three methylxanthines (caffeine, theobromine, and theophylline) in beverages and urine samples based on reversed-phase high-performance liquid chromatography. Separation is made with a Bondesil C18 column using methanol-water-acetic acid or ethanol-water-acetic acid (20:75:5, v/v/v) as the mobile phase at 0.7 mL/min. Identification is made by absorbance detection at 273 nm. Under optimized conditions, the detection limit of the HPLC method is 0.1 pg/mL for all three methylxanthines. This method is applied to urine and to 25 different beverage samples, which included coffee, tea, chocolate, and coconut water. The concentration ranges determined in the beverages and urine are: < 0.1 pg/mL to 350 microg/mL and 3.21 microg/mL to 71.2 microg/mL for caffeine; < 0.1 pg/mL to 32 microg mL and < 0.1 pg/mL to 13.2 microg/mL for theobromine; < 0.1 pg/mL to 47 microg/mL and < 0.1 pg/mL to 66.3 microg/mL for theophylline. The method proposed in this study is rapid and suitable for the simultaneous quantitation of methylxanthines in beverages and human urine samples and requires no extraction step or derivatization. 相似文献
292.
Dvornikovs V House BE Kaetzel M Dedman JR Smithrud DB 《Journal of the American Chemical Society》2003,125(27):8290-8301
Host-[2]rotaxanes, containing a diarginine-derivatized dibenzo-24-crown-8 (DB24C8) ether as the ring and a cyclophane pocket or an aromatic cleft as one blocking group, are cell transport agents. These hosts strongly associate with a variety of amino acids, dipeptides, and fluorophores in water (1 mM phosphate buffer, pH 7.0), DMSO, and a 75/25 (v/v) buffer to DMSO solution. All peptidic guests in all solvent systems have association constants (K(A)'s) in the range of 1 x 10(4) to 5 x 10(4) M(-)(1), whereas the K(A) range for the fluorophores is 1 x 10(4) to 9 x 10(5) M(-)(1). Association constants for the cyclophane itself, cyclophane 3, are smaller. These values are in the 1 x 10(3) to 5 x 10(3) M(-)(1) range, which shows that the rotaxane architecture is advantageous for guest binding. Cyclophane-[2]rotaxane 1 efficiently transports fluorescein and a fluorescein-protein kinase C (PKC) inhibitor into eukaryotic COS-7 cells, including the nucleus. Interestingly, cleft-[2]rotaxane 2 does not transport fluorescein as efficiently, even though the results from the fluorescence assays show that both [2]rotaxanes bind fluorescein with the same ability. 相似文献
293.
A new computer program called MoCalc (Molecular Calculations) has been designed to help the computational chemistry practitioner in the task of performing and analyzing molecular calculations. MoCalc is a graphical user interface for the MO calculation programs Gamess and Mopac, and uses Rasmol and Babel for molecule display and file conversion, respectively. In its initial version, MoCalc can execute the following operations: (a) create and handle Gamess and Mopac input files; (b) import any kind of molecular geometry supported by Babel and paste it as Cartesian, internal, or Gaussian-type coordinates on the input file; (c) convert Gamess and Mopac output files to inputs of both programs; (d) edit and validate the keywords that control the Gamess and Mopac calculation procedure; (e) display the input (Mopac) and output (Gamess and Mopac) molecular geometries; (f) run single or multiple (batch) calculations, either interactively or in background; (g) automatically open the output files as soon as the calculation finishes; (h) extract results from the output files, such as energy, charges, dipole, population analysis, wave function, bond orders, and valence analysis, and display them in spreadsheets; (i) calculate reactivity indices derived from the frontier orbital theory and the root-mean-square (rms) deviation of input and output geometries. All the results generated by MoCalc can be promptly transferred to text editors and electronic spreadsheets, which facilitate a detailed subsequent analysis and the publication of the results. MoCalc can also perform graphical and numerical comparative analysis of the some results when more than one output file is loaded. The program was coded in Visual Basic and runs in Windows 95/98/NT4/ME/2000/XP environments. 相似文献
294.
Marcia M. de O. Buanafina Tim Langdon Barbara Hauck Sue J D Alton Phil Morris 《Applied biochemistry and biotechnology》2006,130(1-3):416-426
In grass cell walls, ferulic acid esters linked to arabinosyl residues in arabinoxylans play a key role in crosslinking hemicellulose.
Although such crosslinks have a number of important roles in the cell wall, they also hinder the rate and extent of cell wall
degradation by ruminant microbes and by fungal glycohydrolyase enzymes. Ferulic acid esterase (FAE) can release both monomeric
and dimeric ferulic acids from arabinoxylans making the cell wall more susceptible to further enzymatic attack. Transgenic
plants of Lolium multiflorum expressing a ferulic acid esterase gene from Aspergillus niger, targeted to the vacuole under a constitutive rice actin promoter, have been produced following microprojectile bombardment
of embryogenic cell cultures. The level of FAE activity was found to vary with leaf age and was highest in young leaves. FAE
expression resulted in the release of monomeric and dimeric ferulic acids from cell walls on cell death and this was enhanced
severalfold by the addition of exogenous β-1,4-endoxylanase. We also show that a number of plants expressing FAE had reduced
levels of cell wall esterified monomeric and dimeric ferulates and increased in vitro dry-matter digestibility compared with
nontransformed plants. 相似文献
295.
Roger Bishop Gary a. Downing Donald C. Craig Marcia L. Scudder 《Journal of inclusion phenomena and macrocyclic chemistry》1998,31(2):145-160
The alicyclic diol 2 is known to adopt five different types of crystallattice: the pure solid, cocrystalline compounds with water or certainphenols, and two different types of lattice inclusion system (helicaltubulates and ellipsoidal clathrates). Self-resolution only occurs onforming the helical tubulate inclusion compounds. Its close analogue2,7-bis(trifluoromethyl)tricyclo[4.3.1.13,8]un-decane-syn-2,syn-7-diol 4 was synthesised to examine the influence ofreplacing CH3- by CF3-groups. It no longer formslattice inclusion compounds but does form a cocrystalline solid withdimethyl sulfoxide (DMSO). Crystal structures of 4[C13H16O2F6,P21/c, a 7.8636(6), b 13.1020(7), c 25.319(2) Å, 101.526(4)°, Z 8, R 0.044] and (4)V_2.VD/MSO[(C13H16O2F6)2⋅C2H6SO,P212121, a 7.249(1), b 16.064(3),c 25.347(4) Å, Z 4, R 0.033] were determined. In solid diol 4 themolecules are linked through (–-O–-H)4 rings toproduce layers of chirally pure enantiomers but the net crystal structure,which comprises layers of alternating handedness, is achiral. In contrast,complexation of 4 with DMSO by means of two -–O-–H.O=Shydrogen bonds induces complete enantiomeric self-resolution. 相似文献
296.
For one-dimensional unimodal mapsh
(x):I I, whereI=[x
0,x
1] when =max, a binary tree which includes all the periodic windows in the chaotic regime is constructed. By associating each element in the tree with the superstable parameter value of the corresponding periodic interval, we define a different unimodal map. After applying a certain renormalization procedure to this new unimodal map, we find the period-doubling fixed point and the scaling constant. The period-doubling fixed point depends on the details of the maph
(x), whereas the scaling constant equals the derivative
. The thermodynamics and the scaling function of the resulting dynamical system are also discussed. In addition, the total measure of the periodic windows is calculated with results in basic agreement with those obtained previously by Farmer. Up to 13 levels of the tree have been included, and the convergence of the partial sums of the measure is shown explicitly. A new scaling law has been observed, i.e., the product of the length of a periodic interval characterized by sequenceQ and the scaling constant ofQ is found to be approximately 1. 相似文献
297.
Gabriel Rocha Martins Douglas Guedes Urbano Luiz Marques de Paula Maria do Socorro Padilha de Oliveira Marcia Teresa Soares Lutterbach Leila Yone Reznik Eliana Flvia Camporese Srvulo Celuta Sales Alviano Antonio Jorge Ribeiro da Silva Daniela Sales Alviano 《Molecules (Basel, Switzerland)》2021,26(11)
Euterpe oleracea Mart. (Arecaceae) is an endogenous palm tree from the Amazon region. Its seeds correspond to 85% of the fruit’s weight, a primary solid residue generated from pulp production, the accumulation of which represents a potential source of pollution and environmental problems. As such, this work aimed to quantify and determine the phytochemical composition of E. oleracea Mart. seeds from purple, white, and BRS-Pará açaí varieties using established analytical methods and also to evaluate it as an eco-friendly corrosion inhibitor. The proanthocyanidin quantification (n-butanol/hydrochloric acid assay) between varieties was 6.4–22.4 (w/w)/dry matter. Extract characterization showed that all varieties are composed of B-type procyanidin with a high mean degree of polymerization (mDP ≥ 10) by different analytical methodologies to ensure the results. The purple açaí extract, which presented 22.4% (w/w) proanthocyanidins/dry matter, was tested against corrosion of carbon steel AISI 1020 in neutral pH. The crude extract (1.0 g/L) was effective in controlling corrosion on the metal surface for 24 h. Our results demonstrated that the extracts rich in polymeric procyanidins obtained from industrial açaí waste could be used to inhibit carbon steel AISI 1020 in neutral pH as an abundant, inexpensive, and green source of corrosion inhibitor. 相似文献
298.
299.
An important phenomenon in supply chain management, known as the bullwhip effect, suggests that demand variability increases as one moves up a supply chain. This paper examines the influence of different replenishment policies on the occurrence of the bullwhip effect. The paper demonstrates that certain replenishment policies can in themselves be inducers of the bullwhip effect, while others inherently lower demand variability. The main causes of increase in variability are projections of future demand expectations, which result in over-exaggerated responses to changes in demand. We suggest that through appropriate selection and use of certain replenishment rules, the bullwhip effect can be avoided, subsequently allowing supply chain management costs to be lowered. 相似文献
300.
Roummel F. Marcia Julie C. Mitchell J. Ben Rosen 《Computational Optimization and Applications》2005,32(3):285-297
An algorithm for finding an approximate global minimum of a funnel shaped function with many local minima is described. It
is applied to compute the minimum energy docking position of a ligand with respect to a protein molecule. The method is based
on the iterative use of a convex, general quadratic approximation that underestimates a set of local minima, where the error
in the approximation is minimized in the L1 norm. The quadratic approximation is used to generate a reduced domain, which is assumed to contain the global minimum of
the funnel shaped function. Additional local minima are computed in this reduced domain, and an improved approximation is
computed. This process is iterated until a convergence tolerance is satisfied. The algorithm has been applied to find the
global minimum of the energy function generated by the Docking Mesh Evaluator program. Results for three different protein
docking examples are presented. Each of these energy functions has thousands of local minima. Convergence of the algorithm
to an approximate global minimum is shown for all three examples. 相似文献