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Linear Fit (linear + fit)
Selected AbstractsBacterial metabolism in small temperate streams under contemporary and future climatesFRESHWATER BIOLOGY, Issue 12 2007KAJ SAND-JENSEN Summary 1. We examined the detailed temperature dependence (0,40 °C) of bacterial metabolism associated with fine sediment particles from three Danish lowland streams to test if temperature dependence varied between sites, seasons and quality of organic matter and to evaluate possible consequences of global warming. 2. A modified Arrhenius model with reversible denaturation at high temperatures could account for the temperature dependence of bacterial metabolism and the beginning of saturation above 35 °C and it was superior to the unmodified Arrhenius model. Both models overestimated respiration rates at very low temperatures (<5 °C), whereas Ratkowsky's model , the square root of respiration , provided an excellent linear fit between 0 and 30 °C. 3. There were no indications of differences in temperature dependence among samples dominated by slowly or easily degradable organic substrates. Optimum temperature, apparent minimum temperature, Q10 -values for 0,40 °C and activation energies of bacterial respiration were independent of season, stream site and degradability of organic matter. 4. Q10 -values of bacterial respiration declined significantly with temperature (e.g. 3.31 for 5,15 °C and 1.43 for 25,35 °C) and were independent of site and season. Q10 -values of bacterial production behaved similarly, but were significantly lower than Q10 -values of respiration implying that bacterial growth efficiency declined with temperature. 5. A regional warming scenario for 2071,2100 (IPCC A2) predicted that mean annual temperatures will increase by 3.5 °C in the air and 2.2,4.3 °C in the streams compared with the control scenario for 1961,1990. Temperature is expected to rise more in cool groundwater-fed forest springs than in open, summer-warm streams. Mean annual bacterial respiration is estimated to increase by 26,63% and production by 18,41% among streams assuming that established metabolism,temperature relationships and organic substrate availability remain the same. To improve predictions of future ecosystem behaviour, we further require coupled models of temperature, hydrology, organic production and decomposition. [source] Accurate prediction of proton chemical shifts.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2001Abstract Forty-five proton chemical shifts in 14 aromatic molecules have been calculated at several levels of theory: Hartree,Fock and density functional theory with several different basis sets, and also second-order Møller,Plesset (MP2) theory. To obtain consistent experimental data, the NMR spectra were remeasured on a 500 MHz spectrometer in CDCl3 solution. A set of 10 molecules without strong electron correlation effects was selected as the parametrization set. The calculated chemical shifts (relative to benzene) of 29 different protons in this set correlate very well with the experiment, and even better after linear regression. For this set, all methods perform roughly equally. The best agreement without linear regression is given by the B3LYP/TZVP method (rms deviation 0.060 ppm), although the best linear fit of the calculated shifts to experimental values is obtained for B3LYP/6-311++G**, with an rms deviation of only 0.037 ppm. Somewhat larger deviations were obtained for the second test set of 4 more difficult molecules: nitrobenzene, azulene, salicylaldehyde, and o -nitroaniline, characterized by strong electron correlation or resonance-assisted intramolecular hydrogen bonding. The results show that it is possible, at a reasonable cost, to calculate relative proton shieldings in a similar chemical environment to high accuracy. Our ultimate goal is to use calculated proton shifts to obtain constraints for local conformations in proteins; this requires a predictive accuracy of 0.1,0.2 ppm. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1887,1895, 2001 [source] Radiotracer-based method for determining water solubility of highly insoluble compoundsJOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 9 2006Ketai Wang Abstract Ascertaining the aqueous solubility of compounds is important in the selection of drug candidates. We describe a radiotracer method for estimating water solubility of compounds that are soluble in dimethyl sulfoxide (DMSO). Various volumes of DMSO, saturated with 127I-labeled compound and spiked with the corresponding 125I-labeled derivative, are mixed in de-ionized water and the tubes centrifuged to remove insoluble material. Since (i) the iodine-127 and iodine-125 compounds have the same solubilities and are equally distributed in the DMSO,water solution, and (ii) the nonradioactive compound is accurately weighed, dissolved in a known volume of DMSO, and then further diluted as required, the concentration of compound in solution can be calculated and plotted as a function of the DMSO-to-water ratio. Water solubility of the compound is then determined by extrapolation of the linear fit of data points to zero DMSO. As proof of the methodology, 5-iodo-2,-deoxyuridine (IUdR) and 2-iodo-8-methyl-8H -quino[4,3,2- kl]acridine (IMAc), water-soluble compounds, were assessed using 125IUdR and 125IMAc, respectively. The solubility values obtained by the radiotracer method were similar to those acquired by spectrophotometry. Values calculated for several water-insoluble compounds indicate that the radiotracer method can accurately quantify the solubility of low-molecular-weight compounds (1000,2000 Da) in the pg,ng/ml range. Copyright © 2006 John Wiley & Sons, Ltd. [source] Bacteriochlorophyll e Monomers, but Not Aggregates, Sensitize Singlet Oxygen: Implications for a Self-photoprotection Mechanism in Chlorosomes,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2002Juan B. Arellano ABSTRACT Sensitization of singlet delta oxygen (O2(1,g)) by bacteriochlorophyll e (BChle) has been investigated to gain a better understanding of the photoprotection mechanism(s) operating in chlorosomes of green photosynthetic bacteria. The sensitization process has been studied in media where BChle forms monomers (acetone and aqueous solutions containing 0.5% Triton X-100 [TX]) and in systems where BChle aggregates, namely, aqueous solutions containing 0.003% monogalactosyl diglyceride (MGDG) and chlorosomes (control as well as hexanol perturbed) from Chlorobium phaeobacteroides strain CL1401. In Ar-purged acetone, BChle triplets (BChle,) have a lifetime of a few tens of microseconds; however, in air-saturated acetone, quenching of BChle, by ground-state oxygen (O2(3,,g)) and formation of O2(1,g) take place. The O2(1,g) so formed is susceptible to quenching by BChle0, a ground-state BChle molecule. A Stern,Volmer analysis reveals a linear fit between the decay rate of O2(1,g) and the BChle concentration. The rate constants for the quenching of O2(1,g) by BChle0 and for the deactivation of O2(1,g) by the solvent come out to be kq= (1.4 ± 0.1) × 109 M,1 s,1 and k0= (18.5 ± 0.7) × 103 s,1, respectively. The absolute quantum yield of O2(1,g) sensitization by BChle monomers is 0.65 ± 0.15 in air-saturated acetone. In aqueous phase, the triplet lifetime of BChle aggregates in native or hexanol-perturbed chlorosomes shortens by more than two orders of magnitude when compared with the triplet lifetime of BChle monomers in 0.5% TX solution (a few hundreds of microseconds). Quenching by carotenoids (Car) makes only a minor contribution to the decay of BChle, in aggregates. Because O2(1,g) sensitization by BChle, could be detected neither in MGDG aggregates nor in chlorosomes (control as well as hexanol perturbed), it is concluded that (1) this process is highly likely when BChle is present as a monomer but not when it is tightly packed in artificial aggregates or in chlorosomes; and (2) Car, though vital for the baseplate BChla, are dispensable for BChle. [source] Methylprednisolone Exposure in Pediatric Renal Transplant PatientsAMERICAN JOURNAL OF TRANSPLANTATION, Issue 6 2006P. Seikku Glucocorticoid (GC) dosing is commonly based on body mass or surface area in children, although the drug effects appear to correlate with steroid exposure, rather than dose. We compared the area under the serum concentration,time curve (AUC) of methylprednisolone (MP) with a recombinant cell bioassay measuring serum glucocorticoid bioactivity (GBA), in prediction of side effects in 16 pediatric patients (5.4,18.4 years of age) 2.0,14.9 years after renal transplantation (TX). They received 0.3 mg/kg of MP orally and timed blood samples were drawn up to 8 h postdose. Serum MP concentrations correlated moderately with GBA (r= 0.65, p < 0.0001) with best linear fit at 6 and 8 h (r= 0.72, 0.79, respectively, p < 0.001). MP-AUCt = 0,8 and GBAt = 6 were significantly greater in patients who gained excessive weight soon after TX. Change in growth after TX was inversely correlated with MP-AUC (r= 0.73, p < 0.05) and GBAt = 6 (r= 0.62, p < 0.05). No correlation of MP-AUC or GBA was found with blood glucose or serum lipid concentrations, glomerular filtration rate, bone mineral density or graft histology. In conclusion, GC exposure varies individually and dosing should be adjusted accordingly to control the adverse effects. GBA might provide a complementary tool for monitoring GC exposure but further studies are needed. [source] Independent Effects of Nitrogen Substitution for Oxygen and Yttrium Substitution for Magnesium on the Properties of Mg-Y-Si-Al-O-N GlassesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2003Michael J. Pomeroy Glasses of composition (in equivalent percent) (28 ,x)Mg:xY:56Si:16Al:(100 ,y)O:yN, with x= 0, 14, or 28 for y= 0 and 15 and with x= 0, 7, 14, 21, or 28 for y= 10, were prepared by melting and casting. For glasses where the nitrogen content was varied for a constant cationic ratio, the glass molar volume (MV), compactness (C), Young's modulus (E), glass-transition temperature (Tg), and dilatometric-softening temperature (Tds) varied linearly as the nitrogen content increased, with MV decreasing and the other properties increasing. From the incremental changes in these properties with nitrogen content, for glasses with x= 0, 14, and 28, good linear fits (R2 > 0.99) were obtained, and best-fit slopes are reported here. The property changes and their linearity were consistent with the increased cross-linking of the glass network by tricoordinated nitrogen. The replacement of magnesium by yttrium led to a nonlinear decrease in glass compactness and to nonlinear increases in MV, Tg, and Tds. However, linear correlations were found for MV and ionic volume and for Tg, Tds, and the coordination of (Si,Al)(O,N) tetrahedra of the glass structural units to the modifier cations not involved in charge compensating aluminum ions in fourfold coordination. The replacement of magnesium by yttrium had little effect on Young's modulus, and this result was related to similar changes in the compactness, C. The present results showed that the effects of substituting nitrogen for oxygen and yttrium for magnesium are independent and additive; thus, no synergistic effects of anion and cation substitutions were observed. [source] Prediction and nonparametric estimation for time series with heavy tailsJOURNAL OF TIME SERIES ANALYSIS, Issue 3 2002PETER HALL Motivated by prediction problems for time series with heavy-tailed marginal distributions, we consider methods based on `local least absolute deviations' for estimating a regression median from dependent data. Unlike more conventional `local median' methods, which are in effect based on locally fitting a polynomial of degree 0, techniques founded on local least absolute deviations have quadratic bias right up to the boundary of the design interval. Also in contrast to local least-squares methods based on linear fits, the order of magnitude of variance does not depend on tail-weight of the error distribution. To make these points clear, we develop theory describing local applications to time series of both least-squares and least-absolute-deviations methods, showing for example that, in the case of heavy-tailed data, the conventional local-linear least-squares estimator suffers from an additional bias term as well as increased variance. [source] Molecular Junctions Composed of Oligothiophene Dithiol-Bridged Gold Nanoparticles Exhibiting Photoresponsive PropertiesCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2006Wei Huang Dr. Abstract Three oligothiophene dithiols with different numbers of thiophene rings (3, 6 or 9) have been synthesized and characterized. The X-ray single crystal structures of terthiophene 2 and sexithiophene 5 are reported herein to show the exact molecular lengths, and to explain the difference between their UV-visible spectra arising from the different packing modes. These dithiols with different chain lengths were then treated with 2-dodecanethiol-protected active gold nanoparticles (Au-NPs) by means of in situ thiol-to-thiol ligand exchange in the presence of 1,,m gap Au electrodes. Thus the molecular junctions composed of self-assembled films were prepared, in which oligothiophene dithiol-bridged Au-NPs were attached to two electrodes by means of AuS bonded contacts. The morphologies and current,voltage (I,V) characteristics of these films were studied by SEM and AFM approaches, which suggested that the thickness of the films (3,4 layers) varied within the size of one isolated Au-NP and typical distance-dependent semiconductor properties could be observed. Temperature dependent I,V measurements for these molecular junctions were performed in which the films served as active elements in the temperature range 6,300 K; classical Arrhenius plots and subsequent linear fits were carried out to give the activation energies (,E) of devices. Furthermore, preliminary studies on the photoresponsive properties of these devices were explored at 80, 160, and 300 K, respectively. Physical and photochemical mechanisms were used to explain the possible photocurrent generation processes. To the best of our knowledge, this is the first report in which oligothiophene dithiols act as bridging units to link Au-NPs, and also the first report about functionalized Au-NPs exhibiting photoresponse properties in the solid state. [source] | |||||||||||||