| 1. |
- Abelev, Betty, et al.
(författare)
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Long-range angular correlations on the near and away side in p-Pb collisions at root S-NN=5.02 TeV
- 2013
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Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 719:1-3, s. 29-41
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Tidskriftsartikel (refereegranskat)abstract
- Angular correlations between charged trigger and associated particles are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV for transverse momentum ranges within 0.5 < P-T,P-assoc < P-T,P-trig < 4 GeV/c. The correlations are measured over two units of pseudorapidity and full azimuthal angle in different intervals of event multiplicity, and expressed as associated yield per trigger particle. Two long-range ridge-like structures, one on the near side and one on the away side, are observed when the per-trigger yield obtained in low-multiplicity events is subtracted from the one in high-multiplicity events. The excess on the near-side is qualitatively similar to that recently reported by the CMS Collaboration, while the excess on the away-side is reported for the first time. The two-ridge structure projected onto azimuthal angle is quantified with the second and third Fourier coefficients as well as by near-side and away-side yields and widths. The yields on the near side and on the away side are equal within the uncertainties for all studied event multiplicity and p(T) bins, and the widths show no significant evolution with event multiplicity or p(T). These findings suggest that the near-side ridge is accompanied by an essentially identical away-side ridge. (c) 2013 CERN. Published by Elsevier B.V. All rights reserved.
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| 2. |
- Abelev, Betty, et al.
(författare)
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Measurement of prompt J/psi and beauty hadron production cross sections at mid-rapidity in pp collisions at root s=7 TeV
- 2012
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Ingår i: Journal of High Energy Physics. - 1029-8479. ; :11
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Tidskriftsartikel (refereegranskat)abstract
- The ALICE experiment at the LHC has studied J/psi production at mid-rapidity in pp collisions at root s = 7 TeV through its electron pair decay on a data sample corresponding to an integrated luminosity L-int = 5.6 nb(-1). The fraction of J/psi from the decay of long-lived beauty hadrons was determined for J/psi candidates with transverse momentum p(t) > 1,3 GeV/c and rapidity vertical bar y vertical bar < 0.9. The cross section for prompt J/psi mesons, i.e. directly produced J/psi and prompt decays of heavier charmonium states such as the psi(2S) and chi(c) resonances, is sigma(prompt J/psi) (p(t) > 1.3 GeV/c, vertical bar y vertical bar < 0.9) = 8.3 +/- 0.8(stat.) +/- 1.1 (syst.)(-1.4)(+1.5) (syst. pol.) mu b. The cross section for the production of b-hadrons decaying to J/psi with p(t) > 1.3 GeV/c and vertical bar y vertical bar < 0.9 is a sigma(J/psi <- hB) (p(t) > 1.3 GeV/c, vertical bar y vertical bar < 0.9) = 1.46 +/- 0.38 (stat.)(-0.32)(+0.26) (syst.) mu b. The results are compared to QCD model predictions. The shape of the p(t) and y distributions of b-quarks predicted by perturbative QCD model calculations are used to extrapolate the measured cross section to derive the b (b) over bar pair total cross section and d sigma/dy at mid-rapidity.
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| 3. |
- Abelev, Betty, et al.
(författare)
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Underlying Event measurements in pp collisions at root s=0.9 and 7 TeV with the ALICE experiment at the LHC
- 2012
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Ingår i: Journal of High Energy Physics. - 1029-8479. ; :7
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Tidskriftsartikel (refereegranskat)abstract
- We present measurements of Underlying Event observables in pp collisions at root s = 0 : 9 and 7 TeV. The analysis is performed as a function of the highest charged-particle transverse momentum p(T),L-T in the event. Different regions are defined with respect to the azimuthal direction of the leading (highest transverse momentum) track: Toward, Transverse and Away. The Toward and Away regions collect the fragmentation products of the hardest partonic interaction. The Transverse region is expected to be most sensitive to the Underlying Event activity. The study is performed with charged particles above three different p(T) thresholds: 0.15, 0.5 and 1.0 GeV/c. In the Transverse region we observe an increase in the multiplicity of a factor 2-3 between the lower and higher collision energies, depending on the track p(T) threshold considered. Data are compared to PYTHIA 6.4, PYTHIA 8.1 and PHOJET. On average, all models considered underestimate the multiplicity and summed p(T) in the Transverse region by about 10-30%.
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| 4. |
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| 5. |
- Barros, Raúl J., et al.
(författare)
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Effect of mass-transfer limitations on the selectivity of immobilized α-chymotrypsin biocatalysts prepared for use in organic medium
- 2000
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Ingår i: Biotechnology and Bioengineering. - 0006-3592. ; 67:3, s. 319-326
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Tidskriftsartikel (refereegranskat)abstract
- The selectivity of preparations of α-chymotrypsin immobilized on Celite or polyamide and carrying out syntheses of di- and tripeptides in acetonitrile medium were studied. The study concerns the effect of mass- transfer limitations on three different kinds of selectivity: acyl donor, stereo- and nucleophile selectivities, defined respectively as the ratio of initial rates with different acyl donors; the enantioselectivity factor (E); and the ratio of initial rates of peptide synthesis and hydrolysis of the acyl donor. Strong mass-transfer limitations caused by increased enzyme loading had a very strong effect on acyl donor selectivity, with reductions of up to 79%, and on stereoselectivity, with reductions of up to 77% in relation to optimum values, both on Celite. Nucleophile selectivity was not affected as strongly by mass-transfer limitations. Using a small molecule (AlaNH2) as nucleophile, the onset of these limitations caused only minor reductions in selectivity, while when using a larger nucleophilic species (AlaPheNH2) it was reduced by up to 60% when increasing enzyme loading on Celite from 2 to 100 mg/g. The different way these kinds of selectivity are affected by the onset of mass-transfer limitations can be explained by a combination of different aspects: the kinetic behavior of the enzyme toward nucleophile and acyl donor concentrations, the relative concentrations of reagents used in the reaction media, and their relative diffusion coefficients. In short, higher concentrations of nucleophile than acyl donor are generally used, and the nucleophile most often used in the experiments hereby described (AlaNH2) diffuses faster than the acyl donors employed. These factors combined are expected to give rise to concentration gradients inside porous biocatalyst particles higher for acyl donor than for nucleophile under conditions of mass-transfer limitations. This explains why acyl donor selectivity and stereoselectivity are much more influenced by mass transfer limitations than nucleophile selectivity.
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| 6. |
- Barros, Raúl J., et al.
(författare)
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Enhancement of immobilized protease catalyzed dipeptide synthesis by the presence of insoluble protonated nucleophile
- 1999
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Ingår i: Enzyme and Microbial Technology. - 0141-0229. ; 24:8-9, s. 480-488
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Tidskriftsartikel (refereegranskat)abstract
- α-Chymotrypsin immobilized on celite catalyzing a kinetically controlled dipeptide synthesis reaction in acetonitrile medium showed an odd behavior in response to additions of triethylamine to the reaction mixture. This base is used to deprotonate the nucleophilic reagent, l-alaninamide hydrochloride, in order to increase its nucleophilicity and solubility. However, the enzyme performance is apparently enhanced by additions of triethylamine below one equivalent (in the range 15-20 mm) while the used concentration of nucleophilic reagent is 30 mm. Under these conditions, the initial rate is up to 2.5 times higher and the nucleophile specificity is approximately 30% better than when one equivalent is added. The activating effect on initial rates of dipeptide synthesis was not observed when polyamide was used as support. Unlike polyamide, celite is a material with quite low porosity. Improvement of nucleophile specificity was observed using both supports. It is shown that this activation arises due to the presence of a separate dense liquid phase of insoluble l-alaninamide hydrochloride that intimately contacts with the enzyme preparation, and does not depend on the addition of triethylamine itself. Additions of l-alaninamide hydrochloride improved initial rates of synthesis more than 2.5-fold, and nucleophile specificity more than threefold. The initial rate activation was also observed when using non-porous glass beads to immobilize the enzyme at a loading of 5 mg enzyme g-1 glass but not at 1 mg enzyme g-1 glass when no mass transfer limitations in the immobilized enzyme layer are expected to occur. The results suggest that the presence of the separate phase helps to relieve mass transfer limitations on the system caused by overloading at the supports. One possible mechanism for the initial rate activation might be that the enzyme is partially desorbed from the support particles into the separate phase of nucleophile, and the better nucleophile specificity observed is due to increased local concentrations of the nucleophile within this phase. Copyright (C) 1999 Elsevier Science Inc. All rights reserved.
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| 7. |
- Barros, Raúl J
(författare)
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Influence of Internal Diffusional Limitations on the Performance of an Immobilized Enzyme in Organic Medium
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The influence of mass-transfer limitations on the performance of immobilized a-chymotrypsin operating in acetonitrile medium has been investigated. The enzyme was immobilized by deposition on various porous carrier materials such as celite, polyamides of different particle sizes, controlled-pore glasses, polypropylene or polyacrylamides. When carrying out kinetically controlled peptide synthesis reactions catalyzed by these preparations, mass-transfer effects have been observed. It has been shown that internal diffusional limitations are the mechanism responsible such mass-transfer effects. A correlation between observed activity towards a given reaction at a given enzyme loading and the morphological characteristics of the support material has been developed, taking into account the effects of internal diffusion. The factors that affect substrate diffusion and enzyme kinetics are discussed, because both processes must happen at the same rate in simultaneous reaction and diffusion. The extent of internal diffusional limitations can be controlled by changing enzyme loading, support material, or the intrinsic velocity of the reaction carried out. The intrinsic kinetics of the studied reactions have been studied in the absence of mass-transfer effects by immobilizing the enzyme on nonporous glass beads. Internal diffusional limitations influence the performance of the immobilized enzyme in many ways: Strong diffusional limitations reduce the specific activity of the enzyme preparations. The initial rate per amount of preparation can even decrease with increased enzyme loadings in the case of very low porosity supports, due to reduced available space for substrate diffusion. Internal diffusional limitations can be relaxed by the presence of insoluble hydrochlorinated nucleophile, presumably by promoting the desorption of part of the enzyme from "overloaded" supports. Internal diffusional limitations also decrease the apparent specificity of the chemical transformations. Acyl donor specificity and enantioselectivity in the conversion of a racemic acyl donor are much more affected than nucleophile specificity. The extent of the effect on nucleophile specificity depends on the size of the used nucleophile. Strong internal diffusional limitations still increase the apparent stability, both storage and operational, of the enzyme preparations. A numeric model has been developed to describe the effects of internal diffusional on initial activity and nucleophile specificity. The model accurately describes experimental observations such as plateau-shaped initial rate profiles, the stronger influence on initial rate than on nucleophile specificity and reduction of initial rate with increased enzyme loading with low porosity supports. Besides showing that internal diffusional limitations influence all aspects of enzyme performance, conceptual and numerical explanations to the observed effects have also been provided.
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| 8. |
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| 9. |
- Barros, Raúl J., et al.
(författare)
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Mass transfer studies on immobilized α-chymotrypsin biocatalysts prepared by deposition for use in organic medium
- 1998
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Ingår i: Biotechnology and Bioengineering. - 0006-3592. ; 59:3, s. 364-373
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Tidskriftsartikel (refereegranskat)abstract
- Mass transfer limitations were studied in enzyme preparations of α- chymotrypsin made by deposition on different porous support materials such as controlled pore glasses, Celite, and polyamides of different particle sizes. It is the onset of mass transfer limitations that determines the position of the activity optimum with respect to enzyme loading on each support. The evidence of various experiments indicates that internal diffusional limitations are the important mechanism for the observed mass transfer limitations. External diffusion was not found to play an important role under the conditions used, and it was also found that when immobilizing multilayers of enzyme the buried enzyme molecules are active to a large extent. An extreme situation is observed on Celite at very high loadings. Under these conditions, this support is expected to have its pores completely filled with packed enzyme molecules, and then it is the diffusion within the enzyme layer that determines the observed rate. As the enzyme loading increases, the area of contact between the deposited enzyme layers and the liquid solution inside the pores diminishes, causing a decrease on the observed rate of an intrinsically fast reaction which apparently is incongruous with the presence of more enzyme in the system. This work shows that mass transfer limitations can be an important factor when working with immobilized enzymes in organic media, and its study should be carried out in order to avoid undesired reduced enzyme activities and specificities.
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| 10. |
- Barros, Raúl J., et al.
(författare)
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Modeling the performance of immobilized α-chymotrypsin catalyzed peptide synthesis in acetonitrile medium
- 2001
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Ingår i: Journal of Molecular Catalysis - B Enzymatic. - 1381-1177. ; 11:4-6, s. 841-850
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Tidskriftsartikel (refereegranskat)abstract
- A model was developed which describes simultaneous reaction and internal diffusion for kinetically controlled, immobilized α-chymotrypsin-catalyzed, oligopeptide synthesis in acetonitrile medium. The model combines the equations that describe the intrinsic kinetics of four different reactions and the physical characteristics of three different support materials, as determined experimentally, to predict the apparent initial activity and nucleophile selectivity of the immobilized biocatalyst. The model is able to predict reasonably well the experimentally observed initial rate and nucleophile selectivity vs. enzyme loading profiles. The reduction in observed initial rate with enzyme loading when fast reactions are carried out with α-chymotrypsin immobilized on celite, and the larger influence of mass transfer limitations on the initial reaction rates than on nucleophile selectivities are correctly predicted by the numerical calculations. The model is general in terms of its application to other systems - enzymes, reactions, support materials and/or kinetic schemes - as long as the intrinsic kinetics and the characteristics of the enzyme and support material are known.
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