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1539 Publications visible to you, out of a total of 1539
Photometric Redshift Estimation of Quasars: Local versus Global Regression

Abstract (Expand)

The task of estimating an object's redshift based on photometric data is one of the most important ones in astronomy. This is especially the case for quasars. Common approaches for this regression … task are based on nearest neighbor search, template fitting schemes, or combinations of, e.g., clustering and regression techniques. As we show in this work, simple frameworks like k-nearest neighbor regression work extremely well if one considers the overall feature space (containing patterns of all objects with low, middle, and high redshifts). However, such methods naturally fail as soon as only very few or even no training patterns are given in the appropriate region of the feature space. In the literature, a wide range of other regression techniques can be found. Among the most popular ones are regularized regression schemes like ridge regression or support vector regression. In this work, we show that an out-of-the-box application of this type of schemes for the whole feature space is difficult due to the involved computational requirements and the specific properties of the data at hand. However, in contrast to nearest neighbor search schemes, such methods can be employed to extrapolate, i.e., they can be used to predict redshifts for patterns in new, unseen regions of the feature space.

Authors: F. Gieseke, Kai L. Polsterer, Peter-Christian Zinn

Date Published: 1st Sep 2012

Publication Type: InProceedings

A Single Mutation in a Tunnel to the Active Site Changes the Mechanism and Kinetics of Product Release in Haloalkane Dehalogenase LinB

Abstract

Not specified

Authors: Lada Biedermannová, Zbyněk Prokop, Artur Gora, Eva Chovancová, Mihály Kovács, Jiří Damborský, Rebecca C. Wade

Date Published: 17th Aug 2012

Publication Type: Journal

New Insights into the Mechanism of Amine/Nitroxide Cycling during the Hindered Amine Light Stabilizer Inhibited Oxidative Degradation of Polymers

Abstract (Expand)

High-level ab initio molecular orbital theory calculations are used to identify the origin of the remarkably high inhibition stoichiometric factors exhibited by dialkylamine-based radical-trapping … antioxidants. We have calculated the free energy barriers and reaction energies at 25, 80, and 260 °C in the gas phase and in aqueous solution for a broad range of reactions that might, potentially, be involved in amine/nitroxide cycling, as well as several novel pathways proposed as part of the present work, including that of N-alkyl hindered amine light stabilizer activation. We find that most of the literature nitroxide regeneration cycles should be discarded on either kinetic or thermodynamic grounds; some are even inconsistent with existing experimental observations. We therefore propose a new mechanistic cycle that relies on abstraction of a β-hydrogen atom from an alkoxyamine (R1R2NOCHR3R4). Our results suggest that this cycle is energetically feasible for a range of substrates and provides an explanation for previously misinterpreted or unexplained experimental results. We also explore alternative mechanisms for amine/nitroxide cycling for cases where the alkoxyamines do not possess an abstractable β-hydrogen.

Authors: Ganna Gryn’ova, K. U. Ingold, Michelle L. Coote

Date Published: 24th Jul 2012

Publication Type: Journal

Computational Design of Cyclic Nitroxides as Efficient Redox Mediators for Dye-Sensitized Solar Cells

Abstract (Expand)

Cyclic nitroxide radicals represent promising alternatives to the iodine‐based redox mediator commonly used in dye‐sensitized solar cells (DSSCs). To date DSSCs with nitroxide‐based redox mediators … have achieved energy conversion efficiencies of just over 5 % but efficiencies of over 15 % might be achievable, given an appropriate mediator. The efficacy of the mediator depends upon two main factors: it must reversibly undergo one‐electron oxidation and it must possess an oxidation potential in a range of 0.600–0.850 V (vs. a standard hydrogen electrode (SHE) in acetonitrile at 25 °C). Herein, we have examined the effect that structural modifications have on the value of the oxidation potential of cyclic nitroxides as well as the reversibility of the oxidation process. These included alterations to the N‐containing skeleton (pyrrolidine, piperidine, isoindoline, azaphenalene, etc.), as well as the introduction of different substituents (alkyl‐, methoxy‐, amino‐, carboxy‐, etc.) to the ring. Standard oxidation potentials were calculated using high‐level ab initio methodology that was demonstrated to be very accurate (with a mean absolute deviation from experimental values of only 16 mV). An optimal value of 1.45 for the electrostatic scaling factor for UAKS radii in acetonitrile solution was obtained. Established trends in the values of oxidation potentials were used to guide molecular design of stable nitroxides with desired equation image, and a number of compounds were suggested for potential use as enhanced redox mediators in DSSCs.

Authors: Ganna Gryn'ova, Jonathan M. Barakat, James P. Blinco, Steven E. Bottle, Michelle L. Coote

Date Published: 11th Jun 2012

Publication Type: Journal

The Interaction Properties of the Human Rab GTPase Family – A Comparative Analysis Reveals Determinants of Molecular Binding Selectivity

Abstract

Not specified

Authors: Matthias Stein, Manohar Pilli, Sabine Bernauer, Bianca H. Habermann, Marino Zerial, Rebecca C. Wade

Date Published: 16th Apr 2012

Publication Type: Journal

Reversible Addition–Fragmentation Chain Transfer Polymerization of Vinyl Chloride

Abstract (Expand)

Controlled/“living” radical polymerization (CLRP) of vinyl chloride (VC) via the reversible addition–fragmentation chain transfer (RAFT) process is reported for the first time. The cyanomethyl … methyl(phenyl)carbamodithioate (CMPCD) was found to be an efficient RAFT agent enabling the CLRP polymerization of VC monomer under certain experimental conditions. Two different radical initiators, having very distinct half-life times at room temperature, were employed in this study. The kinetic studies of RAFT polymerization of VC show a linear increase of the molecular weight with the monomer conversion and the lowest polydispersity (PDI) ever reported for poly(vinyl chloride) (PVC) synthesized with CLRP method (PDI ∼ 1.4). The resulting PVC was fully characterized using the matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), 1H nuclear magnetic resonance spectroscopy (1H NMR), and gel permeation chromatography (GPC) techniques. The 1H NMR and MALDI-TOF-MS analysis of PVC prepared via RAFT polymerization method have shown the absence of structural defects and the presence of chain-end functional groups. The “livingness” of the PVC was also confirmed by a successful reinitiation experiment. The suitability of the RAFT agent was also confirmed via high-level ab initio molecular orbital calculations.

Authors: Carlos M. R. Abreu, Patrícia V. Mendonça, Arménio C. Serra, Jorge F. J. Coelho, Anatoliy V. Popov, Ganna Gryn’ova, Michelle L. Coote, Tamaz Guliashvili

Date Published: 27th Feb 2012

Publication Type: Journal

Atomic Detail Brownian Dynamics Simulations of Concentrated Protein Solutions with a Mean Field Treatment of Hydrodynamic Interactions

Abstract

Not specified

Authors: Paolo Mereghetti, Rebecca C. Wade

Date Published: 8th Feb 2012

Publication Type: Journal

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