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991.
Dr. Peter Friedrich Dr. Ulrich Baisch Dr. Ross W. Harrington Fredrick Lyatuu Kai Zhou Dr. Felix Zelder Prof. Dr. William McFarlane Prof. Dr. Wolfgang Buckel Prof. Dr. Bernard T. Golding 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(50):16114-16122
Coenzyme B12 can assist radical enzymes that accomplish the vicinal interchange of a hydrogen atom with a functional group. It has been proposed that the Co? C bond homolysis of coenzyme B12 to cob(II)alamin and the 5′‐deoxyadenosyl radical is aided by hydrogen bonding of the corrin C19? H to the 3′‐O of the ribose moiety of the incipient 5′‐deoxyadenosyl radical, which is stabilized by 30 kJ mol?1 (B. Durbeej et al., Chem. Eur. J. 2009 , 15, 8578–8585). The diastereoisomers (R)‐ and (S)‐2,3‐dihydroxypropylcobalamin were used as models for coenzyme B12. A downfield shift of the NMR signal for the C19? H proton was observed for the (R)‐isomer (δ=4.45 versus 4.01 ppm for the (S)‐isomer) and can be ascribed to an intramolecular hydrogen bond between the C19? H and the oxygen of CHOH. Crystal structures of (R)‐ and (S)‐2,3‐dihydroxypropylcobalamin showed C19? H???O distances of 3.214(7) Å (R‐isomer) and 3.281(11) Å (S‐isomer), which suggest weak hydrogen‐bond interactions (?ΔG<6 kJ mol?1) between the CHOH of the dihydroxypropyl ligand and the C19? H. Exchange of the C19? H, which is dependent on the cobalt redox state, was investigated with cob(I)alamin, cob(II)alamin, and cob(III)alamin by using NMR spectroscopy to monitor the uptake of deuterium from deuterated water in the pH range 3–11. No exchange was found for any of the cobalt oxidation states. 3′,5′‐Dideoxyadenosylcobalamin, but not the 2′,5′‐isomer, was found to act as a coenzyme for glutamate mutase, with a 15‐fold lower kcat/KM than 5′‐deoxyadenosylcobalamin. This indicates that stabilization of the 5′‐deoxyadenosyl radical by a hydrogen bond that involves the C19? H and the 3′‐OH group of the cofactor is, at most, 7 kJ mol?1 (?ΔG). Examination of the crystal structure of glutamate mutase revealed additional stabilizing factors: hydrogen bonds between both the 2′‐OH and 3′‐OH groups and glutamate 330. The actual strength of a hydrogen bond between the C19? H and the 3′‐O of the ribose moiety of the 5′‐deoxyadenosyl group is concluded not to exceed 6 kJ mol?1 (?ΔG). 相似文献
992.
We present the use of hydrazone dynamic combinatorial libraries (DCLs) to identify macrocyclic receptors that are selective for alkaline earth metal ions over alkali metal ions. In particular, the toxic heavy metal ions Sr(2+) and Ba(2+) induce characteristic changes in the DCLs. Four macrocycles were isolated and characterised by LCMS, HRMS, NMR and X-ray crystallography; binding studies by UV-Vis spectroscopy confirm the selectivity observed in the DCLs. 相似文献
993.
Andreas Helmstedt Marc D. Sacher Aaron Gryzia Alexander Harder Armin Brechling Norbert Müller Ulrich Heinzmann Veronika Hoeke Erich Krickemeyer Thorsten Glaser Samuel Bouvron Mikhail Fonin 《Journal of Electron Spectroscopy and Related Phenomena》2012,184(11-12):583-588
X-ray absorption spectroscopy studies of the [MnIII6CrIII]3+ single-molecule magnet deposited as a microcrystalline layer on gold substrates are presented. The oxidation state of the manganese centers changes from MnIII to MnII due to irradiation with soft X-rays. The influence of the charge-neutralizing anions on the stability of [MnIII6CrIII]3+ against soft X-ray exposure is investigated for the different anions tetraphenylborate (BPh4?), lactate (C3H5O3?) and perchlorate (ClO4?). The exposure dependence of the radiation-induced reduction process is compared for [MnIII6CrIII]3+ with the three different anions. 相似文献
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996.
Flight gate scheduling with respect to a reference schedule 总被引:1,自引:0,他引:1
Ulrich?Dorndorf Florian?Jaehn Erwin?PeschEmail author 《Annals of Operations Research》2012,194(1):177-187
This paper considers the problem of assigning flights to airport gates. We examine the general case in which an aircraft serving
a flight may be assigned to different gates for arrival and departure processing and for optional intermediate parking. Restrictions
to this assignment include gate closures and shadow restrictions, i.e., the situation where certain gate assignments may cause
blocking of neighboring gates. The objectives include maximization of the total assignment preference score, a minimal number
of unassigned flights during overload periods, minimization of the number of tows, maximization of a robustness measure as
well as a minimal deviation from a given reference schedule. We show that in case of a one period time horizon this objective
can easily be integrated into our existing model based on the Clique Partitioning Problem. Furthermore we present a heuristic
algorithm to solve the problem for multiple periods. 相似文献
997.
Hoffmann I Oppel C Gernert U Barreleiro P von Rybinski W Gradzielski M 《Langmuir : the ACS journal of surfaces and colloids》2012,28(20):7695-7703
We present a novel method for the measurement of polymer adsorption on fibers by employing fluorescently labeled polymers. The method itself can be used for any compound that either shows fluorescence or can be labeled with a fluorescent dye, which renders it ubiquitously applicable for adsorption studies. The main advantage of the method is that the choice of adsorbent is not limited to flat surfaces, thereby allowing the investigation of fibrous and porous systems. As an example of high interest for application we determined the adsorption isotherms of various polysaccharide-based polymers with different charges and different substituents on cotton fibers. These experiments show that the extent of adsorption depends not only on the charge conditions but also very much on the specific interactions between the polymer and fiber. For instance, the cationic hydroxyethyl cellulose can become bound to an extent similar to that of the anionic alginate, while the anionic carboxymethyl cellulose of similar charge density adsorbs much less under these conditions. This shows that the adsorption of polymers depends subtly on the details of the interaction between the polymer and fiber but can be determined with good precision with our direct fluorescence method. 相似文献
998.
DeBond N Oakes RL Paytan A Wortmann UG 《Isotopes in environmental and health studies》2012,48(1):180-194
Current carbon and sulphur isotope ratios (δ(13)C and δ(34)S) suggest there were major shifts in partitioning between reduced and oxidised reservoirs of carbon and sulphur during the Early Cretaceous. However, the δ(13)C and δ(34)S records are composed from different Ocean Drilling Program sites and are hard to correlate at high resolution. We present high-resolution Aptian δ(13)C(org) and δ(34)S(barite) values derived from the same set of samples, enabling a higher certainty correlation than previously possible. Two major hypotheses aim to explain the Early Aptian S-isotope excursion: increased volcanic degassing and/or fluctuations in the marine sulphate concentration. Our S-isotope data provide tight constraints on the timing and magnitude of volcanic flux required. We show that the observed S-isotope signature can be explained by a 2?Ma pulse of increased volcanic flux, injecting ~4.5×10(18)?mol C into the atmosphere. Further work is needed to evaluate whether these fluxes are compatible with the existing C-isotope record. 相似文献
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1000.