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981.
Widin JM Schmitt AK Schmitt AL Im K Mahanthappa MK 《Journal of the American Chemical Society》2012,134(8):3834-3844
Controlled/"living" polymerizations and tandem polymerization methodologies offer enticing opportunities to enchain a wide variety of monomers into new, functional block copolymer materials with unusual physical properties. However, the use of these synthetic methods often introduces nontrivial molecular weight polydispersities, a type of chain length heterogeneity, into one or more of the copolymer blocks. While the self-assembly behavior of monodisperse AB diblock and ABA triblock copolymers is both experimentally and theoretically well understood, the effects of broadening the copolymer molecular weight distribution on block copolymer phase behavior are less well-explored. We report the melt-phase self-assembly behavior of SBS triblock copolymers (S = poly(styrene) and B = poly(1,4-butadiene)) comprised of a broad polydispersity B block (M(w)/M(n) = 1.73-2.00) flanked by relatively narrow dispersity S blocks (M(w)/M(n) = 1.09-1.36), in order to identify the effects of chain length heterogeneity on block copolymer self-assembly. Based on synchrotron small-angle X-ray scattering and transmission electron microscopy analyses of seventeen SBS triblock copolymers with poly(1,4-butadiene) volume fractions 0.27 ≤ f(B) ≤ 0.82, we demonstrate that polydisperse SBS triblock copolymers self-assemble into periodic structures with unexpectedly enhanced stabilities that greatly exceed those of equivalent monodisperse copolymers. The unprecedented stabilities of these polydisperse microphase separated melts are discussed in the context of a complete morphology diagram for this system, which demonstrates that narrow dispersity copolymers are not required for periodic nanoscale assembly. 相似文献
982.
Moeser B Janoschka A Wolny JA Paulsen H Filippov I Berry RE Zhang H Chumakov AI Walker FA Schünemann V 《Journal of the American Chemical Society》2012,134(9):4216-4228
In this work, we present a study of the influence of the protein matrix on its ability to tune the binding of small ligands such as NO, cyanide (CN(-)), and histamine to the ferric heme iron center in the NO-storage and -transport protein Nitrophorin 2 (NP2) from the salivary glands of the blood-sucking insect Rhodnius prolixus. Conventional M?ssbauer spectroscopy shows a diamagnetic ground state of the NP2-NO complex and Type I and II electronic ground states of the NP2-CN(-) and NP2-histamine complex, respectively. The change in the vibrational signature of the protein upon ligand binding has been monitored by Nuclear Inelastic Scattering (NIS), also called Nuclear Resonant Vibrational Spectroscopy (NRVS). The NIS data thus obtained have also been calculated by quantum mechanical (QM) density functional theory (DFT) coupled with molecular mechanics (MM) methods. The calculations presented here show that the heme ruffling in NP2 is a consequence of the interaction with the protein matrix. Structure optimizations of the heme and its ligands with DFT retain the characteristic saddling and ruffling only if the protein matrix is taken into account. Furthermore, simulations of the NIS data by QM/MM calculations suggest that the pH dependence of the binding of NO, but not of CN(-) and histamine, might be a consequence of the protonation state of the heme carboxyls. 相似文献
983.
O'Reilly ME Ghiviriga I Abboud KA Veige AS 《Journal of the American Chemical Society》2012,134(27):11185-11195
Appending an amine to a C═C double bond drastically increases the nucleophilicity of the β-carbon atom of the alkene to form an enamine. In this report, we present the synthesis and characterization of a novel CF(3)-ONO(3-) trianionic pincer-type ligand, rationally designed to mimic enamines within a metal coordination sphere. Presented is a synthetic strategy to create enhanced nucleophilic tungsten-alkylidene and -alkylidyne complexes. Specifically, we present the synthesis and characterization of the new CF(3)-ONO(3-) trianionic pincer tungsten-alkylidene [CF(3)-ONO]W═CH(Et)(O(t)Bu) (2) and -alkylidyne {MePPh(3)}{[CF(3)-ONO]W≡C(Et)(O(t)Bu)} (3) complexes. Characterization involves a combination of multinuclear NMR spectroscopy, combustion analysis, DFT computations, and single crystal X-ray analysis for complexes 2 and 3. Exhibiting unique nucleophilic reactivity, 3 reacts with MeOTf to yield [CF(3)-ONO]W═C(Me)(Et)(O(t)Bu) (4), but the bulkier Me(3)SiOTf silylates the tert-butoxide, which subsequently undergoes isobutylene expulsion to form [CF(3)-ONO]W═CH(Et)(OSiMe(3)) (5). A DFT calculation performed on a model complex of 3, namely, [CF(3)-ONO]W≡C(Et)(O(t)Bu) (3'), reveals the amide participates in an enamine-type bonding combination. For complex 2, the Lewis acids MeOTf, Me(3)SiOTf, and B(C(6)F(5))(3) catalyze isobutylene expulsion to yield the tungsten-oxo complex [CF(3)-ONO]W(O)((n)Pr) (6). 相似文献
984.
FX Gallat AP Brogan Y Fichou N McGrath M Moulin M Härtlein J Combet J Wuttke S Mann G Zaccai CJ Jackson AW Perriman M Weik 《Journal of the American Chemical Society》2012,134(32):13168-13171
The observation of biological activity in solvent-free protein-polymer surfactant hybrids challenges the view of aqueous and nonaqueous solvents being unique promoters of protein dynamics linked to function. Here, we combine elastic incoherent neutron scattering and specific deuterium labeling to separately study protein and polymer motions in solvent-free hybrids. Myoglobin motions within the hybrid are found to closely resemble those of a hydrated protein, and motions of the polymer surfactant coating are similar to those of the hydration water, leading to the conclusion that the polymer surfactant coating plasticizes protein structures in a way similar to hydration water. 相似文献
985.
Aqueous room temperature synthesis of cobalt and zinc sodalite zeolitic imidizolate frameworks 总被引:1,自引:0,他引:1
Sodalite zeolitic imidazolate frameworks containing Co (ZIF-67) and Zn (ZIF-8) were synthesized at room temperature under aqueous conditions in 10 min. A trialkylamine deprotonated the 2-methylimidazole ligand and nucleated the frameworks. Furthermore, the ligand acted as a structure directing agent in place of an organic solvent. 相似文献
986.
987.
Mehdi Asghari‐Khiavi Bayden R. Wood Pejman Hojati‐Talemi Andrew Downes Don McNaughton Adam Mechler 《Journal of Raman spectroscopy : JRS》2012,43(2):173-180
Tip‐enhanced Raman scattering (TERS) spectroscopy is a promising technique for nanoscale chemical analysis. However, there are several challenges preventing widespread application of this technology, including reproducible fabrication of efficient TERS probes. These problems reflect a lack of clear understanding of the origins of, and the parameters influencing TERS. It is believed that the coating characteristics at the apex of the tip have a major effect on the near‐field optical enhancement and thus the TERS activity of a metalized probe. Here we show that the aspect ratio of the tip can play a significant role in the efficiency of TERS probes. We argue that the electrostatic field arising from the lightning‐rod effect has a substantial role in the observed TERS effect. This argument is supported by ‘edge‐enhanced Raman scattering’ which is shown for a noble metal film. Furthermore, it is reported that an associated tip‐surface‐enhanced Raman scattering effect can be achieved by using a TERS‐inactive metalized probe on a surface‐enhanced Raman spectroscopy‐inactive roughened surface. This observation can be explained by an interparticle enhancement of the electromagnetic field. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
988.
989.
We present a definition of Riemannian manifold in noncommutative geometry. Using products of unbounded Kasparov modules, we show one can obtain such Riemannian manifolds from noncommutative spinc manifolds; and conversely, in the presence of a spinc structure. We also show how to obtain an analogue of Kasparov’s fundamental class for a Riemannian manifold, and the associated notion of Poincaré duality. Along the way we clarify the bimodule and first-order conditions for spectral triples. 相似文献
990.
Adam Os?kowski 《Statistics & probability letters》2012,82(1):186-190
Suppose that f is a martingale taking values in a Banach space B and g is its transform by a deterministic sequence of numbers in {−1,1}, such that supn‖gn‖≥1 almost surely. We show that a certain family of Φ-estimates for f holds true if and only B is a Hilbert space. 相似文献