Field-theoretical study of the Bose polaron Rath, Steffen Patrick; Schmidt, Richard
Physical review. A, Atomic, molecular, and optical physics,
11/2013, Letnik:
88, Številka:
5
Journal Article
New Principles for Glycoside-Bond Formation Zhu, Xiangming; Schmidt, Richard R
Angewandte Chemie (International ed.),
March 2, 2009, Letnik:
48, Številka:
11
Journal Article
Recenzirano
Meeting the demand for sugars: The importance of oligosaccharides and glycoconjugates in biological systems has stimulated a need to access significant amounts of these compounds. Much effort has ...been devoted to the stereoselective generation of the key glycosidic bond, both between carbohydrate residues (see scheme) and to the aglycone. Various methodologies are now available for the efficient synthesis of structurally defined complex oligosaccharides and glycoconjugates.Increased understanding of the important roles that oligosaccharides and glycoconjugates play in biological processes has led to a demand for significant amounts of these materials for biological, medicinal, and pharmacological studies. Therefore, tremendous effort has been made to develop new procedures for the synthesis of glycosides, whereby the main focus is often the formation of the glycosidic bonds. Accordingly, quite a few review articles have been published over the past few years on glycoside synthesis; however, most are confined to either a specific type of glycoside or a specific strategy for glycoside synthesis. In this Review, new principles for the formation of glycoside bonds are discussed. Developments, mainly in the last ten years, that have led to significant advances in oligosaccharide and glycoconjugate synthesis have been compiled and are evaluated.
Inverting glycosyltransferases enforce in the active site an intramolecular, acid–base catalyzed glycosidation that, due to proximity of the donor anomeric carbon and the acceptor hydroxyl group, ...follows an SN2-type reaction. Spacers, tethering donor and acceptor via nonreacting functional groups, led in intramolecular glycosidations to excellent yields and, independent of the donor anomeric configuration, to either the α- or the β-anomer. The requirement of a demanding protecting group pattern confines the application of this efficient method. Only the method where the 2-hydroxyl group of a mannopyranosyl donor is tethered via an acetal spacer to the reacting acceptor functional group is used for β-mannopyranoside synthesis. The most elegant method, tethering donor and acceptor covalently to the spacer via the leaving group and the reacting functional group, was so far not as efficient as hoped. This method is very efficient when donor and acceptor are temporarily assembled through a hydrogen-bond facilitating a stretched hexagon-like transition state. This follows from the stereoselective O-glucopyranosyl trichloroacetimidate transformation into O-glucopyranosyl phosphate with dibenzyl phosphoric acid as acceptor that can be regarded as AB–C–H acceptor type. Generalizing this concept to the use of alcohols as acceptors requires reversible generation of an A–B–C–H adduct where A–H represents the acceptor (RO–H) and BC a catalyst that has to fulfill several criteria. Among these criteria are low affinity to nitrogen, avoiding glycosyl donor activation in the absence of acceptor, and high affinity to oxygen in order to generate the A–B–C–H adduct with increased proton acidity. Thus, hydrogen-bond mediated self-assembly of donor and acceptor and concomitant donor activation via a transition state is available, which enforces an acid–base catalyzed SN2-type reaction. It could be shown that PhBF2, Ph2BF, and PhSiF3 are such catalysts that fulfill the desired four functions: reversible adduct formation with the acceptor, hydrogen-bond mediated tethering of this adduct with the donor, and acid- and base-catalysis of the glycosidation. Also Lewis acidic metal salts, particularly the dimeric gold(III) chloride, turned out to exhibit excellent BC type catalyst properties. Worth mentioning in this context is the ability of gold(III) chloride to regioselectively activate diols. As thioureas have high affinity to anions and also to neutral compounds through strong hydrogen bonds, their binding to alcohols and concomitant activation of O-glycosyl trichloroacetimidates was of interest. Yet, even the acidic N,N′-bis3,5-bis(trifluoromethyl)phenyl-thiourea was unable to catalyze glycosidations. However, as a cocatalyst to acids, thiourea exerts a strong effect that, based on NMR studies, leads first to a hydrogen-bond mediated catalyst–cocatalyst–acceptor complex. This complex activates the donor in an intramolecular, acid–base catalyzed reaction that is again closely related to the action of inverting glycosyltransferases. Thus, from O-(α-glycosyl) trichloroacetimidates, good yields of the inversion products, that is, the β-glycosides, are obtained. This novel conceptual approach to glycosidation revealed that for retention of configuration in addition a catalytic nucleophile is required that enables formation of the α-glucoside from the α-trichloroacetimidate. Preliminary studies with a catalyst possessing this 5-fold function, that is, adduct formation with the acceptor, hydrogen-bonding between the reactants, acid and base catalysis, and a catalytic nucleophile as part of a chiral framework supporting facial selection, exhibited good chances for final success in this endeavor.
We develop a microscopic theory describing a quantum impurity whose rotational degree of freedom is coupled to a many-particle bath. We approach the problem by introducing the concept of an ..."angulon"-a quantum rotor dressed by a quantum field-and reveal its quasiparticle properties using a combination of variational and diagrammatic techniques. Our theory predicts renormalization of the impurity rotational structure, such as that observed in experiments with molecules in superfluid helium droplets, in terms of a rotational Lamb shift induced by the many-particle environment. Furthermore, we discover a rich many-body-induced fine structure, emerging in rotational spectra due to a redistribution of angular momentum within the quantum many-body system.
This book is the first comprehensive guide to the underlying skills embodied in the IEEE's Software Engineering Body of Knowledge (SWEBOK) standard. The author explains the traditional software ...engineering practices recognized for developing projects for government or corporate systems.
Software engineering education often lacks standardization, with many institutions focusing on implementation rather than design as it impacts product architecture. Many graduates join the workforce with incomplete skills, leading to software projects that either fail outright or run woefully over budget and behind schedule.
Additionally, software engineers need to understand system engineering and architecture the hardware and peripherals their programs will run on. This issue will only grow in importance as more programs leverage parallel computing, requiring an understanding of the parallel capabilities of processors and hardware. This book gives both software developers and system engineers key insights into how their skillsets support and complement each other. With a focus on these key knowledge areas, this book offers a set of best practices that can be applied to any industry or domain involved in developing software products.
A thorough, integrated compilation on the engineering of software products, addressing the majority of the standard knowledge areas and topics
Offers best practices focused on those key skills common to many industries and domains that develop software
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Gold(III) chloride as catalyst for O-glycosyl trichloroacetimidate activation revealed low affinity to the glycosyl donor but high affinity to the hydroxy group of the acceptor alcohol moiety, thus ...leading to catalyst–acceptor adduct formation. Charge separation in this adduct, increasing the proton acidity and the oxygen nucleophilicity, permits donor activation and concomitant acceptor transfer in a hydrogen-bond mediated SN2-type transition state. Hence, the sequential binding between acceptor and catalyst and then with the glycosyl donor enables self-organization of an ordered transition-state. This way, with various acceptors, even at temperatures below −60 °C, fast and high yielding glycosidations in high anomeric selectivities were recorded, showing the power of this gold(III) chloride acid–base catalysis. Alternative reaction courses via hydrogen chloride or HAuCl4 activation or intermediate generation of glycosyl chloride as the real donor could be excluded. With partially O-protected acceptors, prone to bidentate ligation to gold(III) chloride, particularly high reactivities and anomeric selectivities were observed. Gold(I) chloride follows the same catalyst–acceptor adduct driven acid–base catalysis reaction course.
During the past 70 years, the quantum theory of angular momentum has been successfully applied to describing the properties of nuclei, atoms, and molecules, and their interactions with each other as ...well as with external fields. Because of the properties of quantum rotations, the angular-momentum algebra can be of tremendous complexity even for a few interacting particles, such as valence electrons of an atom, not to mention larger many-particle systems. In this work, we study an example of the latter: a rotating quantum impurity coupled to a many-body bosonic bath. In the regime of strong impurity-bath couplings, the problem involves the addition of an infinite number of angular momenta, which renders it intractable using currently available techniques. Here, we introduce a novel canonical transformation that allows us to eliminate the complex angular-momentum algebra from such a class of many-body problems. In addition, the transformation exposes the problem’s constants of motion, and renders it solvable exactly in the limit of a slowly rotating impurity. We exemplify the technique by showing that there exists a critical rotational speed at which the impurity suddenly acquires one quantum of angular momentum from the many-particle bath. Such an instability is accompanied by the deformation of the phonon density in the frame rotating along with the impurity.
Exotic phenomena in systems with strongly correlated electrons emerge from the interplay between spin and motional degrees of freedom. For example, doping an antiferromagnet is expected to give rise ...to pseudogap states and high-temperature superconductors. Quantum simulation using ultracold fermions in optical lattices could help to answer open questions about the doped Hubbard Hamiltonian, and has recently been advanced by quantum gas microscopy. Here we report the realization of an antiferromagnet in a repulsively interacting Fermi gas on a two-dimensional square lattice of about 80 sites at a temperature of 0.25 times the tunnelling energy. The antiferromagnetic long-range order manifests through the divergence of the correlation length, which reaches the size of the system, the development of a peak in the spin structure factor and a staggered magnetization that is close to the ground-state value. We hole-dope the system away from half-filling, towards a regime in which complex many-body states are expected, and find that strong magnetic correlations persist at the antiferromagnetic ordering vector up to dopings of about 15 per cent. In this regime, numerical simulations are challenging and so experiments provide a valuable benchmark. Our results demonstrate that microscopy of cold atoms in optical lattices can help us to understand the low-temperature Fermi-Hubbard model.
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