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121.
122.
We introduce a class of function algebras, that we call unimodular,
and study Toeplitz operators on the Hardy spaces associated to representing
measures on these algebras.We show that our class of function algebras is very
extensive and that a number of important results for Toeplitz operators and
their associated C*-algebras extend to the very general setting we consider.
Submitted: July 1. 2001. 相似文献
123.
Let M be a simply connected Riemannian symmetric space, with at most one flat direction. We show that every Riemannian (or
unitary) vector bundle with parallel curvature over M is an associated vector bundle of a canonical principal bundle, with
the connection inherited from the principal bundle. The problem of finding Riemannian (or unitary) vector bundles with parallel
curvature then reduces to finding representations of the structure group of the canonical principal bundle. 相似文献
124.
Reynaud F Mertz D Celestini F Debierre JM Ghorayeb AM Simon P Stepanov A Voiron J Delmas C 《Physical review letters》2001,86(16):3638-3641
We report on the ESR, magnetization, and magnetic susceptibility measurements performed over a large temperature range, from 1.5 to 750 K, on high-quality stoichiometric LiNiO2. We find that this compound displays two distinct temperature regions where its magnetic behavior is anomalous. With the help of a statistical model based on the Kugel'-Khomskii Hamiltonian, we show that below T(of) approximately 400 K, an orbitally frustrated state characteristic of the triangular lattice is established. This then gives a solution to the long-standing controversial problem of the magnetic behavior in LiNiO2. 相似文献
125.
[reaction: see text] To facilitate the access to unique models for biological processes, we examined six different synthetic routes to octi(p-phenylene) rods with lateral and terminal substituents R(L) and R(T). This systematic study allowed us to increase to overall yield for the synthesis of a new class of oligo(p-phenylene) ionophores about 20 times and to provide general insights into the practicability of synthetic routes to multiply substituted molecular rods. 相似文献
126.
Gerard A. van Albada Nora Veldman Anthony L. Spek Jan Reedijk 《Journal of chemical crystallography》2000,30(1):69-72
The title compound, catena-[bis(azido-N)-copper(II)-(bis(2-benzimidazolyl)butane), [Cu(C18H18N4)(N3)2]n, was obtained from the reaction of the ligand bis(2-benzimidazolyl)butane and Cu(N3)2. The x-ray crystal structure is reported. The compound crystallizes in the monoclinic space group P21/c with a = 8.2524(10), b = 12.765(5), c = 9.1125(15) Å, = 106.423(12)°, Z = 2. The Cu(II) ions are square-planar coordinated with trans-oriented end-on binding azido ligands. The structure is a polynuclear chain with the benzimidazole bridging at each end. In addition a N(ligand)-H···N(azido) H-bridge [N(ligand)···N(azido) = 2.994(7) Å] is present, resulting in a pseudo 2-dimensional lattice. The characteristic azido infrared vibrations are found at 2060 and 2077 cm–1 (as(N3)) and 1284 and 1297 cm–1 ((N3)). 相似文献
127.
Biosynthesis of sulfated glycopeptide antibiotics by using the sulfotransferase StaL 总被引:2,自引:0,他引:2
The unique glycopeptide antibiotic A47934, produced by Streptomyces toyocaensis, possesses a nonglycosylated heptapeptide core that is sulfated on the phenolic hydroxyl of the N-terminal 4-hydroxy-L-phenylglycine residue. Genetic and biochemical experiments confirmed that StaL is a sulfotransferase capable of sulfating the predicted crosslinked heptapeptide substrate to produce A47934 both in vivo and in vitro. Incubation of purified His(6)-StaL with various substrates in vitro revealed substrate specificity and yielded two sulfo-glycopeptide antibiotics: sulfo-teicoplanin aglycone and sulfo-teicoplanin. Quantification of the antibacterial activity of desulfo-A47934, A47934, teicoplanin, and sulfo-teicoplanin demonstrated that sulfation slightly increased the minimum inhibitory concentration. This unique modification by sulfation expands glycopeptide diversity with potential application for the development of new antibiotics. 相似文献
128.
We examine the interaction between cytoskeletal F-actin and espin 3A, a prototypical actin bundling protein found in sensory cell microvilli, including ear cell stereocilia. Espin induces twist distortions in F-actin as well as facilitates bundle formation. Mutations in one of the two F-actin binding sites of espin, which have been implicated in deafness, can tune espin-actin interactions and radically transform the system's phase behavior. These results are compared to recent theoretical work on the general phase behavior linker-rod systems. 相似文献
129.
Design, synthesis, and structural and functional studies of rigid-rod ionophores of different axial electrostatic asymmetry are reported. The employed design strategy emphasized presence of (a) a rigid scaffold to minimize the conformational complexity, (b) a unimolecular ion-conducting pathway to minimize the suprastructural complexity and monitor the function, (c) an extended fluorophore to monitor structure, (d) variable axial rod dipole, and (e) variable terminal charges to create axial asymmetry. Studies in isoelectric, anionic, and polarized bilayer membranes confirmed a general increase in activity of uncharged rigid push-pull rods in polarized bilayers. The similarly increased activity of cationic rigid push-pull rods with an electrostatic asymmetry comparable to that of alpha-helical bee toxin melittin (positive charge near negative axial dipole terminus) is shown by fluorescence-depth quenching experiments to originate from the stabilization of transmembrane rod orientation by the membrane potential. The reduced activity of rigid push-pull rods having an electrostatic asymmetry comparable to that in alpha-helical natural antibiotics (a positive charge near the positive axial dipole terminus) is shown by structural studies to originate from rod "ejection" by membrane potentials comparable to that found in mammalian plasma membranes. This structural evidence for cell membrane recognition by asymmetric rods is unprecedented and of possible practical importance with regard to antibiotic resistance. 相似文献
130.
Moore DT Oomens J Eyler JR von Helden G Meijer G Dunbar RC 《Journal of the American Chemical Society》2005,127(19):7243-7254
Infrared spectra were recorded for a series of gas-phase Cr+ complexes using infrared multiphoton dissociation (IRMPD) in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The functionalized aromatic ligands (acetophenone, anisole, aniline, and dimethyl aniline) offer a choice of either aromatic ring-pi or n-donor-base binding sites. Use of the FELIX free electron laser light source allowed convenient, rapid scanning of the chemically informative wavelength range from approximately 500 to 1800 cm(-1), which in many cases characterized the preferred site of metal binding, as well as the electronic spin state of the complex. Mono-complex ions, Cr+(ligand), for anisole, aniline, and dimethyl aniline and bis-complex ions, Cr+(ligand)(2), for anisole, aniline, and acetophenone were produced by ligand attachment to laser-desorbed Cr+ ions in the FT-ICR cell. The photodissociation yields plotted as a function of wavelength were interpreted as approximations to the infrared absorption spectra and were compared with computed spectra of different possible geometries and spin states. Clear-cut diagnostic features in the spectra of the acetophenone, anisole, and aniline complexes showed the sites of Cr+ attachment to be the carbonyl oxygen site for acetophenone (bis-complex) and the ring-pi site for anisole and aniline (both mono- and bis-complexes). The bis-complexes of aniline and anisole are low-spin (probably doublet) states, while the mono-complexes of these same ligands are high-spin (sextet) states. The dimethyl aniline complex gave a cluttered spectrum in poor agreement with calculations, which may reflect a mixture of binding-site isomers in this case. 相似文献