Descripción: In this paper we generalize an interpolation result due to J.-O. Strömberg and A. Torchinsky to the case of one-sided Hardy spaces. This generalization is important in the study of the weak type (1,1) for lateral strongly singular operators. We shall need an atomic decomposition in which for every atom there exists another atom supported contiguously at its right. In order to obtain this decomposition we have developed a rather simple technique to break up an atom into a sum of others atoms. © 2006 by Institut Mittag-Leffler. All rights reserved.

Descripción: The title compounds, tetraphenylphosphonium chlorobis(diethyldithiocarbamato-S,S)cadmium(II), (C24H20P)-[CdCl(C5H10NS 2)2], (1), and tetraphenylphosphonium bromobis(diethyldithiocarbamato-S,S)cadmium(II), (C24-H20P)[CdBr(C5H10NS 2)2], (2), are isomorphous. The compexes are mononuclear with coordination spheres of the types S4Cl and S4Br, respectively. The central Cd atom is linked to four S atoms and two bidentate dithiocarbamate ligands and to the halide atom. The resulting Cd-atom coordination geometry is halfway between trigonal bipyrimidal and square pyramidal. Principal dimensions include: Cd-S 2.573 (1)-2.682 (1) in (1) and 2.571 (3)-2.736 (2) Å in (2); Cd-Cl and Cd-Br 2.462 (1) and 2.626 (1) Å, respectively.

Descripción: In this work we investigate experimentally and theoretically angular distributions of swift multi-electronic atoms after colliding with a LiF(001) surface under axial surface channeling conditions. We use the surface eikonal approximation to describe the quantum interference of scattered projectiles. The atom-surface interaction is represented by means of a pairwise additive potential that includes the contribution of the projectile polarization. The aim is to use the experimental spectra to test the range of validity of the proposed potential model. © Published under licence by IOP Publishing Ltd.

Descripción: We report a study of 3,3-dimethyl substituted acylthioureas. X ray data and quantum mechanical calculations demonstrated that the "S" conformation is the most stable both for the acylthiour eas and the corresponding anions. The high regioselectivity towards S-alkylation is explained on the basis of the localization of the HOMO mainly over the sulfur atom.

Descripción: The Kustaanheimo-Stiefel transformation together with the well-known expansion of the kernel of an isotropic harmonic oscillator is used to generate the atomic orbitals of the nonrelativistic hydrogen atom in a four-dimensional Riemann space through the path integral formalism. Group theoretical implications of the present problem are briefly discussed. © 1983 American Institute of Physics.

Descripción: 17O chemical shifts are found to be highly sensitive probes in the study of keto-enol tautomeric equilibria due to the strong sensitivity of these shifts to the coordination of the oxygen atom. The large shielding effect observed, both experimentally and theoretically, for the carbonyl oxygen atom in 2-pyridone suggests that the carbonyl π-electronic system is undergoing a strong conjugation with the formal C3=C4 double bond and/or the nitrogen lone pair. A Cl or NH2 substitution at position 6 of the pyridine ring shifts the tautomeric equilibrium toward the 2-hydroxypyridine form, while a CH3 substitution results in the keto form being predominant, as is the case in the parent compound. © 1992 American Chemical Society.

Descripción: We analyze the doubly-differential momentum distributions of electrons ejected at the interaction of strong IR laser pulses with atoms. With the help of a semiclassical model we clarify the interplay between intracycle and intercycle interferences. The model results are compared with numerical solutions of the time-dependent Schrödinger equation for atoms with long-range potentials. Similarities and differences will be discussed. © Published under licence by IOP Publishing Ltd.

Descripción: The post-collisional interactions in ion-atom ionization collisions are studied around the electron capture to the continuum (ECC) process. For this purpose, a suitable double differential cross section is introduced, involving the longitudinal recoil-ion momentum and the projectile transverse momentum transfer. Using the fact that the ECC process is closely related to the threshold in the longitudinal momentum distribution, we study this distribution as a function of the projectile scattering angle. Using the CDW-EIS approximation we theoretically find a focusing (defocusing) effect as we get closer to the distribution threshold for proton (antiproton) impact on He atoms. © 2007 IOP Publishing Ltd.

Descripción: This paper reports a synthesis of novel diheteroarylethenes functionalized for coupling to biomolecules starting from indole derivatives. The strategy is based on the derivatization at the N-atom in the indole substructure. TBDMS protection proved to be superior over BOC protection schemes, leading to higher yields in the overall synthesis. The suitability of the new derivatives as acceptors for pcFRET was calculated for selected donors.

Descripción: Five disulfated steroids and a mixture of monosulfated steroids were isolated from the ethanolic extract of the antarctic ophiuroid Gorgonocephalus chilensis. The structures were determined by 1H-NMR, 13C-NMR and FABMS.

Descripción: Proximity effects on 1J(CH) couplings are studied from a theoretical point of view in the following systems: CH4/FH (A) and H2O/HCN (B) which form dimers by hydrogen-bond interactions. 1J(CH) couplings for different intermolecular distances are calculated for the C - H bond facing the atom bearing lone pairs. While in the former system, this coupling is increased owing to the proximity to the F atom; in the latter this coupling is decreased owing to the proximity to the O atom. These opposite trends are accompanied by slight shortening and lengthening, respectively, of the corresponding C - H bond lengths. As part of this work, measurements of 1J(CH) couplings in 9-(1,3-dioxolan-2-yl)-1,2,3,4-tetrafluorotriptycene (I) and 1-formyl-2-hydroxy-8-fluoronaphthalene (II) and 1J(NH) in o-fluorobenzamide (III), showing proximity effects as those described above are carried out. These results suggest that 1J(XH) couplings are adequate probes to distinguish two different types of X-H⋯Y hydrogen bonds. © 1994 American Chemical Society.

Descripción: We propose a method to probe dispersive atom-surface interactions by measuring via two-photon Bragg spectroscopy the dynamic structure factor of a Bose-Einstein condensate above corrugated surfaces. This method takes advantage of the condensate coherence to reveal the spatial Fourier components of the lateral Casimir-Polder interaction energy. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Descripción: The title complex, aqua(2,2′-bipyridy-N,N′(3-thiapentanedioate-O,S,O′)zinc(II) tetrahydrate, [Zn(C4H4-O4S)(C10H8N 2)(H2O)].4H2O, is mononuclear with a coordination sphere consisting of one S, two N and three O atoms. The Zn atom is linked to a tridentate thiodiacetate ligand, a bidentate 2,2′-bipyridine ligand and an aqua ligand resulting in a slightly distorted octahedral coordination geometry. The molecular units are linked together in the crystal by an extensive hydrogen-bonding network and by intermolecular S⋯S interactions.

Descripción: In order to increase the understanding of the interactions which define the planar conformation of the methoxy group with respect to the aryl ring in methyl aryl ethers and the effect these interactions have on the methoxy NMR parameters, 17O and 13C spectra were measured and ab initio optimized geometries were calculated for three different conformations of the methoxy group in 2-methoxy-5-X-pyridines (X = H, NO2, and NH2). 17O and 13C chemical shifts were also calculated using the LORG approach. It was found that, contrary to what has been commonly assumed, the oxygen and the carbon of the methoxy group are deshielded when the electronic resonance interaction is increased. Therefore, the large 13C deshielding effect observed for a conformation with an out-of-plane methoxy group in aryl methyl ethers and related compounds has to be attributed to the inhibition of the attractive van der Waals forces between the methyl moiety and the aromatic ortho-cis carbon atom. © 1993 American Chemical Society.

Descripción: This article is a continuation of our previous paper on schemes of energy decompositions of molecular systems in the real space [D. R. Alcoba, J. Chem. Phys. 122, 074102 (2005)] now using correlated state functions. We study, according to physical arguments, the appropriate management of the density cumulant arising from the second-order reduced density matrix at correlated level, whose contributions can be assigned to one-center or to two-center terms in the energy partitioning. Our treatments are applied within two physical space partitioning schemes: the Bader partitioning into atomic basins and the fuzzy atom procedure. The results obtained in selected molecules are analyzed and discussed in detail. © 2007 American Institute of Physics.

Descripción: A recently proposed local Fukui function is used to predict the binding site of atomic hydrogen on silicon clusters. To validate the predictions, an extensive search for the more stable Sin H (n=3-10) clusters has been done using a modified genetic algorithm. In all cases, the isomer predicted by the Fukui function is found by the search, but it is not always the most stable one. It is discussed that in the cases where the geometrical structure of the bare silicon cluster suffers a considerable change due to the addition of one hydrogen atom, the situation is more complicated and the relaxation effects should be considered. © 2005 American Institute of Physics.

Descripción: In contrast to the wide spectrum of cytochrome P450 monooxygenases, there are only 2 heme-based dioxygenases in humans: tryptophan dioxygenase (hTDO) and indoleamine 2,3-dioxygenase (hIDO). hTDO and hIDO catalyze the same oxidative ring cleavage reaction of L-tryptophan to N-formyl kynurenine, the initial and rate-limiting step of the kynurenine pathway. Despite immense interest, the mechanism by which the 2 enzymes execute the dioxygenase reaction remains elusive. Here, we report experimental evidence for a key ferryl intermediate of hIDO that supports a mechanism in which the 2 atoms of dioxygen are inserted into the substrate via a consecutive 2-step reaction. This finding introduces a paradigm shift in our understanding of the heme-based dioxygenase chemistry, which was previously believed to proceed via simultaneous incorporation of both atoms of dioxygen into the substrate. The ferryl intermediate is not observable during the hTDO reaction, highlighting the structural differences between the 2 dioxygenases, as well as the importance of stereoelectronic factors in modulating the reactions.

Descripción: Frataxin (FXN) is an α/β protein that plays an essential role in iron homeostasis. Apparently, the function of human FXN (hFXN) depends on the cooperative formation of crucial interactions between helix α1, helix α2, and the C-terminal region (CTR) of the protein. In this work we quantitatively explore these relationships using a purified recombinant fragment hFXN90-195. This variant shows the hydrodynamic behavior expected for a monomeric globular domain. Circular dichroism, fluorescence, and NMR spectroscopies show that hFXN90-195 presents native-like secondary and tertiary structure. However, chemical and temperature induced denaturation show that CTR truncation significantly destabilizes the overall hFXN fold. Accordingly, limited proteolysis experiments suggest that the native-state dynamics of hFXN90-195 and hFXN90-210 are indeed different, being the former form much more sensitive to the protease at specific sites. The overall folding dynamics of hFXN fold was further explored with structure-based protein folding simulations. These suggest that the native ensemble of hFXN can be decomposed in at least two substates, one with consolidation of the CTR and the other without consolidation of the CTR. Explicit-solvent all atom simulations identify some of the proteolytic target sites as flexible regions of the protein. We propose that the local unfolding of CTR may be a critical step for the global unfolding of hFXN, and that modulation of the CTR interactions may strongly affect hFXN physiological function. © 2012 Roman et al.

Descripción: We investigate the importance of relativistic effects on NMR shielding constants and chemical shifts of linear HgL2 (L = Cl, Br, I, CH 3) compounds using three different relativistic methods: the fully relativistic four-component approach and the two-component approximations, linear response elimination of small component (LR-ESC) and zeroth-order regular approximation (ZORA). LR-ESC reproduces successfully the four-component results for the C shielding constant in Hg(CH3)2 within 6 ppm, but fails to reproduce the Hg shielding constants and chemical shifts. The latter is mainly due to an underestimation of the change in spin-orbit contribution. Even though ZORA underestimates the absolute Hg NMR shielding constants by ∼2100 ppm, the differences between Hg chemical shift values obtained using ZORA and the four-component approach without spin-density contribution to the exchange-correlation (XC) kernel are less than 60 ppm for all compounds using three different functionals, BP86, B3LYP, and PBE0. However, larger deviations (up to 366 ppm) occur for Hg chemical shifts in HgBr 2 and HgI2 when ZORA results are compared with four-component calculations with non-collinear spin-density contribution to the XC kernel. For the ZORA calculations it is necessary to use large basis sets (QZ4P) and the TZ2P basis set may give errors of ∼500 ppm for the Hg chemical shifts, despite deceivingly good agreement with experimental data. A Gaussian nucleus model for the Coulomb potential reduces the Hg shielding constants by ∼100-500 ppm and the Hg chemical shifts by 1-143 ppm compared to the point nucleus model depending on the atomic number Z of the coordinating atom and the level of theory. The effect on the shielding constants of the lighter nuclei (C, Cl, Br, I) is, however, negligible. © 2011 American Institute of Physics.