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991.
992.
In recent experiments of resonance‐enhanced laser ionization, large differences between circular dichroism measured for molecular and fragment ions were found by several research groups for different molecular systems. In the case of 3‐methylcyclopentanone (3‐MCP) we attributed this effect to a large circular dichroism of the molecular ion. In the work presented here, this effect in 3‐MCP is studied by ion spectroscopy, by varying the neutral intermediate excited state involved in resonance‐enhanced multiphoton ionization (REMPI) and by performing REMPI‐induced measurements of circular dichroism at different laser pulse energies. It turns out that the dynamics of structural changes in the ionic ground state strongly influences the observed ionic circular dichroism. 相似文献
993.
Wei R Martin TG Rant U Dietz H 《Angewandte Chemie (International ed. in English)》2012,51(20):4864-4867
DNA has it covered: DNA origami gatekeeper nanoplates convert nanopores in solid-state membranes into versatile devices for label-free macromolecular sensing applications. The custom apertures in the nanoplates can be chemically addressed for sequence-specific detection of DNA. 相似文献
994.
I Jakovkin M Klipfel C Muhle-Goll AS Ulrich B Luy U Sternberg 《Physical chemistry chemical physics : PCCP》2012,14(35):12263-12276
Although difficult to analyze, NMR chemical shifts provide detailed information on protein structure. We have adapted the semi-empirical bond polarization theory (BPT) to protein chemical shift calculation and chemical shift driven protein structure refinement. A new parameterization for BPT amide nitrogen chemical shift calculation has been derived from MP2 ab initio calculations and successfully evaluated using crystalline tripeptides. We computed the chemical shifts of the small globular protein ubiquitin, demonstrating that BPT calculations can match the results obtained at the DFT level of theory at very low computational cost. In addition to the calculation of chemical shift tensors, BPT allows the calculation of chemical shift gradients and consequently chemical shift driven geometry optimizations. We applied chemical shift driven protein structure refinement to the conformational analysis of a set of Trypanosoma brucei (the causative agent of African sleeping sickness) tryparedoxin peroxidase Px III structures. We found that the interaction of Px III with its reaction partner Tpx seems to be governed by conformational selection rather than by induced fit. 相似文献
995.
Bag P Dutta S Biswas P Maji SK Flörke U Nag K 《Dalton transactions (Cambridge, England : 2003)》2012,41(12):3414-3423
A series of mononuclear lanthanide(III) complexes [Ln(LH(2))(H(2)O)(3)Cl](ClO(4))(2) (Ln = La, Nd, Sm, Eu, Gd, Tb, Lu) of the tetraiminodiphenolate macrocyclic ligand (LH(2)) in 95 : 5 (v/v) methanol-water solution fix atmospheric carbon dioxide to produce the carbonato-bridged trinuclear complexes [{Ln(LH(2))(H(2)O)Cl}(3)(μ(3)-CO(3))](ClO(4))(4)·nH(2)O. Under similar conditions, the mononuclear Y(III) complex forms the dimeric compound [{Y(LH(2))(H(2)O)Cl}(μ(2)-CO(3)){Y(LH(2))(H(2)O)(2)}](ClO(4))(3)·4H(2)O. These complexes have been characterized by their IR and NMR ((1)H, (13)C) spectra. The X-ray crystal structures have been determined for the trinuclear carbonato-bridged compounds of Nd(III), Gd(III) and Tb(III) and the dinuclear compound of Y(III). In all cases, each of the metal centers are 8-coordinate involving two imine nitrogens and two phenolate oxygens of the macrocyclic ligand (LH(2)) whose two other imines are protonated and intramolecularly hydrogen-bonded with the phenolate oxygens. The oxygen atoms of the carbonate anion in the trinuclear complexes are bonded to the metal ions in tris-bidentate μ(3)-η(2):η(2):η(2) fashion, while they are in bis-bidentate μ(2)-η(2):η(2) mode in the Y(III) complex. The magnetic properties of the Gd(III) complex have been studied over the temperature range 2 to 300 K and the magnetic susceptibility data indicate a very weak antiferromagnetic exchange interaction (J = -0.042 cm(-1)) between the Gd(III) centers (S = 7/2) in the metal triangle through the carbonate bridge. The luminescence spectral behaviors of the complexes of Sm(III), Eu(III), and Tb(III) have been studied. The ligand LH(2) acts as a sensitizer for the metal ions in an acetonitrile-toluene glassy matrix (at 77 K) and luminescence intensities of the complexes decrease in the order Eu(3+) > Sm(3+) > Tb(3+). 相似文献
996.
Assran AS Izarova NV Banerjee A Rabie UM Abou-El-Wafa MH Kortz U 《Dalton transactions (Cambridge, England : 2003)》2012,41(33):9914-9921
Interaction of potassium antimony(iii) tartrate hydrate K(2)(SbC(4)H(2)O(6))(2)·3H(2)O with the trilacunary Keggin derivatives [A-α-XW(9)O(34)](10-) (X = Si(IV), Ge(IV)) and [A-α-PW(9)O(34)](9-) in aqueous acidic medium (pH 4.8) resulted in three novel polyanions, [Sb(3)(A-α-XW(9)O(34))(2)](11-) (X = Si(IV) (1), Ge(IV) (2)) and [Sb(6)O(2)(A-PW(6)O(26))(A-α-PW(9)O(34))(2)](15-) (3), which were isolated as the hydrated potassium salts K(11)[Sb(3)(A-α-XW(9)O(34))(2)]·31H(2)O (X = Si(IV) (K-1), Ge(IV) (K-2)) and the mixed potassium-sodium salt K(14)Na[Sb(6)O(2)(A-PW(6)O(26))(A-α-PW(9)O(34))(2)]·61H(2)O (KNa-3) salts, respectively, and characterized by single-crystal X-ray diffraction, IR spectroscopy, as well as elemental and thermogravimetric analyses. The Sb(III)-containing polyanions 1-3 possess unique structural features, as they represent the first examples of sandwich-type POMs with trigonal-pyramidal Sb(III)O(3) linkers. The stability of 1-3 in aqueous media was investigated by multinuclear ((183)W, (31)P) NMR and UV-Vis spectroscopy. 相似文献
997.
Dang L Shibl MF Yang X Alak A Harrison DJ Fekl U Brothers EN Hall MB 《Journal of the American Chemical Society》2012,134(10):4481-4484
The binding of an alkene by Ni(tfd)(2) [tfd = S(2)C(2)(CF(3))(2)] is one of the most intriguing ligand-based reactions. In the presence of the anionic, reduced metal complex, the primary product is an interligand adduct, while in the absence of the anion, dihydrodithiins and metal complex decomposition products are preferred. New kinetic (global analysis) and computational (DFT) data explain the crucial role of the anion in suppressing decomposition and catalyzing the formation of the interligand product through a dimetallic complex that appears to catalyze alkene addition across the Ni-S bond, leading to a lower barrier for the interligand adduct. 相似文献
998.
Franz J. Schmied Christian Teichert Lisbeth Kappel Ulrich Hirn Robert Schennach 《Cellulose (London, England)》2012,19(3):1013-1021
A method is presented which enables analysis of lignin precipitated on the surface of kraft pulp fibers. As experimental input,
high-resolution atomic force microscopy phase images of the fiber surfaces have been recorded in tapping mode. A digital image
analysis procedure—based on the watershed algorithm—is applied to distinguish between cellulose fibrils and the precipitated
lignin. In this way, size distributions for the diameter of lignin precipitates on pulp fiber surfaces can be obtained. In
an initial application of the method, three softwood kraft pulps were analyzed: a black liquor cook with a very high content
of precipitated lignin, a bleached pulp where nearly no precipitated lignin is visible and an unbleached industrial pulp.
The proposed method is suggested as an appropriate tool to investigate the kinetics of lignin precipitation and the structure
of lignin precipitates in pulping and bleaching. 相似文献
999.
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
1000.
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