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Upfield Shift (upfield + shift)
Selected AbstractsTowards Understanding of the Selective Precipitation of Alkali Metal Cations in Presence of Dipicrylamine AnionEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2005Suresh Eringathodi Abstract Dipicrylamine anion (DPA,) precipitates out [K(DPA)] with high selectivity from salt bitterns containing Na+, K+, and Mg2+, whereas the same ligand shows poor selectivity towards K+ , and much higher selectivity towards Cs+ , in studies conducted with a mixture of K+, Rb+, and Cs+. Their single-crystal structures reveal that the K+ and Rb+ salts have similar layered structures, with 8 oxygen atoms from seven DPA, anions encapsulating the metal cation, whereas the Cs+ salt possesses a channel-like structure with the metal ion encapsulated by ten oxygen atoms from six DPA,. The conformation of DPA, in the [Cs(DPA)] single crystal matches closely that of DPA in crystalline state. M···O and intermolecular C,H···O interactions together stabilize the structures. The 133Cs NMR spectrum of the poorly soluble [Cs(DPA)] shows an upfield shift of the peak with respect to CsCl as a result of the interaction with the oxygen atoms of DPA,, whereas 23Na NMR spectrum of the highly soluble [Na(DPA)] shows no such upfield shift compared to NaCl. Powder XRD patterns of bulk [M(DPA)] (M = K+, Rb+, and Cs+) precipitates show that these are similar to the patterns obtained by simulation of the single-crystal X-ray data. The selectivity of precipitation correlates qualitatively with the size and hydration enthalpies of the ions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Synthesis and Reactivity of 23 - tert -Butyl- and 23 -Phenyltetraarylazuliporphyrins: an Analysis of the Effect of Bulky Substituents on Oxidative Ring Contractions to Benzocarbaporphyrins,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 24 2007Jessica A. El-Beck Abstract 6- tert -Butyl- and 6-phenylazulene reacted with pyrrole and benzaldehyde in a molar ratio of 1:3:4 in the presence of BF3·Et2O in chloroform, followed by oxidation with DDQ, to give 23 -substituted tetraphenylazuliporphyrins in 15,20,% yield. Slightly higher yields of the related meso -tetrakis(4-chlorophenyl)azuliporphyrins were obtained using 4-chlorobenzaldehyde. The presence of an electron-donating tert -butyl substituent increased the diatropic character of the azuliporphyrin system as determined by the proton NMR chemical shifts for the internal CH resonance, while intermediary results were noted for 23 -phenylazuliporphyrins. Addition of TFA afforded dications with increased aromatic ring currents, but electron-donating substituents (tBu,>,Ph) again produced a larger upfield shift for the internal CH signal due to stabilization of the tropylium character that is required so that the system can attain carbaporphyrin-type aromaticity. The substituted azuliporphyrins reacted with nickel(II) acetate or palladium(II) acetate to give the corresponding organometallic derivatives. In addition, oxidations with tBuOOH and KOH afforded benzocarbaporphyrin products in approximately 50,% yield. The presence of tert -butyl or phenyl substituents did not block these oxidative ring contraction processes, and the rate of reaction was slightly increased compared to 23 -unsubstituted azuliporphyrins. The major products were 22 - tert -butyl or phenyl-substituted benzocarbaporphyrins and minor products with an additional formyl substituent were also isolated. These products are consistent with an initial nucleophilic addition occurring at the position adjacent to the R group on the azulene ring. Detailed mechanisms are proposed to explain these observations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Toward direct determination of conformations of protein building units from multidimensional NMR experiments VI.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2005Chemical shift analysis of his to gain 3D structure, protonation state information Abstract NMR,chemical shift structure correlations were investigated by using GIAO-RB3LYP/6-311++G(2d,2p) formalism. Geometries and chemical shifts (CSI values) of 103 different conformers of N,-formyl-L-histidinamide were determined including both neutral and charged protonation forms. Correlations between amino acid torsional angle values and chemical shifts were investigated for the first time for an aromatic and polar amino acid residue whose side chain may carry different charges. Linear correlation coefficients of a significant level were determined between chemical shifts and dihedral angles for CSI[1H,]/,, CSI[13C,]/,, and CSI[13C,]/,. Protonation of the imidazole ring induces the upfield shift of CSI[13C,] for positively charged histidines and an opposite effect for the negative residue. We investigated the correspondence of theoretical and experimental 13C,, 13C,, and 1H, chemical shifts and the nine basic conformational building units characteristic for proteins. These three chemical shift values allow the identification of conformational building units at 80% accuracy. These results enable the prediction of additional regular secondary structural elements (e.g., polyProlineII, inverse ,-turns) and loops beyond the assignment of chemical shifts to ,-helices and ,-pleated sheets. Moreover, the location of the His residue can be further specified in a ,-sheet. It is possible to determine whether the appropriate residue is located at the middle or in a first/last ,-strand within a ,-sheet based on calculated CSI values. Thus, the attractive idea of establishing local residue specific backbone folding parameters in peptides and proteins by employing chemical shift information (e.g., 1H, and 13C,) obtained from selected heteronuclear correlation NMR experiments (e.g., 2D-HSQC) is reinforced. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1307,1317, 2005 [source] 1H-NMR Studies of Duplex DNA Decamer Containing a Uracil Cyclobutane Dimer: Implications Regarding the High UV Mutagenecity of CC Photolesions,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2002Hyun Mee Lee ABSTRACT To determine the origin of the UV-specific CC to TT tandem mutation at the CC site, we made a duplex DNA decamer containing a uracil cis,syn cyclobutane dimer (CBD) as the deaminated model of a cytosine dimer. Two-dimensional 1H-NMR spectroscopy studies were performed on this sequence where two adenines (Ade) were opposite to the uracil dimer. Two imino protons of the uracil dimer were found to retain Watson,Crick hydrogen bonding with the opposite Ade, although the 5,-U(NH) of the dimer site showed unusual upfield shift like that of the 5,-T(NH) of the TT dimer, which seemed to be associated with deshielding by the flanking base rather than with reduced hydrogen bonding. (McAteer et al. 1998, J. Mol. Biol. 282:1013,1032). Hydrogen bondings at the dimer site were also supported by detecting typical strong nuclear Overhauser effects (NOE) between two imino protons and the opposite Ade H2 or NH2. But sequential NOE interactions of base protons with sugar protons were absent at the two flanking nucleotides of the 5, side of the uracil dimer and at the intradimer site, contrasting with its thymine analog where sequential NOE was absent only at the A4,T5 step. In addition, NOE cross peak for U5(NH) , A4(H2) was detected, although the NOE interactions of U6(NH) with A7(H2) and A17(H2) were not observed in contrast to the thymine dimer duplex. This different local structural alteration may be affected by the induced right-hand twisted puckering mode of cis,syn cyclobutane ring of the uracil dimer in the B-DNA duplex, even though the isolated uracil dimer had left-hand twisted puckering rigidly. In parallel, these observations may be correlated with observed differences in mutagenic properties between cis,syn UU dimer and cis,syn TT dimer. [source] Four Generations of Water-Soluble Dendrimers with 9 to 243 Benzoate Tethers: Synthesis and Dendritic Effects on Their Ion Pairing with Acetylcholine, Benzyltriethylammonium, and Dopamine in WaterCHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2008Elodie Boisselier Abstract Water-soluble benzoate-terminated dendrimers of four generations (from G0 with 9 branches to G3 with 243 branches) were synthesized and fully characterized. They form water-soluble assemblies by ion-pairing interactions with three cations of medicinal interest (acetylcoline, benzyltriethylammonium, and dopamine), which were characterized and investigated by 1H,NMR spectroscopy, whereas such interactions do not provoke any significant shift of 1H,NMR signals with the monomeric benzoate anion. The calculated association constants confirm that the dendritic carboxylate termini reversibly form ion pairs and aggregates. Diffusion coefficients and hydrodynamic diameters of the dendrimers, as well as changes thereof on interaction with the cations, were evaluated by DOSY experiments. The lack of increase of dendrimer size on addition of the cations and the upfield shifts of the 1H,NMR signals of the cation indicate encapsulation within the hydrophobic dendrimer interiors together with probable backfolding of the benzoate termini. [source] | ||||||||||