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We have used ab initio calculations to compute all of the tensor elements of the electric field gradient for each carbon-deuterium bond in the ring of deuterated 3-methyl-indole. Previous analyses have ignored the smaller tensor elements perpendicular to principal component Vzz which is aligned with the C-2H bond (local bond z-axis). At each ring position, the smallest element Vxx is in the molecular plane and Vyy is normal to the plane of the ring. The asymmetry parameter η = (∣Vyy∣-∣Vxx∣)/∣Vzz∣ ranges from 0.07 at C4 to 0.11 at C2. We used the perpendicutar (off-bond) tensor elements, in concert with an improved understanding of the indole ring geometry, to analyze prototype 2H NMR spectra from well- oriented, hydrated peptide/lipid samples. For each of the four tryptophans of membrane-spanning gramicidin A (gA) channels, the inclusion of the perpendicular elements changes the deduced ring tilt by nearly 10° and increases the ring principal order parameter Szz for overall "wobble" with respect to the membrane normal (molecular z-axis). With the improved analysis, the magnitude of Szz for the outermost indole rings of Trp13 and Trp15 is indistinguishable from that observed previously for backbone atoms (0.93 ± 0.03). For the Trp9 and Trp11 rings, which are slightly more buried within the membrane, Szz is slightly lower (0.86 ± 0.03). The results show that the perpendicular elements are important for the detailed analysis of 2H NMR spectra from aromatic ring systems. [ABSTRACT FROM AUTHOR]

Neutral triple-decker complexes of the heavy alkaline earth metals and ytterbium with tetraiso-propylcyclopentadienide anions as terminal ligands and a cyclooctatetraene dianion as a middle deck have been synthesized from tetraisopropylcyclopentadienyl metal halide precursors and disodium cyclooctatetraenide. The pentaisopropylcyclopentadienyl analogue [{(C5iPr5)Yb}2(C8H8)] was prepared from ytterbium metal, cyclooctatetraene, and the free pentaisopropylcyclopentadienyl radical. X-ray crystal structure determinations for the barium and the calcium derivative show an almost linear arrangement of ring centers and metal atoms in both cases with metal-ring center distances of 2.33 Å (Ca-Cp), 1.99/1.98 Å (Ca-COT) and 2.71 Å (Ba-Cp), 2.40 Å (Ba-COT). The geometrical features of these molecules could be modeled quite accurately with density functional calculations. With potassium cyclononatetraenide, sparingly soluble bis(cyclononatetraenyl)barium could be prepared and characterized by 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. Cyclononatetraenyl(tetraisopropylcyclopentadienyl)- barium was obtained from ((C5HiPr4)Bal(THF)2]2 and KC9H9 as a 1:1 mixture with octaisopropylbarocene. Density functional calculations predict sandwich structures with parallel rings and a 2.37 Å Ba-ring distance for [Ba(C9H9)2] and a 174° bending with metal-ring distances of 2.72 Å (Ba-Cp) and 2.35 Å (Ba-CNT) for [(C5HiPr4)Ba(C9H9)]. All alkaline earth sandwich and triple-decker complexes mentioned above have been heated to 250 °C without decomposition and have been sublimed in oil pump vacuum. [ABSTRACT FROM AUTHOR]

A refocused INEPT through-bond coherence transfer technique is demonstrated for NMR of rigid organic solids and is shown to provide a valuable building block for the development of NMR correlation experiments in biological solids. The use of efficient proton homonuclear dipolar decoupling in combination with a direct spectral optimization procedure provides minimization of the transverse dephasing of coherences and leads to very efficient through-bond 1H-13C INEPT transfer for crystalline organic compounds. Application of this technique to 20 heteronuclear correlation spectroscopy leads to up to a factor of 3 increase in sensitivity for a carbon-13 enriched sample in comparison to standard through-bond experiments and provides excellent selectivity for one-bond transfer. The method is demonstrated on a microcrystalline sample of the protein Crh (2 x 10.4 koa). [ABSTRACT FROM AUTHOR]

The article reports that nonisotropic averaging of the nuclear chemical shift Hamiltonian arising from paramagnetic alignment results in residual anisotropic chemical shifts (RACS) which must be taken into account for an accurate analysis of pseudocontact shift data of heteronuclear spins with pronounced chemical shift anisotropy (CSA). It suggests that CSA tensors of 1HN, 15N, and 13C' nuclei have been determined from ΔδRACS observed in the presence of a liquid crystalline medium, from cross-correlated relaxation,and from density functional theory calculations.

The article reports on synthesis and characterization of 1,3-substituted triazole oligomers from amino acids. It suggests that triazole oligomers are a new class of distinctly folded nonpeptidic oligomers in which the amide bond is replaced by aromatic rings yet the chiral main chain and amino acid side chains are maintained. It argues that these oligomers afford specific conformations featuring a diverse set of side chains without the limitations imposed by the secondary amide bond. It also reports that solution NMR studies on trimers and tetramers suggested that the oligomers adopt zigzag conformations reminiscent of peptide β-strands.