15N,(1)H Heteronuclear correlation NMR of gramicidin A in DMPC-d(67)

We have studied gramicidin A, an environmentally sensitive polymorphic pentadecapeptide, fully 15N-labelled and dispersed in a highly deuterated phospholipid bilayer system. By submitting the sample to fast magic angle spinning, we were able to reduce the polypeptide amide hydrogen linewidths to 160 Hz, and hence to partially resolve them. By correlating these resonances with the 40 Hz wide dipolar coupled 15N in a 2D-CROPSY (cross-polarization spectroscopy) experiment, it was possible to observe the 20 partially overlapping 1H-15N signal pairs from the polypeptide backbone and sidechains. Both chemical shift distributions closely match those of the same peptide in SDS micelles, but only poorly match those of conformationally different gramicidin A in trifluoroethanol, dimethylsulfoxide, or methanol/chloroform mixture. Our results are indicative of the N-to-N right-handed beta6.3-helix conformation of gramicidin A and offer sufficient resolution to encourage development of experiments to measure orientational or distance restraints using through-space dipolar couplings.     

Chemistry of polyfunctional molecules-123. Reactions of BiBr3, SbI3 and AsI3 with LiN(PPh2)2; X-ray structure of a cyclophosphazene salt containing arsenic(I)

BiBr₃ or SbI₃ react at 20°C with LiN(PPh₂)₂ (1) to give elementary Bi or Sb and the P---P coupled phosphazene ligand Ph₂P---N=PPh₂---PPh₂=N---PPh₂ (2). The reaction of AsI₃ with 1 at room temperature formed yellow needles of the eight-membered heterocycle (3), whereas AsI₃ interacted at 80°C with 1 in the molar ratio of 1:3 to give elementary arsenic and 2. Treatment of AsI₃ and 1 at 20°C in a 1:2 stoichiometry yielded the seven-membered, cyclic arsenium(I) salt

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I·4THF (5·4THF), which was characterized by elemental analysis, conductivity, mass, IR and NMR spectroscopy and single-crystal X-ray structural analysis.     

Solution conformation by NMR and molecular modeling of three sulfide-free somatostatin octapeptide analogs compared to angiopeptin

Summary The conformation in dimethylsulfoxide of the somatostatin derivative angiopeptin and of three disulfide-free analogs was estimated by two-dimensional nuclear magnetic resonance spectroscopy at room temperature. The resulting 3D molecular graphics were compared and shown to reflect the observed differences in the inhibition of restenosis after rat aorta balloon injury by these octapeptide inhibitors. Angiopeptin and its active analog 2 displayed a relatively rigid conformation of the cyclic hexapeptide backbone due to the presence of two well-defined hydrogen bonds, further stabilized by a third hydrogen bond outside the ring. No such constraints were detected for the two biologically inactive analogs, which, compared to 2, had a two-atom longer or shorter hexapeptide ring. The well-defined structure of compound 2 may serve as an improved pharmacophore for this new class of drugs.     

Chemie der lithium-enolat-heptafulvene: regiospezifische aldol-addition

Aldehydes and ketones add to the cycloheptatriene ester anion $\text{2a}$ and to the carboxylic acid dianion $\text{2b}$ at C-7 (α-attack) or C-2 (C-5) (γ-attack), depending on the starting compounds.