- ROUTES TO OPTICALLY ACTIVE ELECTRON-RICH OLEFINS (L*2) AND SOME DERIVED CARBENEMETAL COMPLEXES; X-RAY STRUCTURES OF *)(NO)(PPh3)> AND cis-*)> *=, L'*=, COD = 1,5-C8H12>
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Optically active electron-rich olefins L2* are obtained from readily available chiral starting materials , and are converted into carbenemetal (RhI or CoI) complexes with retention of optical integrity.
- Coleman, Anthony W.,Hitchcock, Peter B.,Lappert, Michael F.,Maskell, Robin K.,Mueller, Joachim H.
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- Competitive formation of 10- and 7-membered hydrogen-bonded rings of proline-containing model peptides
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Intramolecularly hydrogen-bonded structures of proline-containing model peptides with a sequence of N-tert-butoxycarbonyl-prolyl-Xaa-NHCH3 [Xaa = Gly (glycyl), Ala (alanyl), Phe (phenylalanyl), Leu (leucyl), Ile (isoleucyl), and Val (valyl)] were studied by proton nuclear magnetic resonance and infrared spectroscopy. Variation of chemical shifts of amide protons with composition change of DMSO-d6/CDCl3 mixed solvents were found to be a good measure of intramolecular hydrogen bonding of peptides in CDCl3 solution. It has been shown that 10- and 7-membered hydrogen-bonded rings, which should have the β- and γ-turn like structures in proteins, respectively, form competitively with each other. It is suggested that the equilibrium between the two hydrogen-bonded rings is determined by steric hindrance due to a side chain of the Xaa residue. Free energies for formation of the 10- and 7-membered hydrogen-bonded rings, ΔG10 and ΔG7, were estimated from the solvent composition-dependent change of the chemical shifts. A good correlation between ΔG10 and the occurrence frequencies of residues Xaa at the (i + 2)th position for the β-turns in proteins has been found.
- Jin, Yusuke,Tonan, Kenji,Ikawa, Shun-Ichi
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p. 2795 - 2802
(2007/10/03)
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