Studies on the Conformation of Boc‐Protected (<i>S</i>)‐(+)‐Isovaline Homopeptide Methyl Esters in the Solid State and in Solution

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<jats:title>Abstract</jats:title><jats:p>X‐Ray diffraction analyses of the fully protected peptides Boc‐[(<jats:italic>S</jats:italic>)‐Iva]<jats:sub>n</jats:sub>‐OMe (<jats:italic>n</jats:italic> = 3, 4, 6) reveal two independent molecules in the asymmetric unit. The structures of these can be described as β‐turns or 3<jats:sub>10</jats:sub> helices (depending on the length of the oligopeptide) of alternating screw sense (<jats:italic>M</jats:italic> and <jats:italic>P</jats:italic>) in a head to tail alignment. This structure is stabilized by hydrogen bonds between the NH(1) of the (<jats:italic>M</jats:italic>)‐helix and the OC(ω‐1) of the (<jats:italic>P</jats:italic>)‐helix and the NH(2) (<jats:italic>M</jats:italic>) and the ester carbonyl group (<jats:italic>P</jats:italic>). Low temperature <jats:sup>1</jats:sup>H‐NMR spectra of the hexamer in CD<jats:sub>2</jats:sub>Cl<jats:sub>2</jats:sub> solution show two interchanging species in a ratio of 4:1; NOESY experiments prove that these are the two helical conformers found in the crystal (<jats:italic>P</jats:italic>:<jats:italic>M</jats:italic>, 4:1). The NOESY spectrum at −90°C indicates the pairing of (<jats:italic>P</jats:italic>) and (<jats:italic>M</jats:italic>) helices. Thermodynamic and kinetic parameters for the helix transformation <jats:italic>P</jats:italic> ⇌ <jats:italic>M</jats:italic> (unfolding/folding) are presented.</jats:p>

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