Journal of the American Chemical Society, Vol.120, No.33, 8448-8460, 1998
Demonstration of endo-cis-(2S,3R)-bicyclo[2.2.1]hept-5-en-2,3-dicarbonyl unit as a reverse-turn scaffold and nucleator of two-stranded parallel beta-sheets : Design, synthesis, crystal structure, and self-assembling properties of norborneno peptide analogues
endo-cis-(2S,3R)-Bicyclo[2.2.1]hept-5-en (norbornene) dicarbonyl unit with a built-in U-architecture has been demonstrated to be an excellent reverse-turn molecular scaffold. A large variety of endo-cis-(2S,3R)-norborneno bispeptides containing almost all of the coded amino acids were synthesized and examined for conformational preferences by H-1 NMR, FT-IR, CD, and X-ray crystallographic studies. While FT-IR and H-1 NMR variable-temperature studies ruled out the presence of any significant amount of intramolecular hydrogen bonding in simple bispeptides (3a-h) (except in Aib bispeptide), the CD studies were clearly in favor of a beta-turn type structure. Single-crystal X-ray studies on Aib, Val and Leucontaining norborneno bispeptides (3b-d) provided convincing proof for the presence of reverse-turn conformation. While the interstrand C-alpha-C-alpha' distances (5.2-5.7 Angstrom) were well within the range of those for beta-turn structures, no interstrand intramolecular hydrogen bonding was seen in Val and Leu bispeptides; the Aib bispeptide forms a seven-membered hydrogen-bonded ring, thus, showing that the norbornene (2S,3R)-dicarbonyl template assembles peptide chains in reverse-turn conformation by virtue of its built-in U-shaped architecture at these positions, and hydrogen bonding may not be necessary to stabilize the turn structure.
Keywords:SECONDARY STRUCTURE MIMETICS;DE-NOVO DESIGN;PEPTIDOMIMETIC INHIBITORS;SOLUTION CONFORMATIONS;CHEMICAL SYNTHESIS;IMPROVED TRANSPORT;LOCAL STRUCTURES;PROTEIN DESIGN;DENOVO DESIGN;ALPHA-HELICES