| An Unusual C–H . . . O Hydrogen Bond Mediated Reversal of Polypeptide Chain Direction in a Synthetic Peptide Helix (2000) | |||||||||
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| An unusual C-terminal conformation has been detected in a synthetic decapeptide designed to analyze the stereochemistry of helix termination in polypeptides. The crystal structure of the decapeptide Boc-Leu-Aib-Val-Ala-Leu-Aib-Val-DAla-DLeu-Aib-OMe reveals a helical segment spanning residues 1–7 and helix termination by formation of a Schellman motif, generated by DAla(8) adopting the left-handed helical (aL) conformation. The extended conformation at DLeu(9) results in a compact folded structure, stabilized by a potentially strong C-H . . . O hydrogen bond between Ala(4) CaH and DLeu(9) CO. The parameters for C-H . . . O interaction are Ala(4) CaH. . O=C DLeu(9) distance 3.27 Å, Ca-H . . O angle 176°, and O . . Ha distance 2.29 Å. This structure suggests that insertion of contiguous D-residues may provide a handle for the generation of designed structures containing more than one helical segment folded in a compact manner. | |||||||||
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