Design of Peptides with β-Carbon Dehydroresidues: Syntheses, Crystal Structures and Molecular Conformations of Two Peptides, (I) \u3cem\u3eN\u3c/em\u3e‐Carbobenzoxy‐ΔVal‐Ala‐Leu‐OCH\u3csub\u3e3\u3c/sub\u3e and (II) \u3cem\u3eN\u3c/em\u3e‐Carbobenzoxy‐ΔIle‐Ala‐Leu‐OCH\u3csub\u3e3\u3c/sub\u3e

Highly specific structures can be designed by inserting dehydro-residues into peptide sequences. The conformational preferences of branched β-carbon residues are known to be different from other residues. As an implication it was expected that the branched β-carbon dehydro-residues would also induce different conformations when substituted in peptides. So far, the design of peptides with branched β-carbon dehydro-residues at (i + 1) position has not been reported. It may be recalled that the nonbranched β-carbon residues induced β-turn II conformation when placed at (i + 2) position while branched β-carbon residues induced β-turn III conformation. However, the conformation of a peptide with a nonbranched β-carbon residue when placed at (i + 1) position was not found to be unique as it depended on the stereochemical nature of its neighbouring residues. Therefore, in order to induce predictably unique structures with dehydro-residues at (i + 1) position, we have introduced branched β-carbon dehydro-residues instead of nonbranched β-carbon residues and synthesized two peptides: (I) N-Carbobenzoxy-ΔVal-Ala-Leu-OCH3 and (II) N-Carbobenzoxy-ΔIle-Ala-Leu-OCH3 with ΔVal and ΔIle, respectively. The crystal structures of peptides (I) and (II) have been determined and refined to R-factors of 0.065 and 0.063, respectively. The structures of both peptides were essentially similar. Both peptides adopted type II β-turn conformations with torsion angles; (I): φ1 = -38.7 (4)°, ψ1 = 126.0 (3)°; φ2 = 91.6 (3)°, ψ2 = -9.5 (4)° and (II): φ1 = -37.0 (6)°, ψ1 = 123.6 (4)°, φ2 = 93.4 (4), ψ2 = -11.0(4)° respectively. Both peptide structures were stabilized by intramolecular 4→1 hydrogen bonds. The molecular packing in both crystal structures were stabilized in each by two identical hydrogen bonds N1…O1′ (-x, y + 1/2, -z) and N2…O2′ (-x + 1, y + 1/2, -z) and van der Waals interactions