Structural versatility of peptides from Cα,α-disubstituted glycines: Preferred conformation of the chiral isovaline residue

The molecular structures of four protected isovaline- (Iva-) containing peptides to the pentamer level have been determined by x-ray diffraction. The peptides are t-Boc-Ala-(S)-Iva-Ala-OMe (t-Boc : tert-butyloxycarbonyl; OMe : methoxy) and its (R)-Iva diastereomer, and t-Boc-[Ala-(R)-Iva]₂-Ala-OH and its (S)-Iva diastereomeric methyl ester analogue. The two tripeptides are folded in an open type II β-bend conformation. The fully developed right-handed 3₁₀-helix formed by the (R)-Iva pentapeptide, which includes an unusual intramolecular (acid) O? H ?O?C(peptide) H bond, is partially unfolded (near the C-terminus) in the (S) -Iva pentapeptide. ¹H-nmr and Fourier transform ir absorption studies suggest that in CDCl₃ solution (a) the two tripeptides maintain a type II β-bend conformation of comparable stability and (b) both diastereomeric pentapeptide sequences adopt a fully developed 3₁₀-helix. A comparison with the preferred conformation of other extensively investigated C^α,α-disubstituted glycines is made and the implications for the use of the Iva residue in designing conformationally constrained analogues of bioactive peptides are briefly discussed.