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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 Cα,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.