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 beta-bend conformation. The fully developed right-handed 3(10)-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. H-1-nmr and Fourier transform ir absorption studies suggest that in CDCl3 solution (a) the two tripeptides maintain a type II beta-bend conformation of comparable stability and (b) both diastereomeric pentapeptide sequences adopt a fully developed 3(10)-helix. A comparison with the preferred conformation of other extensively investigated C-alpha,alpha-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.