Crystal structures of Boc-D- and L-Iva-L-Pro-OBZl: Unturned conformation of Aib-Pro sequence unaffected by replacement of Me with Et in Aib

The crystal structures of the isovaline (Iva) containing dipeptides, Boc-D -Iva-L -Pro-OBz l and Boc-L -Iva-L -Pro-OBz l, were determined by x-ray diffraction. The diastereomeric peptides were shown to adopt unturned conformations closely similar to each other (?_Iva 52°, ψ_Iva 46°, ?_Pro–65°, and ψ_Pro 143° for D -Iva-L -Pro sequence and ?_Iva 52°, ψIva 44°, ?_Pro ?63°, and ψ_pro 148° for L -Iva-L -Pro sequence). The Pro ring of each peptide was in C^γ-endo conformation. The unusually large ∠C_Iva-N_Pro-C values (131° in both peptides) were observed, that was due to steric repulsion between the δ-methylene of Pro and the alkyl side chain of Iva residue. These conformations were essentially the same as that of the corresponding α-aminoisobutyric acid (Aib)-containing peptide Boc-Aib-L -Pro-OBz l. The result has demonstrated that replacement of either one of the two methyl groups of the Aib residue in Boc-Aib-L -Pro-OBz l with an ethyl group does not cause any significant change in the unturned conformation of the dipeptide. © 1993 John Wiley & Sons, Inc.