CRYSTAL-STRUCTURES OF BOC-D-PRO-OBZL 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, B oc-D-Iva-L-Pro-OBzl and Boc-L-Iva-L-Pro-OBzl, were determined by x-ray diffraction. The diastereomeric peptides were shown to adopt unturned conformations closely similar to each other (phi(Iva) 52-degrees, psi (Iva) 46-degrees, phi(Pro) -65-degrees, and phi(Pro) 143-degrees for D -Iva-L-Pro sequence and phi(Iva) 52-degrees, ps(Iva) 44-degrees, phi(P ro) -63-degrees, and psi(Pro) 148-degrees for L-IVa-L-Pro sequence). T he Pro ring of each peptide was in C(gamma)-endo conformation. The unu sually large angle C(Iva)-N(Pro)-C(Pro)delta values (131-degrees in bo th peptides) were observed, that was due to steric repulsion between t he delta-methylene of Pro and the alkyl side chain of Iva residue. The se conformations were essentially the same as that of the correspondin g alpha-aminoisobutyric acid (Aib) -containing peptide Boc-Aib-L-Pro-O Bzl. The result has demonstrated that replacement of either one of the two methyl groups of the Aib residue in Boc-Aib-L-Pro-OBzl with an et hyl group does not cause any significant change in the unturned confor mation of the dipeptide. (C) 1993 John Wiley & Sons, Inc.