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

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Abstract
The crystal structures of the isovaline (Iva) containing dipeptides, Boc‐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 (ϕIva 52°, ψIva 46°, ϕPro–65°, and ψPro 143° for D‐Iva‐L‐Pro sequence and ϕIva 52°, ψIva 44°, ϕPro −63°, and ψpro 148° for L‐Iva‐L‐Pro sequence). The Pro ring of each peptide was in Cγ‐endo conformation. The unusually large ∠CIva‐NPro‐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‐OBzl. 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 ethyl group does not cause any significant change in the unturned conformation of the dipeptide. © 1993 John Wiley & Sons, Inc.