Two alternative conformational states of α,α-dialkylglycyl-L-prolyl sequences governed by presence/absence of an NH group directly following the proline residue. X-Ray crystal and molecular structures of Boc-D-Iva-L-Pro-NHBzl and Boc-L-Iva-L-Pro-NHBzl

Abstract

The crystal structures of the isovaline-containing dipeptides, Boc-D-Iva-L-Pro-NHBzl 4 and Boc-L-Iva-L-Pro-NHBzl 5 were determined by X-ray diffraction. The diastereoisomeric peptides adopt intramolecular hydrogen-bonded β-turn conformations closely similar to each other (4:φ_Iva–51°, ψ_Iva–38°, φ_Pro–70° and ψ_Pro–17° and 5: φ_Iva–53°, ψ_Iva–35°, φ_Pro–72° and ψ_Pro–14°). The Pro ring of each peptide is in C^γ-exo conformation. These conformations are essentially the same as those in the reported crystal structures of the Aib-L-Pro sequence possessing an NH group directly attached to the carbonyl of the L-Pro, indicating that replacement of either one of the two methyl groups of the Aib moiety with an ethyl group does not cause any significant change in the β-turn conformation of the Aib-L-Pro sequence in the crystalline state.

CD spectral analysis of the terminal chromophoric group-carrying peptides Dnp-Gly-X-L-Pro-Gly-pNA (X = Aib 6 and D/L-Iva 7/8) has shown that these three tetrapeptides in CHCl₃ and THF solutions also adopt a β-turn-type conformation. CD spectra of glycolic acid residue-containing analogues in place of the fourth Gly residue revealed a lack of β-turn tendency in these analogues, indicating the importance of intramolecular hydrogen bonding for the β-turn conformation of the central dipeptide moieties. The results are consistent with the reported unturned crystal structures of Aib-L-Pro arid D/L-Iva-L-Pro sequence-containing peptides lacking the NH group which directly follows the Pro residue available for intramolecular hydrogen bonding.